Choosing a Tree: Issues of Structure and Growth

Right Plant Right Place covers a lot. Usually when you hear it used the speaker is referring to matching the plant with the growing conditions: soil, sun, climatic conditions, size, etc. Here, I’m going to address the physical structure and growth of a tree as it relates to place, Carpinus betulus ‘Fastigiata’, in this case, as it is often used as a ‘street tree’. The Upright European Hornbeam has proven its value as a street tree. It is very adaptable to conditions in our region that exist at the curb. That being said there are questions of structure and growth that need to be asked if such a planting is going to be aesthetically successful over the longer term.

Hornbeams across the street from my house after pruning

Hornbeams across the street from my house after pruning

Carpinus betulus ‘Fastigiata is a more upright form of the species typically growing to 40’ tall and 30’ across. The species grows more rounded into a ‘gumdrop’ shape while this tree will have a more ‘teardrop’ shape. Many broadleafed trees, like Carpinus, are decurrent….they don’t exhibit strong apical dominance. This means that while they may or may not have a strong central leader the side branches grow with vigor and may in some cases out compete the leader for dominance giving it a rounder overall shape. Decurrent trees may have multiple leaders.

Conifers, on the other hand, tend to be excurrent, having strong apical dominance, the main central leader dominates all side branching, releasing a hormone from its apical bud that suppresses all of the growth below resulting in the typically pyramidal shape common to conifers. In its extreme some species become narrowly upright with short weak side growth. These trees don’t form competing leaders unless the leader has been damaged or removed which ‘releases’ the side shoots allowing them to grow more vigorously as the hormone that suppresses them is absent until a new leader can take over.

Again, Carpinus, are decurrent. As in most things, apical dominance varies across species. There are not two simple forms of trees, excurrent and decurrent. You should think of these as two polar points on a continuum. Carpinus betulus would be near the extreme end of the decurrent pole. Apical dominance is weak. Many of its branches grow vigorously. There is no strong central leader structure. Without our intervention these trees will branch low and these early branches will grow with considerable vigor adding caliper, diameter, and length that may exceed the growth of the early young leader. It forms a relatively dense teardrop shaped silhouette.

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In the above photo the lateral branching begins at about 36” above the root flare at the ground. You can see how these laterals are vigorously challenging each other for dominance. There is little hormone produced by this plant to suppress its laterals. These branches will continue over the years to add wood and caliper up to an ever greater diameter. As you can see this will put them into more conflict with both the parked cars at the curb and with pedestrians on the sidewalk.

This is a very strong tendency in this tree…a genetic imperative. When choosing a tree you must be aware of this. These trees were planted four-ish years ago. These branches were there then and, this is significant, they were smaller caliper. Because of this tree’s strongly decurrent growth somebody should have been paying attention and annually removing the lowest branches before they had much of a chance to compete with the leader. Some people might have made this a two stage process one year cutting them back and then removing them the next working up the tree from year to year. The intent should be to grow a clean branch free trunk up to the height where side branching would neither conflict with pedestrians or parked vehicles and their users egress/ingress.

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You can see from the photo above that this didn’t happen. The landscape service hired by the adjacent apartment owners has made a belated attempt to address the problem. The result is where the branches were removed larger than desirable wounds were left on the tree. The remaining branches having had the smaller branching growth removed from them leave an awkward remaining structure. In the photos you can see that all of the laterals are growing upward with as much vigor as the original leader. They will continue to caliper up over the years, broadening the tree, pressing out into the street and sidewalk.

This result is quite common around town on other species of decurrent trees. It is a result of deferred maintenance and a reluctance to remove larger structural wood from the growing tree. The nurseryman, growing the tree, should know better than to leave such low branching. Trees commonly sold as street or larger shade trees should have had this work done to them in the field. Having said this unless a tree is being grown to larger caliper size like 3.5” or bigger this kind of structural pruning would continue to be necessary after planting it out in its site. You can hope that homeowners are aware of this need, but it is obvious that they don’t, they need to be taught, and you would think that landscape maintenance companies would know, but often they don’t as training takes time and effort.

C. betulus 'Fastigiata'

C. betulus ‘Fastigiata’

The above tree on SE 7th near its intersection with Sellwood Blvd grows in a parking strip adjacent to the north parking lot in Sellwood Park. These are not small trees. While some might argue the laterals should begin even higher, I, at 6’2”, can easily walk down the sidewalk without ducking or moving out my line of direction. This will not be the case of the aforementioned trees just down the street from my house.

Carpinus betulus 'Fastigiata' w/ 16" diameter trunk and massive heavy branching

Carpinus betulus ‘Fastigiata’ w/ 16″ diameter trunk and massive heavy branching

This tree has massive branching beginning at about 5′ high.  The only reason it’s not a problem is because this low branching is parallel with the street and sidewalk.  This illustrates how the almost delicate looking branching shown in the earlier photographs bulks up and can become a major problem.

Genetics determines to a large degree what any living thing will look like. It will define how it grows and responds physically to damage and stimulus.  It can do nothing else. If you want to grow a decurrent tree in a narrow space like a parking strip you will have to take some informed action and prune it in a timely manner to reduce injury to the tree. There are thousands of trees around Portland that have been planted and forgotten. I dodge them all the time when I go on walks. It is as if people believe the as a tree grows it pushes up out of the ground lifting all of the branches with it. No!!! The position of the heart of the branch is static. As wood is added over the years it effectively gets ‘lower’ and because of a trees growth response to gravity a branch actually adds more bulk to its underside to support the increasing weight of the branch. Keep in mind that a branch that is a problem now will be a larger problem in years to come. The lesson is to know how your tree grows and to make ‘training’ cuts at a young age to minimize the size of the wound.

Some people seem to live with idea that because trees evolved without our continuous intervention that trees we plant in an urban situation shouldn’t need any care.  That is just ignorant and wrong. We have changed the world and placed them in it.  We should not be surprised to see our plans go awry especially when we ignore the plants.

Inclusion on the City’s Street Tree List or as a choice offered by Friends of Trees does not mean that any randomly selected tree of that species or variety will work in your space, especially without attention and follow-up structural pruning. Right Plant Right Place. Know the plant you intend to plant. A successful landscape is not one in which all the plants simply survive. Gardens and landscapes are not fixed in time. They grow and may outgrow the space they’re allotted. Be careful with your choices and take responsibility for them.

An 'abandoned' 20 year old Friends of Trees planting that never received structural pruning now with low heavy branching & big wounds from removing conflicting branches.

A decurrent, Pacific Sunset Maple.  An ‘abandoned’ 20 year old Friends of Trees planting that never received structural pruning now with low heavy branching & big wounds from removing conflicting branches.

Purple Leaf Plum pruned up showing the bare limbs of its decurrent structure

Purple Leaf Plum, a decurrent grower, pruned up showing the bare limbs of its structure when squeezed into a narrow space and open sight-lines are desired.  This was a bad choice.

The decurrent Big Leaf Maple responding to damage with vigorous shooting.  Growing in a natural closed canopy this would have a more sinuous form having shed some of the shaded out lower branching

The decurrent Big Leaf Maple responding to damage with vigorous shooting. Growing in a natural closed canopy this would have a more sinuous form having shed some of the shaded out lower branching.  As is it tends to grow as a giant bush.

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