Helping Homeowners Choose Trees Wisely: what you need to know

Trees originate in a particular environments, not an urban one. This landscape of California Interior Live Oaks creates a beautiful natural alle'e through the woods. These native Oaks can soften a street scene over time, are well adapted to our street environment requiring little effort on our part beyond structural pruning.

Trees originate in a particular environment, not an urban one. This landscape of California Oaks creates a beautiful natural alle’e through the woods. These native Oaks can soften a street scene over time, are well adapted to our street environment requiring little effort on our part beyond structural pruning.

The urban environment can be an extremely stressful one to live in.  This is no less true for plants than it is for us, the people, who created and maintains this place for our own use.  It is no less naive to believe that a tree, planted out by someone, no matter how much they may love at least the idea of trees, in a random parking strip or next to their place of business, will thrive after a year or two of well intentioned irrigation, on its own than it is to think that a child will grow up to be strong, happy and successful simply by having its first few years of nutrition provided for….Cities are economic and social constructs.  They did not rise ‘organically’ from the soil supporting a diverse and complex community of species.  Life has had to ‘fit’ in where ever it can.  Much has been unable to.  Many of us plant trees because we feel the loss, the absence of life, and realize that these places are less for it, that we ‘suffer’ because of this.  But we cannot simply add trees and stir.  These are ‘broken’ places and we have to pay more attention to our choices and provide better care than this place alone can provide…otherwise it would be like turning out our children, still unformed, on their own.  Even if we were Spartans and believed that only the ‘strong’ deserved to live, we would be dooming them in these modern, contrived and, in many ways, diminished cities.  As responsible parents and tree stewards, we are bound to them.  We owe them our best.  Without it they will fail and the world that we have built around us will be less as well.

The City of Portland has a web page that discusses the selection and planting of street trees.  Its stated criteria for successful plantings are limited to the presence or absence of overhead power lines and the width of the planting strip.  Toward this end the City has compiled a list of suggested trees, trees that are sorted to ‘fit’ within these criteria and, presumably, meet the basic growing conditions in this region, but just what are the conditions these street trees will be facing and do they ‘match-up’ with the trees on the list.

Planting the Right Tree in the Right Place

“The best street tree is one that fits well in the available space. To help property owners choose the right tree, the City maintains lists of approved street trees based on the width of the planting strip and the presence/absence of overhead high voltage power lines. Generally only primary lines (the power lines usually located above transformers) affect tree selection because trees must not come into contact with these high voltage carriers.” (From the City’s website.)

Skimming through the several lists of trees might give the well intentioned tree planter the idea that any of the listed tress would be a ‘successful’ choice.  Those using the list would likely assume that other important factors, whatever they might be, have been substantially considered by the compilers.  With such a list our choice simply comes down a matter of aesthetics. Water it the first couple of years and success is assured!  As a horticulturist I know that this is not true.  Each of these trees originated in a particular environment with a set of growing conditions and within a plant community that it is best adapted to.  Experience can tell us which ones may be tolerant of other conditions, even those that have proven to do well in much different conditions, but these are not things that you can be easily assumed.  Not all sites are created equal, nor is each tree.  In fact, urban conditions can be very different, read harsh and stressful, than even a local native tree may have evolved with.  They may not be adaptable.  A walk around any neighborhood will reveal how difficult these conditions are with dead, declining and stunted trees all around us (at least in my trips around SE and NE Portland).  There are identifiable reasons for this.

Urban Growing Conditions

Unless you live in a ‘cave’ you’ve heard of ‘climate change’ a phenomenon that has been accelerated by human activity.  For reputable scientists and most of the rest of us, this is a given.  Even a cursory study of our local weather history shows how over the last several decades our low and high temperatures have been trending up.  Common human activities effect the physical and biological world.  Climate change aside, at a more local scale, the phenomenon of an urban heat island has been demonstrated in every city as the built environment, our structures and hardened urban surfaces, have come to dominate.  Winds swirl around buildings in ways more like twisting canyons than the grasslands, savannah and woodlands that were here before them.  Most changed of all are the plant communities and wildlife that previously occupied these places.  Change the life above the surface and the life beneath it changes, they do not change independently, in fact so does the very soil itself.  In modern cities soils have been stripped, graded and ‘filled’, transformed into base material to support our structures.  It has been drained and covered, sealed off from air, water and life that once permeated it, that was in fact a living part of it.  The microbial community, an essential element in any landscape, supporting the plants that grow upon it and even largely determining the characteristics of the soil itself, its ability to allow for the gaseous exchange of healthy root growth, the percolation of water down through its layers, the ability of roots themselves to penetrate more deeply seeking out what they need, all of these things are diminished.  Built structures and hard surfaces like streets, parking lots, sidewalks and plazas were preceded by heavy compaction to assure that their bases remain stable and their integrity uncompromised.  These same surfaces reflect light and heat greatly increasing their intensity on any plant material that is set into this space, amplifying the stress.  And everywhere we walk or roll we compact the soil beneath the surface making it difficult for any of the necessary root/soil transactions to occur.  The urban world is vastly different than the natural one that preceded it.  Our landscapes and trees suffer when we fail to consider just how much things have changed.  What then, are we to do?

Toward this end I will be looking here at one of the City’s tree lists and begin to add in information that I think is necessary to make better tree choices for an owner’s particular site.  It is a beginning place.  Much more discussion is needed on this.  My intent is not to overwhelm, but to inform owners, to make them more aware of the complexities out there so that our urban forest is more thoughtfully planted and the result more healthy, functional and therefore beautiful than it is today.  I don’t think that it is possible to do this without a greater effort at public education.  The public needs to become more engaged in the world around us.  If that is naive, so be it.  Things won’t improve without making the effort.

What We Need to Know to Make Good Choices

Description – Each tree includes a short description describing its aesthetic characteristics, silhouette and mature size.  Many nursery catalogs do a more than adequate job with this, at least regarding a tree’s ornamental characteristics.  Necessary and helpful cultural care and site conditions are often less fully covered.  While we have a mild temperate and quite forgiving climate we should still look more closely at which plants are a better match for our site given the conditions on our site and the care that we are likely to give them.  Ignoring this will compromise our choices and the trees we plant.

The following will be included.  Prospective owners could then ‘score’ a tree by its characteristics, how well it will fulfill their longer term aesthetic desires and how well it will meet their actual growing conditions giving them a better basis for making their selections:

Cold Hardiness –  There is an over reliance on cold hardiness when selecting plants, so much so that there is a tendency to think that it is the most important consideration, above that of other requirements, leading planters and gardeners to select plants even more cold tolerant than they are ever likely to experience, as if this will assure their success….  I don’t know if this is true in other regions, or if this is because people move here from colder regions and they don’t trust that our milder weather won’t fail them!!!  The ‘Portland Metro area ranges from zn7a-9a (I’ve lived in inner SE Portland for 27 years and am consistently zn 8a or milder having never experienced minimum temperatures below 10.  It is common here to remain above 20F (zn 9a) through the winter. know your conditions).  The nursery industry here grows plants for all over the country, many areas that are much colder.  Much of our population comes from colder regions to our east and have developed a sentimental attachment to trees back ‘home’, trees often adapted to summer wet climates.  The local and west coast markets are much smaller and large growers tend to look to the east for the bulk of their customers.  The industry tends to be very conservative when it come to cold hardiness. Zn 7a provides a more than adequate cushion for the entire Portland Metro and Willamette Valley area.  Insisting on colder hardy plants greatly limits choice and often results in inclusion of trees from colder climates with different precipitation patterns that are problematic here in our warmer, summer dry, climate.

(Na) Native – North Willamette Valley.  This brings up the issue of adaptability of our native plants.  Our local region contains many different types of landscapes.  No one specific native plant/tree is well adapted to all of them.  Native designation is not a planter’s ‘silver bullet’. Commonly deciduous trees are planted as street trees yet relatively few of our native trees are such.  The bulk of these come from predominantly summer wet regions.  Peruse any list of ornamental deciduous trees and most of them will come from summer wet areas.  Our relative few such natives tend to occur on lowlands or near rivers and bodies of water where soils remain moist for much of the dry summer period.  Planting these on exposed sites with limited mineral soils, without long term summer irrigation will stress these as well.  Our native Garry White Oak is an exception as it tends to occur on higher/drier sites.  Our Pacific Madrone, a ‘picky’ evergreen tree prefers higher, well drained and exposed sites in the Willamette Valley.  Neither of these are easily available, especially in size, as they are slow growing and generally intolerant of ‘normal’ landscape care, especially of irrigation, which makes them, paradoxically, in some ways very good urban choices.

(Xe) Xeric, mediterranean, Chaparral, Summer Dry – Here I use the word xeric to mean plants that in our region are intolerant of summer irrigation.  These are often of western North American, Mediterranean, Chilean, eastern Australian or Cape South African origin. These are often evergreen.  In some cases they are summer dormant, losing their leaves during the extremes of heat and drought in summer and returning in the Fall.  Trees from other climatic regions may be severely compromised by this level of stress.  When cold tolerant here these are easily adaptable to parking or ‘hell-strip’ environment with no supplemental water after establishment.  Fall plantings, taking advantage of our seasonal rainfall, are often enough to get these established.  These also tend to be tolerant of low nutrient mineral soils, as long as they are reasonably well drained.  An abundance of water will be a limiting factor in their survival.  Planting them in compacted urban soils, though they are adapted to our precipitation pattern, won’t assure success either.  Ideally, soil conditions, would be addressed as well as light.

(LaI) Lawn tree Intolerant – Some summer wet trees, and many mediterranean/summer dry trees, are not tolerant of the frequent shallow wetting that turf requires.  For example, ornamental Cherries.

(SW) Summer Wet – perform best with regular rain/irrigation for healthy growth in summer.  These trees often originate in the eastern US and summer rainfall areas of northern Europe and Asia.  While some may endure under our summer dry conditions it can be very stressful for them especially on the exposed sites that many street trees are confronted with.  Planting strips additionally offer more limited and compacted soil conditions limiting a plants access to water during our dry summers.

(WS) Wet Soils – adapted to wetter lower oxygen winter and summer soils. Often of eastern North American, northern European or Asian origin

Trees/Plants originating from significantly different growing conditions often perform poorly if planted in urban conditions.  These categories go to the conditions beyond cold that effect the growth of plants.  These go to the relationships that occur within the different types of plant communities.  These relationships are often of primary importance.  Life, whatever its form, effects the very conditions within which it and its neighbors grow.  Change the community and you change the conditions.  Some of these relationships are necessary, some of them are only modifiers, but overall, they need to be considered.  A plant removed from its community type will perform differently and may even fail.

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Along the street, in full sun, these Magnolia ‘Galaxy’ will have a tougher time in full sun with plenty of reflected heat, not somethings they ‘like’. Magnolias tend to be woodland dwellers with their mild conditions and loose humus rich soils and summer rain. Remember, if these last and grow with vigor they will reach 40′ tall and a width of 25′. Perhaps the designers are counting on them failing before that. That would be a safe bet. There are a lot of cables above these as well.

(Wd) Woodland – often perform poorly in urban settings with limited, mineral soils and reflected light and heat common.  Woodlands moderate the temperatures within them often protecting community members from their full intensity.  They have humusy, rich, well drained soils

(WE) Woodland Edge – these communities are ‘blended’ transitions between woodlands and grasslands providing an intermediate level of light and warmth.  Siting these plants should take this into consideration.  Both these and Woodland plants should not be planted in full exposure, especially not where they will receive reflected light and heat. Other times, such as on the basaltic hills of the Willamette Valley, these ‘edges’ can be quite exposed, hot and dry, providing the conditions for plants like Madrone to thrive. Planting conditions should mimic those that they come from.

(Cp) Chaparral (mixed evergreen sclerophyllous trees and shrubs, hot and dry) Not intuitively, many of our urban physical sites today mimic those found in California’s chaparral country and our own shrubby landscapes found in parts of southern Oregon and those dry shrubby exposed sites in the Cascades dominated by Manzanita, Ceanothus and other sclerophyllous shrubs with their dry, mineral, nutrient poor soils.

(Sv) Savannah (open mixed grassland)  Trees originating in exposed settings like savannah, chaparral and desert, are tolerant of exposure if they are within their cold and moisture tolerances.

Growth characteristics

(SSR) Shallow and Surface Roots – can lift and push curbs and sidewalks.  Trees with such roots should be recommended only for wider planting strips whether they have overhead power lines or not, as they will invariably conflict with and heave sidewalks, etc. Having said this any tree can have ‘shallower’ roots in poor soil conditions.

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This Stewartia pseudocamellia is around 20 years old having struggled in a parking strip where it has been broken/hacked several times resulting in congested growth with short internodes suggestive of its slower than normal growth. Very congested and truncated with none of the species elegance. It illustrates the need for structural pruning when your trees are young.

(SCT) Small/Low Canopy Trees – may conflict with sidewalks and streets.  Sidewalks require 7’ of clearance beneath overhanging branches while on the street side it is 14′.  Small trees often branch low on the trunk and many of these tend to grow somewhat horizontal.  In narrower ‘strips’ this issue should eliminate many smaller growing trees because it will require heavy pruning that is aesthetically undesirable and may cause health or structure issues with the tree itself.  Such trees need a wider planting strip to accommodate their low branching habit.  ‘Fitting’ under power lines is not the only requirement.

Caliper Consideration – While trees may have no power-line conflicts, larger growing trees may still cause damage to curbs and sidewalks in cases where the strip is too narrow their diameter and root flares lifting and pushing them.  Other trees that tend to have shallow and surface roots of larger size require even broader planting spaces.

(PI) Intolerant of regular and/or the heavy pruning often needed to grow in a narrow strips.  Such trees often respond with heavy sprouting and/or may begin to prematurely decline.

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The portion of this ‘bush’ blooming white to the right, is a grafted ‘Autumn Brilliance’ Serviceberry tree. The bloomless multiple stems to the left are from the Crataegus rootstock that the owner failed to prune off. I’m not sure how common such combinations are, but some rootstocks are heavy spouters.

(G/S) Grafted to a rootstock – Many trees are grafted to a separately grown rootstock because the rootstock possesses disease resistance advantages, maybe more tolerant of poor soil conditions, contributes desired dwarfing effects or simply is a more economical method of ‘production’.  ‘Scions’, are pieces of the plant grafted to the rootstock that possess the desirable characteristics customers want that cannot otherwise be produced by rooting cuttings or by growing seeds.  Some rootstocks have a tendency to produce suckers that if left uncontrolled can out perform and dominate, overwhelm, the scion causing it and its desired characteristics to be lost.

Growth Rate – Slow <1’; Med 1’ > 2’; Fast 2’+

Size at Maturity – Mature size refers to height, canopy spread and the thickness or massiveness of the trunk.  These three characteristics tend to increase or decrease together

Tree Form – The basic forms are Round; Oval; Vase; Weeping; Pyramidal; and Columnar or Fastigate.  These are the classic forms when grown as single specimen and/or in open settings where sunlight is unobstructed and branches are not ‘shaded out’ by adjacent trees.  Trees growing in a woodland setting or group will tend to cause trees to grow faster and narrower in competition for sunlight resulting in lower branches shaded out, shedding over time creating a higher canopy than a tree in an open setting.  Slower and smaller growing trees may be completely shaded out in a grouped setting if they are intolerant of shade.

Pruning and Training

Pruning and ‘training’ of younger trees after being planted out in the landscape is always necessary and is often ignored.  Parking strips are not forests or woodlands.  The trees planted in them will not naturally grow into high canopied specimen without our assistance.  Trees grown in parking strips are exposed to light levels comparable to that found on a savannah causing them to retain their branches low on the trunk where they can conflict with regular use of sidewalks, curb side and bicycle lanes.  Limbs are often left on until they are of large caliper when their removal creates large, slow to close, wounds and the late removal of which can leave a poorly formed and compromised tree.  Tree needs should be met in a timely manner.  Many residents are ignorant of how trees grow in both ‘communities’ and as individuals out in the open.  They need to be educated and supported to improve their practice.  The City’s Forestry Division is not staffed to do this work and the public, largely ignorant of the need, does not hire professional tree care services for such.  The work mostly goes undone.  Low branches are often torn and broken by passing pedestrians and vehicles damaging and compromising trees.

Seattle has their ‘Plant Amnesty’ organization that works across the community to train and educate the public about proper pruning for the health and beauty of their trees and shrubs.  Portland has no similar program.  There is a void here filled by neither the City nor groups like Friends of Trees (FoT).  FoT is very active with increasing the urban forest.  Now we need to increase awareness and concern for its health and what we can do to improve it!  Some of these decisions, such as those I’ve mentioned above, come before we plant, other consideration must be made to improve the health of those trees already in the ground.  Both are needed.

 

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A small growing tree is not an automatic in a narrow parking strip.

A small growing tree is not an automatic in a narrow parking strip.

 

 

 

 

 

 

 

 

 

The City maintains a list of trees they deem appropriate for street tree use based on the two criteria discussed above, the width of the planting space and the presence or absence of overhead power lines.   

Following is the City’s List for:  3.0-3.9 foot sites with or without high voltage power lines as it appears on their webpage, slightly reformatted, with my additional notations so that trees can be better evaluated for the conditions on a particular site.

In general this is a difficult space to fit trees successfully as they are pinched between both street and sidewalk while the branches are, by law, not to encroach, as they impair vision and impede safe movement.  Planting will tend to require heavy pruning which attempts to force a small growing tree into something it is not.  Small trees for narrow spaces are not a slam dunk.  Small trees tend to branch low on the trunk which will, unless a tree is columnar, conflict with sidewalk and street side clearance.  Removing conflicting branches will leave an out of balance tree structure.  These can also impede access in and out of parked vehicles difficult if that is an issue.

All of the red below is added by me.

Cascara (Rhamnus purshianaNa, Wd>Cp, Xe, zn 4a-9b

30’ x 25’ Round

A small native tree with black berries that attract birds.  Wd, woodland and Xe, xeric, this tree performs best in a dry woodland.  There are shrub forms that do well with more exposure.

 

Chokecherry, Canada Red (Prunus virginiana ‘Canada Red’) Wd to Cp, Xe zn 3b-9b

25’ x 20’ Round

Purple leaves turn orange or red in the fall. Fruit attracts wildlife.  In harsher conditions this tends to grow more as a shrub.  Has a tendency to sucker and spread.

 

Crabapple, Purple Prince (Malus ‘Purple Prince’) SW zn 4-8 SCT PI G/S

20’ x 20’ Round

Purple to bronze foliage, pink flowers, good disease resistance, fast growing.  As a SW, summer wet, tree, this will perform best with regular summer water throughout its life and may suffer and decline without it, especially on more exposed sites, or with limiting soil conditions.  As a SCT, a small low canopied tree, especially one that is PI, pruning intolerant, these will be problematic to maintain in a narrow hellstrip.  Additionally, as a G/S, grafted to a rootstock tree, these tend to be on suckering rootstocks that can require annual control over their life.

 

Crabapple, Royal Raindrops® (Malus ‘JFS-KW5’) SW zn 4-8 SCT PI G/S

20’ x 15’ Upright

Bright fall color complements deep purple cutleaf foliage. Magenta pink blossoms.  As a SW, summer wet, tree, this will perform best with regular summer water throughout its life and may suffer and decline without it, especially on more exposed sites, or with limiting soil conditions.  As a SCT, a small low canopied tree, especially one that is PI, pruning intolerant, these will be problematic to maintain in a narrow hellstrip.  Additionally, as a G/S, grafted to a rootstock tree, these tend to be on suckering rootstocks that can require annual control over their life.

 

Crabapple, Tschonoskii (Malus tschonoskii) SW zn 4-9 Pyr

30’ x 15’ Oval

Striking silvery green foliage. White flowers. Outstanding fall color.  As a SW, summer wet, tree, this will perform best with regular summer water throughout its life and may suffer and decline without it, especially on more exposed sites, or with limiting soil conditions.

 

Crape Myrtle (Lagerstroemia cultivars)

20’ x 20’ Various It is very important to pay attention to cultivar names as these trees can vary widely, in hardiness, size and flower color.  Please refer to Xera Plants site for a list of proven cultivars and for cultural, growing, information.

Showy, long-lasting summer flowers. Interesting exfoliating bark. Beautiful Fall color. Vase shaped cultivars are best to plant.  These have proven to be quite drought tolerant after establishment, but perform best, flowering more heavily and consistently, with some summer water

 

Franklinia (Franklinia alatamahaSW Wd zn 5-8

20’ x 15’ Round

Large, fragrant white spring flowers. Long, glossy green leaves turn shades of orange, red, and purple.  A beautiful choice, but one that has been proven to be very difficult here, even in the best of sites.  As a Wd, woodland, tree they will not perform well in exposed sites and should be planted where they get at least afternoon shade away from reflected light and heat.  Afternoon sun is the most intense.  Will struggle in mineral soils without a developed humus layer.  As a SW, summer wet, tree, this will perform best with regular summer water throughout its life and may suffer and decline without it, especially on more exposed sites, or with limiting soil conditions.

 

Laurel, Bay (Laurus nobles)  xe, SCT, zn 8

30’ x 20’ Pyramidal

Fragrant evergreen leaves can be used in cooking. Fruit attracts birds.  This is a low branching, shrubby tree, SCT, that can be problematic in a narrow parking strip.  May be susceptible to loss of leaves when exposed to freezing winds.  Best in a sunny warm location.  Not shade tolerant.

 

Linden, Summer Sprite® (Tilia cordata ‘Halka’ PP 10589)  SCT, SW , zn 4, 

20’ x 15’ Pyramidal

Dense, compact form and dwarf size are perfect for small planting strips.  This tree, SCT, grows into a compact and dwarf pyramid with relatively broad horizontal lower branches that will conflict with street and sidewalk.  Though these are of European origin the species tends to be somewhat drought tolerant here (Be aware that drought tolerance varies between regions.)  As a SW, summer wet, tree it performs best with some summer water throughout its life.

 

Magnolia, Butterflies (Magnolia ‘Butterflies’) Sw Wd PI zn 5-9

20’ x 15’ Pyramidal

Tulip-like yellow flowers with a light lemon oil aroma. Hardy to both heat and cold.  Magnolias as a genus tend to be rather intolerant of exposed street conditions and are not drought tolerant.  They also don’t respond well to hard pruning. As a Wd, woodland, tree they tend not to perform well in exposed sites and should be planted where they get at least afternoon shade away from reflected light and heat.  Afternoon sun is the most intense.  Will have a tougher time in mineral soils without a developed humus layer.  As a SW, summer wet, tree, this will perform best with regular summer water throughout its life and may suffer and decline without it, especially on more exposed sites, or with limiting soil conditions.

This is my 20+ year old Autumn Brilliance Serviceberry growing on the east side of my home where it receives afternoon shade. I give it periodic soakings in the summer. It often suffers severe rust in the spring which can even distort the new growth making the tree lower and more squat. Clearances over the sidewalk are around 7', those on the street side require regular pruning to meet the standard and result in an out of balance structure, flat on the street side in this narrow strip.

This is my 20+ year old Autumn Brilliance Serviceberry growing on the east side of my home where it receives afternoon shade. I give it periodic soakings in the summer. It can often suffer severe rust in the spring which can even distort the new growth making the tree lower and more squat. Unbeknownst to me this was grafted on Hawthorn rootstock and suckers every year requiring that I remove them.  Clearances over the sidewalk are around 7′, those on the street side require regular pruning to meet the standard and result in an out of balance structure, flat on the street side in this narrow strip.  We keep it for the Cedar Waxwings that feed on the fruit, but whom seem to have abandoned our neighborhood the last several years.

Serviceberry, Autumn Brilliance® (Amelanchier grandiflora ‘Autumn Brilliance’) Sw, SCT, Wd, WE, zn 4-9

25’ x 20’ Round

Outstanding red fall leaf color and showy, white spring flowers. Low maintenance tree. Prone to severe rust disease.  This is a hybrid of two eastern species, woodland species, that stress in exposed dry sites here.  As a Wd, woodland, tree they tend not to perform well in exposed sites and should be planted where they get at least afternoon shade away from reflected light and heat.  As a WE, woodland edge, tree this is somewhat more tolerant of sun.  Will have a tougher time in mineral soils without a developed humus layer.  As a SW, summer wet, tree, this will perform best with regular summer water throughout its life and may suffer and decline without it, especially on more exposed sites, or with limiting soil conditions.

 

Serviceberry, Spring Flurry® (Amelanchier laevis ‘JFS-Arb’ PP 15304) Sw, Wd, WE, SCT zn 4-8

30’ x 20’ Oval

An exceptional tree form supports pure white blossoms in the spring and orange fall foliage.  As a Wd, woodland, tree they tend not to perform well in exposed sites and should be planted where they get at least afternoon shade away from reflected light and heat.  As a WE, woodland edge, tree this is somewhat more tolerant of sun.  Will have a tougher time in mineral soils without a developed humus layer.  As a SW, summer wet, tree, this will perform best with regular summer water throughout its life and may suffer and decline without it, especially on more exposed sites, or with limiting soil conditions.

 

Snowbell, Bigleaf (Styrax obassiaSW, Wd, WE, SCT, zn 5-8

35’ x 25’ Round

Also called fragrant snowbell. Perfect white flowers with showy yellow stamens. Interesting bark at maturity. Summer irrigation is essential for good performance.  As a Wd, woodland, tree they tend not to perform well in exposed sites and should be planted where they get at least afternoon shade away from reflected light and heat.  As a WE, woodland edge, tree this is somewhat more tolerant of sun.  Will have a tougher time in mineral soils without a developed humus layer.  As a SW, summer wet, tree, this will perform best with regular summer water throughout its life and may suffer and decline without it, especially on more exposed sites, or with limiting soil conditions.  This is another SCT, small canopied tree, with a form and size that can conflict directly with street and sidewalk.

 

Snowbell, Japanese (Styrax japonicusSW Wd WE zn 5-9 SCT

25’ x 25’ Round

Perfect white, bell-shaped flowers bloom in the late spring.  This species tends to have strong horizontal branching which can conflict with sidewalk and curb space.  Summer irrigation is essential.  As a Wd, woodland, tree they tend not to perform well in exposed sites and should be planted where they get at least afternoon shade away from reflected light and heat.  As a WE, woodland edge, tree this is somewhat more tolerant of sun.  Will have a tougher time in mineral soils without a developed humus layer.  As a SW, summer wet, tree, this will perform best with regular summer water throughout its life and may suffer and decline without it, especially on more exposed sites, or with limiting soil conditions.  This is another SCT, small canopied tree, with a form and size that can conflict directly with street and sidewalk.

 

Snowbell, Pink Chimes Japanese (Styrax japonicus‘Pink Chimes’)   SW Wd We zn 5-9 SCT

20’ x 15’ Round

Fragrant pink flowers. Mature bark fissures, revealing attractive orange inner layer. This cultivar tends to have strong horizontal branching which can conflict with sidewalk and curb space.  Summer irrigation is essential.  As a Wd, woodland, tree they tend not to perform well in exposed sites and should be planted where they get at least afternoon shade away from reflected light and heat.  As a WE, woodland edge, tree this is somewhat more tolerant of sun.  Will have a tougher time in mineral soils without a developed humus layer.  As a SW, summer wet, tree, this will perform best with regular summer water throughout its life and may suffer and decline without it, especially on more exposed sites, or with limiting soil conditions.  This is another SCT, small canopied tree, with a form and size that can conflict directly with street and sidewalk.

 

Snowbell, Snowcone® Japanese (Styrax japonicus‘JFS-D’) SW Wd We zn 5-9 SCT

30’ x 20’ Pyramidal

Dense, symmetrical structure creates a uniform crown. Resistant to twig dieback. Can conflict with sidewalk and curb space.  Summer irrigation is essential.  As a Wd, woodland, tree they tend not to perform well in exposed sites and should be planted where they get at least afternoon shade away from reflected light and heat.  As a WE, woodland edge, tree this is somewhat more tolerant of sun.  Will have a tougher time in mineral soils without a developed humus layer.  As a SW, summer wet, tree, this will perform best with regular summer water throughout its life and may suffer and decline without it, especially on more exposed sites, or with limiting soil conditions.  This is another SCT, small canopied tree, with a form and size that can conflict directly with street and sidewalk.

 

Tree Lilac, Japanese (Syringa reticulataSW We zn 3-7

25’ x 20’ Oval

Low maintenance, disease-resistant tree. Showy flower clusters attract butterflies and humming birds.  May not perform well with the reflected heat on streets as it is a more northerly tree from a summer wet climate.  As a SW, summer wet, tree, this will perform best with regular summer water throughout its life and may suffer and decline without it, especially on more exposed sites, or with limiting soil conditions.  This is another SCT, small canopied tree, with a form and size that can conflict directly with street and sidewalk.

 

Zelkova, City Sprite® (Zelkova serrata ‘JFS-KW1’ P.A.F.)P.A.F.) SW Wd SCT zn 5-8

25’ x 20’ Oval Vase

A low-maintenance city tree. Fine textured foliage is bright green in the summer.  Very tolerant despite its origins, in terms of both exposure and drought though its preference is still for summer water and a woodland setting.  It is still a SCT and is relatively slow growing when compared to the species which means that there can be branching conflicts with the street and sidewalk for several years as a young tree until it reaches more mature size.

The notes that I’ve added for the above should be done for each of the City’s list.  It is disheartening to me in my movement around the City to see so many poorly sited and performing trees.  With a little more thought and care, with better support, home and business owners can do a much better job in selecting and caring for the trees on their particular sites.  There is not one size fits all tree for the City’s stated primary criteria of planting strip width and absence or presence of overhead power lines.  It is not just about increasing tree canopy.  Aesthetics, health and other desired uses should be considered, such as space and light for vegetable gardens, PV and solar hot water systems.

Understandably these lists serve to simplify a complex decision process, one that many homeowners either lack the patience for or are not informed enough that they would then have the capacity to compile such lists themselves and select from it.  Such lists are a good idea in theory.  One problem with this that I see is that there are a significant number of the trees suited for a variety of different conditions included here together with insufficient direction to help homeowners make the best choice for their sites and the level of care, including long term irrigation, that they will realistically commit to.  Planting trees that owners and the community are not committed to caring for, because they are unclear of what will be required of them, assures that we will have an unhealthy and unattractive streetscape.  Each tree, at minimum, that originates from a summer wet climate, should be clearly labeled that they will require an ongoing commitment to summer irrigation…indefinitely.  Our climate is very different than that in the mid-west and eastern parts of this country where many commonly chosen trees originate or are, at least, more successful and better adapted to.  We need to recognize some of the basic differences of our region.  Other trees should be clearly marked as potential sidewalk heavers, an outcome that is relatively predictable given the width of the planting space, the compacted condition of most urban sites and the species or variety of tree.

My intent here is not to lay blame on anyone here but to offer up a way to improve the process.  In the long run our goal should be a healthy ‘forest’ and we cannot accomplish that without paying closer attention to the factors that effect tree health.  All trees are not created equal nor do all regions present the same growing conditions.  We have to recognize that conditions within the City vary as they do with aspect.  Conditions on a north facing street are very different from those facing southward, just as they do on the north or south faces of a hillside, especially when compounded by a broader expanse of hard surfaces which make up our streets, sidewalks, parking lots and plazas.  We have the ability to improve our practice and our landscape.  Criticisms should be made only if no attempt is made.

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One thought on “Helping Homeowners Choose Trees Wisely: what you need to know

  1. Steve Morgan

    Lance,

    Very nice blog on helping homeowners choose trees. You spent a lot of time researching and writing + the photos. Good job!

    Steve

    Like

    Reply

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