Pruning 101: Health & Structure

The Pruning Series, 4.  If you choose to read only one of my posts on pruning, this should be the one.

 

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Pruning cannot save every tree. This large Magnolia appears to have lost its main trunk  long ago and is now surrounded by suckers and sprouts with most of the base rotted out. The section growing to the right is full of rot and is leaning over the adjacent house. It is common for trees like this, heavily damaged and in decline to sprout this way.  Sprouts such as these are weakly attached to the trunk and the structural integrity of this tree is very compromised.  I would not be surprised if spring’s new grew will be enough added weight to cause it to collapse on the house.  The entire ‘landscape’ suffers neglect.  I’m sure no one is monitoring it.  Such is the case with most urban trees.

Whatever your goals for pruning may be you must always keep plant health first and foremost in mind.  In many cases, especially with high value plants in our landscapes, this might be our only reason to prune.  In any good pruning class the instructor will emphasize in some form, the dictum, ’First, do no harm!’ which is often attributed to the medical world’s Hippocratic Oath.  It seems fitting to me to do this as both are dealing with life and promoting good health, only with horticulture and gardening our ‘patients’ are plants.  All organisms have a characteristic, genetically determined structure, that when compromised threatens its health.   All organisms experience stress and, if within limits, respond by strengthening their structure.  Expose them to excessive stress and physical damage occurs.  Storm damage, breakage, vandalism, branch failure following the growth of weak structure, a ‘burden’ of dead wood, diseased tissue, all add to the stresses on a plant and can all be relieved by good pruning…or exacerbated by poor or overly heavy pruning.  Timing can also be a factor as it can disrupt the natural growth cycle causing a delay in the plant’s acclimation to cold process.

When to Prune in Order to Minimize Damage

When beginning to selectively prune it is a good idea to proceed thoughtfully.  Examine the plant.  Before you begin making your shaping or reducing cuts whether they are heading back or thinning type, consider first cutting out/removing the dead, broken and diseased wood.  All of us have jumped right in making the selective cuts we want first only to realize later that one of the branches that we had intended to leave in place was actually dead or diseased and declining leaving us with a plant which is ‘out  of balance’ that we are unhappy with.  Once cut, we can’t replace a branch.  This may happen more often than you might think as we often are doing pruning in the winter/dormant season because we are no longer busied by the have to’s of a summer garden and foliage is absent, the sun, when present, is at a low angle and may be harsh, making it more difficult to discern whether a leafless branch is dead or alive.  Experience teaches us to see the slight shriveled thin bark on dead lateral branches or the ‘swollen’ still healthy collar around its base, which directs us precisely to where we should remove it.  We also learn over time, as we push through branches, that dead and live branches move and feel different .  Sometimes, when I’m not sure, I’ll bend one, and, if its dead it will crack and I’ll follow it back to where it should be removed.  This is an experience thing.

Obviously, it would be better to prune then in the summer when plants are in leaf! Right?  Maybe.  It is also true that the same foliage can make it more difficult to see the whole structure of the plant making it more difficult to make the best structural and aesthetic decisions.  I typically, but not exclusively, prune Japanese Maples when they are in leaf because I am going for a particular ‘feel’ and I prefer that the foliage be there to help determine that.  I ‘use’ the leaves to work my way toward a particular layered look and density of the branching and foliage.  On many other woody plants I’d rather have them dormant and free of foliage.

‘When should one prune?’  When is it best?  Here in the Maritime Pacific Northwest, where our temperatures are relatively mild, the acceptable pruning season is very long.  I’ve often told people, somewhat tongue in cheek, that it is whenever your tools are sharp and you have the time.  There are factors to consider.  In my recent posting on ‘chilling and freezing’ I wrote of how ‘late’ heavy pruning can force growth well into the Fall that will still be soft going into the winter putting all of that new growth at risk for freeze damage.  Woody tissues take time to ‘cure’, to harden, with the proper lignin.  Remember that part of a woody plant’s freeze resistance is in its rigid sturdy structure toughening it to rupture by freezing water in its inter-cellular tissues which leads to the potential dehydration and death of cells.  It may also delay the process of acclimation since the plant will be in a more active growth state rendering the entire plant more subject to freeze damage.  Still, here, in late summer, light corrective pruning would probably be okay, but this needs to be kept in mind.

We also avoid pruning during freezing conditions.  Because plants form physical barriers to wounds when they are ‘active’ they are less able to do so when chilled or freezing.  Again, referring to my ‘chilling freezing’ post, cutting plants opens a wound it will be slower to respond to by sealing off.  A stem that contains ‘supercooled’ water can then freeze in its conductive vessels, while water within cells is more ‘protected, down to the next node or barrier in the stem possibly killing local cells and tissue.  Whether the water is supercooled or not the wound makes nucleation and thus freezing more likely.  I’m thinking particularly of winter pruning roses before the spring push of growth, but wonder about similar damage in other woody plants as well.  On roses I’ve seen resultant freeze damage extend down below the cut to the next node or so.  It seems prudent to not prune under such conditions, and in the case of Roses, maybe to wait until the last chance of freezing passes, though in this yo-yoing climate that could slow things down as you try to avoid damage to new soft growth.

Many gardeners feel compelled to put their gardens to bed by cutting herbaceous plants back and removing what they consider to be unsightly dead or damaged growth from the previous summer.  It is very likely that when we do this with plants that may be borderline in terms of cold-hardiness here that we may be increasing the likelihood of freeze damage or death.  Every plant has only so much ability to ‘acclimate’.  It is genetically fixed.  Cutting such a plant with such limited hardiness response may in fact open it up, literally, to freezing down more severely.  Obviously these more tender plants don’t have the tolerance for super cooling to below zero.  It is also likely that they have less ability to erect barriers to freezing when their vascular systems have been cut down.  Hardiness may not be absolute.  Plants may simply begin failing at a certain temperature as its tolerance and barriers collapse under cold stress, one after the other, down its stems.  Cutting away top growth, then, reduces some of its protective barrier that stands between the cold and its all important meristematic tissue.  Many mediterranean plants seem to be in this group and it is best to leave them alone until they initiate growth in spring before you cut them back to reduce their legginess and keep them more compact.

Very often it is simply expedient to prune in the late winter before new growth initiates both obscuring and slowing the pruning process and also before the chance of damaging/breaking new soft growth.  Fruit trees (always considering whether they flower and fruit on ‘spurs’) are generally dormant pruned as are many ornamental trees and shrubs that bloom on new growth taking care of any shaping or reduction before they bloom.  (On a recent December drive up the Hood River Valley orchard pruning was in process.)  Many other plants bloom on ‘year old’ wood and dormant pruning may sacrifice most of the blooms.  In these cases pruning is often delayed until after blooming, unless the plants are deemed to be so over grown that it is worth sacrificing a year’s bloom now!  Many plants set their ‘buds’ for the next years flowers sometime during the growing season.  Plants like Azalea and Rhododendron set them early, sometimes within a month of their spring blooming so the timing must be watched more carefully.  If you prune much, and you pay attention, you’ll learn this fairly quickly.  You’ll recognize the new flower buds as they form and how they differ from vegetative buds.

Other plants used as hedging or topiary, because blooming is secondary at best, are sheared and shaped whenever needed, depending on one’s standard for ‘fuzziness’ sometimes calling for shearing two or three times a growing season other times, for intricate herbaceous Celtic and knot patterns, even more.  Some people will shear during the dormant season leaving cut stems and sliced leaves exposed for the duration.  This is an especially bad aesthetic choice with larger leaved broad leaved evergreens like English Laurel and Photinia, in my opinion, two of the ‘worst’ choices for hedging due to their rapid growth rate and their ultimate size…and people continue to plant them for a quick visual barrier only to have them claim much of one’s landscape if you aren’t diligent and persistent.

There are several books out there on the cutting back of perennials either to make room for a new ‘flush’ of growth, make the bloom more visible in the case of Helleborus x hybridus or to reduce a plant’s height and force sturdier lower flowering scapes more resistant to flopping.  And of course, there is dead heading done for neatness, disease control and promoting continued flowering on Roses, Perennials and Annuals.  The practice of these are often very specific to each plant and the gardener’s intent.  I used to have a neighbor who dead headed a gorgeous huge hybrid Rhody in his front yard by early summer every year convinced that it encouraged heavier blooming with larger trusses out of the idea that energy, not put into seed production, would all go into next years buds???  I don’t know about that but it was a beautiful plant.  They spent hours on it every year on the ground and up on ladders.

The timing of pruning then is dependent on the plant, your intent and schedule.  Sometimes we simply can’t do a job at its ideal time.  We have to weigh the consequences when we are forced off of schedule.  With pruning, if we are on an annual cycle, missing a year or delaying a shearing, probably won’t be the end of the world.  Shearing of hedges or topiary is probably the most problematic to skip as the ‘shell’ of foliage is very ‘thin’ and cutting a plant harder to make up for the miss will result in ‘holes’ and stubs until the plant can respond with more growth.  Regular shearing to maintain their form and size is important as these fill a formal and rigid role in the garden/landscape and cutting them hard because they’ve gotten too big will detract from the look of the entire landscape.

Decurrent and Excurrent Plants

 

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Two beautifully formed and symmetrical Sweetgum Trees…each recently topped at the same height??? Sweetgums typically exhibit an excurrent form with strong apical dominance. Note the relatively small branching size compared to the trunk. Keep a tree’s natural form in mind when both choosing and pruning your tree. It is common to see Sweetgums with competing leaders and rounded silhouettes resulting from inappropriate pruning, damage at the nursery and storm damage.  It is always a good idea to keep in mind a tree’s ideal form when evaluating your own trees.  Somebody had to really want to do this as the cuts were beyond the reach of any homeowner. It wasn’t Asplundh. The powerlines are on the other side of the street.

Many plants have very strong ‘apical dominance’ and typically, those that do, form a strong central leader that suppresses everything below.  Woody plants, with their more ‘permanent’ structure can be classified by their ‘degrees’ of apical dominance.  This dominance, and genetics, gives most conifers their characteristic excurrent form with narrow columnar trees like Calocedrus decurrens (I know, the species name throws you.  When describing foliage decurrent describes leaves that are held close to the stem.  This can be confusing.), Incense Cedar, Cupressus sempervirens, Italian Cypress, and the many select, columnar forms of a wide variety of species may have examples of this extreme expression.  (Deciduous broad leafed woody plants as well can be considered in terms of their apical dominance.)  The side branching of excurrent trees is weaker.  Removal of the terminal in such plants whether by storms or pruning may greatly alter the form of some of these plants.  In most conifers, like Douglas Fir, losing their terminals, ‘releases’ the buds in the whorl of branches just below them, generally with each of the branches turning upward in competing, individual efforts to gain dominance.  With strongly excurrent plants, those that are narrowly columnar like those mentioned above and Arbor Vitae, their genetics keep the overall form of the plant very narrow so that when the top is removed you tend not to see the once horizontal growing whorls of branches turn up.  Never the less dominance shifts.  Topping such plants in an effort to limit height will result in multiple stems forming congested growth at the top.

Staying with conifers, many or most, of these cannot be pruned ‘hard’ back into old wood.  There are no dormant buds waiting to be released and you will be left with a stub, forever, until the dead branch eventually drops away.  Attempts to control their size must be active and persistent year to year and is most effectively done through ‘candeling’ as I’ve described elsewhere.  Conifers typically grow in forests, sometimes in mixed stands, and the ‘park’ like landscapes we sometimes experience in natural settings is a result of a conifer’s decurrent form with their grand straight trunks and their habit of shedding lower branches due to inadequate light within the stand.  Growing individuals in the open will result in a tree that retains its lower whorls of branches unless we remove them or shade from structures or other plants intrudes.  The tips on these lower branches will continue to grow, though slowly, and can be quite dense.

 

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I don’t know my Willows but this one is really nice. It still has its normal natural upright form and exhibits its tendency to sprout. It has no significant storm damage and no one has headed it back anywhere. Damage and clumsy pruning can stimulate heavy sprouting within the canopies of Willows.  They will also sprout with age as more mature branches leaning out and down more heavily with the sprouts filling in the now more open center.  Cross your fingers. An excellent example of lettin a tree be what it is…let’s hope that this continues!

If conifers and columnar forms define excurrent trees and shrubs, what is the opposite?  Woody plants that grow in a ‘rounder’ form, in which each branch grows with similar vigor, define the other end of this continuum, and are known as ‘decurrent’.  At the extreme they have no central leader and attempting to maintain them as if they did will require continuous work.  Such plants tend to blur the line between trees and shrubs.  We tend to define trees not only by their larger stature but also by their characteristic form…having a trunk.  Like most living organisms plants don’t generally fit cleanly into one category or the other.  Instead they exist along a continuum sharing varying characteristics of both.  Trees at the decurrent end will have a tendency to form competing/multiple leaders.  The will have rounded overall forms as branches compete for light and dominance some resulting in trees that are broader than tall.  Trees less productive of the growth suppressing hormone, auxin, will tend to have a more congested, dense ‘crown’ or canopy with branching in a less ‘ordered’ pattern often crossing through itself as each branch that initiates competes equally.  Plants that are more toward the ‘excurrent’ end will have a more ordered growth pattern with terminals and older branches dominating in their search for light.  Strongly ‘decurrent’ plants will have more tendency to form water sprouts within its canopy and suckers from their base and roots.  They will tend to form thickets.  It is important to know where the plant you are considering to prune is on this continuum…and it should be even more important to the designer who places it in the landscape.  Strongly decurrent plants can quickly become problematic in the controlled and limited more formal garden ‘refusing’ to stay within the bounds defined by the designer making the life of the gardener difficult.

As with most things trees tend to be somewhere in the middle between the two extremes.  Many trees have a somewhat rounded form while having a strong tendency to form a central leader upon which the rest of the structure is built.  These have a tendency to form ‘scaffold’ limbs, larger caliper branches, that remain subordinate to the central leader.  These tend to grow to a third or as much as half of the diameter of the leader, are attached more perpendicularly, with a complete collar surrounding their base.  Such branches are stronger with less tendency to split and tear down.  Scaffold limbs tend to dominate the lateral branches that emerge from along their length.

Wound Response

Pruning of living tissue is a considered infliction of wounds on a plant.  Whatever our intention may be it causes stress for the plant, it reduces its photosynthetic capacity by reducing its foliage and its top growth, pushing it out of balance with its root system.  We should then minimize this if we are concerned about the overall health of the plants under our care.  Wounding is nothing new to plants.  Over the millennia plants have as species developed various strategies to survive wounding.  Like cold responses developed by plants these strategies are related to the whole plant and will vary accordingly.  An annual, which completes its growth cycle over one growing season, producing generally abundant seed to ensure the next generation will have different responses than will a plant that requires many years to attain sexual maturity all the while forming a ‘permanent’ supporting woody structure before it can produce the seed for its next generation.  The longer it takes to attain maturity the more stresses, injuries, it is likely to have to survive.  It follows that mega-flora, like trees, that must maintain their health and vigor, often for many years before they can reproduce, will have developed successful strategies.  These same strategies can insure that many of these plants last considerably longer than the minimum time that it takes to reach maturity.  This longevity would seem somewhat proportional to the investment the plant must make in time and resources to reach maturity.

For some plants the production of viable seed and its germination is only one strategy for survival.  These plants, like Populous tremuloides, Quaking Aspen, effectively clone themselves forming single organisms that can cover many acres as the roots extend into adjacent soil and send new shoots, trunks, up in a spreading genetically identical colony.  This species, along with many others in the genus, and other genera as well, are also capable of responding similarly to mechanical injury resulting in basal sprouting or so called ‘water sprouts’ within the canopy.  This is a very common strategy of many ‘suckering’ shrubs.  Such trees can be difficult to prune and maintain in a form that the gardener may desire as almost any cut results in consequent sprouting.  Such plants can be very difficult to remove should the gardener decide to.  Many such plants respond to ‘heavy’ pruning, even to flush cutting to the ground, with heavy sprouting along many of its roots.  I’ve experienced this even after treating very fresh cuts with an herbicide intended to kill the plant.  These topped trees have no hormone, auxin, suppressing their growth.  Combine this with a huge capacity for growth in their roots and their strong genetic disposition to sprouting, and you can have a swarm of suckers erupting from the roots radiating out from the stump.  It should be kept in mind that terminal buds atop a trees central leader and at its branch tips all release varying amounts of hormone that act to suppress growth ‘downstream’ from them.  Woody plants vary in their ability to do this.  It effectively helps to determine the ultimate ‘shape’ of the mature plant along with its genetic coding that relates more directly to its form.  Remove the terminals, and the buds ‘below’ it, are ‘released’ to grow.  Plants that spread to form clonal communities tend to have buds that produce little of this particular growth repressing hormone.  What the pruner needs to keep in mind is is that pruning always stimulates growth.

Understanding how a Woody Plant Responds to Wounding

All plants respond to pruning/mechanical damage by growing…turf grass, the annual weed, Common Groundsel or Giant Redwood.  It is a genetically wired survival imperative.  To flower and produce seed a plant must be of mature size.  If it is broken or toppled and it has little ability to regrow itself it is less likely to succeed and produce seed.  Growing in response to any wound is a survival strategy.  Buds are ‘released’ that were previously held dormant, suppressed by the growth regulating hormone, auxin, in the terminals, since they weren’t previously needed.  In other cases, depending on the species, plants may have the ability to form ‘adventitious’ buds…, new buds, that were not lying in wait.  This is important to understand when pruning and absolutely essential to the propagator sticking cuttings.  This regrowth is largely determined by a plant’s genetics.  Such sprouting in a decurrent plant will tend to be congested and bushy whereas in a plant which is more excurrent, with a tendency to naturally form a strong central leader, the subsequent sprouting will be more ordered and limited.  If the wounds are well inside the canopy, in a strongly excurrent plant, there may be no sprouting at all as it is being hormonally suppressed or, because it is inside the canopy with limited energy to grow, and is quickly weakened and shaded out by the remaining structure.

Understanding a shrub or tree’s underlying habit, its form, is important in deciding how and even whether you should prune it at all.  Some plants simply put, don’t ‘like’ to be pruned.  These will respond with sprouts and suckers, like many other plants, and be very slow to close the wound. like leading to more extensive rotting thus compromising the structural integrity of the tree.  In these plants it is absolutely imperative that if you choose to prune them you keep your cuts limited in number, make them small in terms of diameter and that they be ‘good’ cuts that respect the natural barriers of the plant you are working on.  Magnolias are one such tree that requires more ‘thrift’ when it comes to pruning than many others.  Removing a leader, in any tree, will tip the growth into a more chaotic/weak pattern.  If you choose to prune you need to understand this.  Plants are not animals, they do not heal! regrowing tissue and reincorporating the damaged tissue back into the living functioning tissue and organs.  Plants, especially woody plants, ‘compartmentalize’ the damage.  They create barriers to protect/wall off the undamaged tissue from the damaged and its consequent death and the rotting that can follow.  They grow around the wound including it within living tissue.  Those of you who are woodworkers, think about knots in lumber.  Knots are where branches originated on the trunk.  ‘Tight knots’ were the bases of a living branch the cambial tissue of the branch and trunk having grown in an interlocking pattern.  You can see the overlapping layers of tissue that joined branch to trunk in an actively growing limb.  When that branch dies or is cut off the branch stops growing and dies while the trunk continues growing its cambium adding layers surrounding it, but separate.  It lies dead within the growing tree or at least it did until the tree was logged and milled giving us a ‘snapshot’ of the trees interior.  The surrounding tissue was living.  The knot, included stub, may have died years before.

Most of us are familiar with bark forming woody plants.  We understand that the cambium is a specialized tissue that exists just below the bark and surrounds the woody interior of dicots, dicotyledonous trees and shrubs. (Woody monocots, like Palms and their relatives don’t have cambium tissue, they don’t add annual girth to their stems in the form of rings and they don’t have ‘bark’.  What I’m discussing here does not pertain to them.)  This living ‘sheath’ during active growth cycles adds a layer to the trunk and limbs each year.  When we remove a branch we can see the annual rings just as we can on the trunk if we were to cut it down.  As in the trunk, the cambium is just below the bark on a branch, though the bark is generally thinner in proportion to its age.  The cut branch is ringed by a thin layer of living cambium tissue.  The vast bulk of the branch is comprised of cells that serve a structural purpose only and are not capable of growth and cell division.  These cells have no ability to grow.  A cut then produces a wound that is open.  It does not ‘scab’ over.  It remains physically exposed to the elements. The cambium on the branch is unable to grow over the wound.  While it is open, air and moisture, along with various rot organisms, can invade the wound and, over time penetrate into the tree following the layers of tissue it ‘shares’ with the trunk.  A two year old branch will share two years of tissue growth with the tree potentially providing a route down the length of the trunk.  An older/larger limb puts that much more of the tree at risk to rot.  The hardened structures of the trees annual growth and vascular system confine any rot to this portion of the plant’s structure unless other damage opens other tissues.  Size of the wound determines the potential problem for the given plant.

The diameter of the branch removed is obviously a factor in how long it will take to close the wound.  When you make the cut you need to minimize its cross-section.  Make your cut perpendicular to the branch not the ground.  No long sloping cuts or wounds.  Because branches tend to grow upwards, cutting vertically to the ground, may lengthen the cut increasing the surface area and the time it will take to close it.  Never make your cuts flush to the trunk!

As long as the wound/cut is open to air and moisture rot can/will proceed down the trunk.  If it ‘seals’ relatively quickly the rot stops.  It must have air and moisture to continue.  Leaving a stub, projecting out from the trunk, will increase the time it will take for the tree to include it, leaving it as an open conduit for rot.

Take a walk in the woods or a large wild Park where the trees are left on their own, no one actively pruning them for structure, health or hazard removal.  These trees ‘self prune’ and they often do this, as I’ve mentioned elsewhere, raggedly, leaving torn branches hanging and many others shaded out but still attached.  This is natural and it also goes to explaining much of a trees response to injury.  Aesthetics don’t matter to the tree, beyond its genetic prescription or pattern to follow.  Its goal is to live long enough to perpetuate the species.  Woody plants have developed two primary abilities to assure this: a capacity to tolerate rot and to stand long enough to produce several generations of seed and the related structures and capacities to physically minimize damage posed by mechanical damage.  Pruning is not necessary for the survival of the species, obviously.

Pruning practices were developed to help fit trees to human needs and aesthetics.  We have developed our pruning practices to take advantage of a tree’s built in responses to its environment.  Wound a tree’s trunk and the cambium has the ability to close it if it’s not too large otherwise it may remain open for decades, hundreds of years in some species, still standing and growing while its core rots away.  Trees like our native Acer macrophyllum, Big Leaf Maple, can loose major scaffold limbs or even a major portion of their leaders and stand for many years.  I once had the experience of taking down a Big Leaf Maple near a play area riddled with rot periodically losing parts of its canopy.  After clearing the area, calculating where it would drop, I went ahead starting my face cut when the whole trunk exploded with the sound of a gun shot the entire tree collapsing in a broken heap.  I had barely begun.  The trunk, 30”+ in diameter was completely rotted out having maybe a one inch+ ring of live healthy tissue beneath its bark around its entire circumference.  It was still capable of producing seed.  Trees, as I said, have a tolerance for rot.  They also have defenses against it.

The woody structure, its natural growth pattern, creates barriers which rot has trouble crossing even if a wound remains open.  Though, again the larger the wound, the older the cut or broken branch, the more of the trunk and supporting structure is susceptible to rot.  You should also remember that because of the way a branch or trunk adds length, remember the alternating or opposite buds on a twig or branch, these nodes where the buds are located create a physical barrier to the movement of rot along its length, though not as strong as those between growth rings or the hardened walls between vessels.  The space between the nodes, the internodes, are exposed.  As you follow a branch back to its attachment at the trunk these more minor barriers continue.  Where the branch actually connects with the trunk tissue there is a much more powerful barrier, part of which is the physical nature of this living joint.  Branch and trunk are two separate, though interlocking, tissue structures.  A branch, cleanly removed at this point, can close the wound and protect the ‘downstream’ structure.

 

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A good example of a bad ‘cut’. The pruner was removing what was likely a comepting leader which was of a diameter similar to the remaining trunk and was attached at a fairly vertical angle. The cut’s probably at least two years old and there’s only a little bit of callous forming near the top. The cut was made almost flush with the trunk disrupting the entire ‘collar’. The cut is very close to the ‘ridge’ and the surface, the cross-section, is elliptical in shape…it was not made perpendicular across the branch. The bark is dying and pulling away from the dead trunk tissue beneath it. Remember the collar is the zone around the base of every branch which is able to best develop callous tissue and close the wound.  This cut will probably result in dead cambium all of the way down the trunk to the root flare.

If you look closely at a woody plant, a tree or shrub, they share common elements at their points of branch attachment.  This may by hard to discern in thin smooth barked plants.  Even so there is often a very visible ‘branch bark ridge’ angling downward and away from the branch and the other branch or trunk it originates from.  As the branch and trunk increase their girth the tissue tends to raise up in a ridge as it is crowded together.  Just outside of it is the collar which marks the limit of the ‘zone’ where the trunk or supporting branch tissue  overlaps the emergent branch you are removing.  This zone often appears slightly swollen and ends in a line perpendicular to the emerging branch.  This collar is a critical point when deciding where to make your cut when removing a branch or stub.  This zone includes cambial tissue from the trunk.  Cut it adjacent but just outside the collar and it can relatively quickly grow over the wound forming specialized ‘callous’ tissue that will close in a uniform ‘bullseye’.  Cut into the collar tissue and you can delay the closing for several years longer.  Cuts that do this can be seen later in the uneven growth of callous tissue around the cut.  Cut too far away and the callous must grow out around the entire stub before it can fully close the wound and end the conditions supportive of rot.

Pruning cuts/wounds also produce less visible responses within a plant.  Because they can be defined by their vascular systems, each year a new flush of growth adds another layer of conductive vessels, running parallel with their length about their entire circumference, vessels that provide a continuous conduit from root tips to the leaves at the top of the canopy, woody plants have developed the ability to relatively quickly shut down select vessels when they are damaged.  It does this with chemical barriers.  These are less sturdy then the rigid lignin reinforced structures of their wood.  Still, they are effective.

Remember that vascular plants draw water up from their roots relying on a combination of forces including negative vacuum pressure, suction.  Plants need to be able to block failures quickly to maintain vacuum pressure and the steady movement of water and nutrients throughout its structure.  Breaches must be blocked.

There is a particular technique used to kill woody plants, though it is used on some herbaceous ones as well, in which the plant is cut down and the cambial tissue is immediately treated with an herbicide.  It greatly limits any off target effects that are more common with spraying and allows you to leave the stump/root structure in the ground thus avoiding the disruption of the rest of your landscape.  If, however, you wait even an hour before you come back to treat the freshly cut stump, the vascular system could already be shutting down in response to the cut and the herbicide won’t be translocated to where it is needed.  The result will be seen next spring when new growth, sprouting, emerges unaffected.

Pruning, particularly the removal of dead, broken and diseased wood, is beneficial to the tree as it reduces the burden of stresses that these place on the tree or shrub, as long as the pruner makes good decisions and is proficient at making accurate good quality cuts.  Bad pruning can result in a weakened structure that is subject to more stresses than you started with.

Trees left on their own in the woods are not pristine or ‘favored’ in some way.  Growing in a uniform stand and having received little storm damage the trunks may be very sound while others may stand despite extensive rot for many many years.  Good pruning should respect the plant’s natural response and barriers.

Landscape trees are often considered high value trees because of our relationship with them.  As good stewards we do what we can to ensure their continued good health, especially given that there are many things that may effect them that are out of our control.

Thinning vs Heading Back Cuts

Where you cut a branch affects how a tree or shrub will respond with growth beyond its wound response.  Thinning cuts involve cutting a branch just above a bud thus ‘releasing’ that bud and perhaps a few below it.  The auxin produced by the now removed terminal is gone.  As the new shoot emerges it will begin to produce the auxin and suppress branches that might otherwise compete with it for dominance. In some cases several sprouts may emerge simultaneously requiring, for structural reasons, that the less desirable ones be removed. A very similar thing happens when a branch is removed to its point of attachment to another branch.  The remaining branch will take over the dominate role.  The pruner must be careful not to make their cut too close to the remaining bud or branch or it can be compromised as well.

By choosing which bud or branch to cut back to we can direct the resulting growth.  By removing competing and weakly attached branches we are favoring a stronger structure and if we do it with foresight, both guiding it away from potential conflicts with power lines, pedestrian paths or other plants, we’ll be preserving or even emphasizing its natural genetic form or elegance.  Our landscape/garden trees need help along the way to fulfill our needs for them and to ensure their health.  Done incorrectly we can either compound a problem or create one where it needn’t have existed.

 

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According to Phyllis Reynolds a few years ago this is one of the largest Magnolia macrophylla in the City, it has a dbh of around 18″….but, it has been heavily and badly pruned over the years with many hard heading back cuts with consquent heavy resprouting. Magnolias tent to be fairly strongly upright growing trees. This one has been ‘forced’ sideways and then had those limbs headed back. The tree is still trying to grow in its genetically driven form.  It has completely lost its inherent grace.

When branches are ‘headed back’, stubbed off, without an adjacent branch or bud to take over, multiple buds can form and produce shoots adventiously from the ring of now exposed cambium tissue.   They will share dominance growing competitively side by side.  These will be weakly attached as they arise from a single point on the cambium and can relatively easily snap off from this point.  The shoots will share tissue only with the actively growing cambium and outward as it grows.  The older interior tissue will begin to rot and contribute nothing to the structural integrity of the emerging sprouts .  The large wound from the cut will be very slow to callous over and rot will grow down through the limb/trunk below further weakening the clustered shoots.  These multiple shoots will move independently in the wind transferring torquing forces to the weakened and compromised limb they’re attached to increasing the likelihood of failure.  This is why ‘topping’ is always a bad idea for any tree.

 

A closer shot of the Magnolia macrophylla looking anything but elegant.

A closer shot of the Magnolia macrophylla looking anything but elegant.

Heading back cuts result in an abrupt end to a branch when done on laterals as well.  The resultant growth will generally be angular and congested detracting from whatever elegance the more natural structure might present.  By making these kinds of cuts we surrender control over the ultimate structure ensuring a more random, congested and weak structure.  Very often these types of cuts are made ignorantly or impatiently to contain growth.  I see this everywhere when people attempt to control the relatively broad spreading, low angled growth of ornamental Cherries, particularly Prunus serrulata cultivars.  Cherries don’t possess much resilience to hard pruning as they tend to be disease prone here and close any wounds slowly.

 

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A typical Portland Cherry, Prunus serrulata. There are thousands of these in Portland including the many Kwanzan, Akebono and Fuji Cherries. Every branch here has been stubbed off…twice. You can see where each cut has been made with its resultant sprouts. These trees are typically grafted at 4 or more feet above ground. The graft union is prominent at the top of the trunk.

If your tree is in this shape as a result of storm damage or from heavy handed attempts to limit its growth you may be further ahead to replace it.  In other cases careful pruning over several years removing stubs, dog legs, dead and crossing branches or selecting one of the crowded competing shoots, can move you back closer to its ideal form.  It becomes a balancing act to  determine which cut will result in greater damage to the integrity of the tree.

Structural Training of Shade Trees

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This Acer x ‘Pacific Sunset’ exhibits this hybrids typical decurrent form, with multiple leaders and a rounded silhouette. Trained up to size in the nursery row its lower branches have been allowed to grow and have taken over the central leader.  It has received no pruning in its 20 years at this site and partially obscures a stop sign.  There was an adjacent one that succumbed to Verticillium Wilt several years ago.

A tree’s structure is important to its long term health and thus to its ability to continue flowering and producing seed over the years.  Through judicious pruning we can help guide a tree as it develops its permanent structure, its framework, that will support it through its life.  Timing is important to the ‘training’ of new trees in the landscape though a missed year here and there isn’t really a problem.  The structural training of trees is a several year process and simply can’t be done at one time.  When we do this we are selecting and guiding the growth of a young tree into a strong structure that won’t conflict with our needs and the uses of the site as the years go by.  Again this is a big topic requiring more space than I have here to do it justice.  Keep in mind that trees naturally grow in a community.

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Parrotia persica on Gladstone w/ heavy rootsuckers and congested low branching. This suckering seems atypical for the forms used as street trees. The root flare is buried under river rock. The collar above the root flare is a sensitive area on most woody plants. It could be damaged or planted too deep. Mine at home has an increasing tendency to form basal suckers at nearly 30 years old, which I annually remove, which then rewards me with more suckers the following season. The congested heavy branching low on the trunk will become more of a problem for pedestrians as they caliper up and broaden taking more of the sidewalk space. These need to be removed higher up the trunk.

When we see a tree in a woodland setting with a high canopy that affords views beneath its scaffold limbs, we need to understand that this did not happen all at once.  Change the plant community, the species and density, the understory and the tree’s form will respond over time.  A relatively dense stand will tend to shade out more of the lower limbs weakening them as those further up are favored with more energy giving light, growing broader and thickening their caliper, while lower branches slip behind, dying and are eventually shed over time and with the vagaries of storms.  Undergrowth will fill in taking space where roots and light allow.  Perhaps deer have ‘browsed’ the small lower limbs weakening them while favoring the stronger growth of limbs higher in the tree.  This form, with a higher canopy and longer straighter trunks, is the ‘classic’ desired shade tree form.

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This is a Tilia next to another well structured one, probably cordata. Normally these tend to have strong central leaders. This one has very poor weak structure. It lost its leader and has been severly headed back with resultant poor shooting. Buds betray its lineage. Trees that have been so brutalized are likely to sucker at the base. Grafted forms so treated can be over taken by the basal suckers of the rootstock.

Homeowners, new gardeners, inexperienced pruners seem to expect that when they plant their Sunset Maple or Zelkova, that it will magically grow into that form on its own.  It won’t.  A branch growing from the trunk 36” up from the ground will still have its core at this same height 25 years later.  They do not ‘push up’ skyward as the tree grows.  Height in trees is gained by adding terminal growth, extending it, and then adding girth by the growth of the cambium tissue.  Homeowners are often at a loss as to what to do so they tend to do nothing, cutting off the occasional limb that juts out in their way and leaving the broken branches left by parked cars and pedestrians tired of ducking or enticed to grab and pull on them because they are within easy reach.

 

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This tree recently failed and was cut down in my neighborhood. The picture shows the several branches just above the point on the trunk where they emerge ‘vertically’ crowding and growing tightly against one another, but separate with bark inbetween. The new lighter colored wood is no longer being added to their shared boundaries instead white mycellium is growing there consuming the bark and creating space for rot to move further down into the trunk. The ‘injured’ cambium on these interior surfaces will begin to rot along with the ‘heartwood’. The gaps can allow moisture and rot to penetrate. Wind can lever the limbs against one another causing a split to form down into the trunk.

This results in a congested pattern of large caliper limbs relatively low on the trunks that become more problematic over time as they add girth and can become serious barriers blocking movement and cutting off sight-lines.  The congested spacing can also lead to weak branch attachment subject to tearing out during storms or when people pull on them and can even result in the leader itself breaking off.  A strong structured tree requires  the adequate spacing of branches along the trunk not of a few inches but of a foot or two and more depending on the tree’s ultimate size.  In a woodland many lower branches get shaded out as I said above.  In your yard or parking strip the branches are all more exposed to sunlight and may exhibit vigorous growth all along the trunk.  Another factor homeowners aren’t aware of is that that woodland tree, that they have pictured in their mind’s eye, is an ideal and a snapshot in time.  These same trees were young once and as small trees went through a continuous series of changes over the years.  They may have had to struggle and been heavy with dead wood until a hole in the shade of the canopy gave them their opportunity.

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This tree started off weakly with the leader arching strongly to the left. It may have been forced in the nursery into rapid vertical growth which is soft and weak allowing it to ‘flop’. By the leader going horizontal buds along its top surface are released to grow strongly vertically. You can see where the original leader was cut off at the left.

[It would seem to be obvious that the structure of the tree that you begin with must be considered.  A young tree with bad form, multiple leaders, congested branching, ‘bad’ angels of attachment (unless they are so low on the trunk that they will be removed latter anyway while training it into a strong structure), wounds, dog legs, etc., will work against you. A ‘bad’ tree is never a bargain no matter the price.  A young tree that requires heavy corrective pruning may not be able to fully recover. It is imperative that you begin with good stock.  This includes root systems, but that’s another story!]

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A poorly structured trunk. All of the main scaffold limbs are attached within 4′ of the ground turning quickly upright weakening the trunk and suppressing other potential scaffold limbs further up that could have otherwise formed a stronger and higher canopy. Tightly bunched like this these scaffold limbs compromise the structural integrity of the trunk.

Where a tree is on the excurrent/decurrent continuum will suggest its characteristic or ideal form.  It will also alert the pruner as to what to watch out for in terms of weak branching that they might want to remove to encourage a stronger structure.  Decurrent trees have a tendency to sprout sending shoots vertically.  Most trees when damaged or clumsily cut will send up shoots at or just below the wound.  These can be very weakly attached and are very likely to fail as they grow.  Decurrent trees, because they produce less of the auxin that suppress, to varying degrees, competing growth below terminal buds will respond to your pruning cuts by more ‘vigorously’ sprouting.  They can have co-dominant, equal, leaders which can lead to a failure as described below.  Strongly decurrent trees may be inherently weaker and as such tend to be shorter lived than those that are excurrent or grow with a strong central leader.  Removing competing leaders or scaffold limbs with weak angles of attachment should be done early before their removal results in a wound that will take several years longer to close.

Keep in mind that branches that are more perpendicular to the trunk are more strongly attached then are those that are more vertical.  Branches that are attached more ‘vertically’ characteristically have a very long branch bark ridge as they are essentially growing side by side.  The branch and trunk, although appearing to be solid and whole are actually separated along the ridge each with its own bark against the other.    Annual growth produces less wood on this common shared side.  This is an inherently weak joint which moves in wind and can split further into otherwise solid tissue.  It is also open to moisture and possibly rot which can eventually lead to failure.

As in most things we find that there are aways exceptions.  This should not dissuade us in our pursuit of good practice.  Manzanita and similar shrubs, often fire resistant burl formers, that typically grow into multi-stemmed shrubs with a gnarly branching habit, did not attain these in the pampered garden.   To achieve these forms it may be necessary to cut them  back in youth to encourage more branches; or, in the case of at least some Eucalypts, cutting them down after a year’s growth to assure heavier rooting before a new stem is trained up to form the ‘permanent’ trunk.  Before attempting these you should consult with someone with experience lest you cause more damage than good.

I also want to note that maturity and vigor should be considered when pruning.  When trees are under stress or in decline, having passed their peak years, they grow with less vigor and enter a phase in which new growth is greatly reduced.  The canopy will show thinning, with smaller leaves and shorter annual growth from its terminals.  Some trees will begin dropping their leaves earlier than is typical.  Such trees are less capable of closing wounds so it becomes even more important to minimize their number and size.  Trees may stand for many decades in this state occasionally dropping major limbs or succumb in a couple of years.  The burden of bad cuts is accumulative and can greatly shorten a trees effective life.  This is compounded by the many stresses placed on trees in the urban environment.

The practice of horticulture recognizes our role in the created/ man made landscape of today.  Eden is no more.  We are powerful actors and creators of our modern landscape.  It is essential that we work within them knowledgeably.  Pruning can make your tree stronger…it can also increase the likelihood that it will fail, catastrophically, down the road.  It is vital that we understand how a plant grows and how it will respond to our efforts if we have any desire to live in healthy and beautiful landscapes.

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