Selecting Tough Street Trees

In the winter, it is so easy to pass by a beautiful tree and not even notice. Bare, grey trunks, branches, and twigs just can’t seem to catch our attention. Of course, we are moving quite a bit faster in the winter when it is cold outside.

I spend a lot of time walking around my neighborhood in the winter months with my dog Georgia. She is a fair-weather dog, so it is difficult to get her to go outside into the garden when it gets cold or if it even looks like it may rain. The only way to get her outside in the winter is to get out the leash and take her for a walk. She keeps our walks moving at a brisk pace, which keeps us both a bit warmer.

Fortunately, we are not walking so fast that I can’t check out all the plants that we pass. This winter, I have been paying more attention to the diverse plantings of street trees in my neighborhood.

Diversity of species with street tree planting is important. The urban/city environment is typically much harsher than in many suburban type neighborhoods or parks. Reduced soil volume, less soil surface to capture rainfall, increased traffic, and risk of injury can take its toll on many urban street trees in the short and long term. It is important to not depend on one species, but to diversify and choose only the toughest species to reduce the risk of widespread failure from environmental conditions, disease, or insect attack.

In the fall, I was on the campus of the University of Kentucky where I attended college. We were on our way to a football game, and as I walked closer to the stadium I noticed the street tree planting of willow oak–Quercus phellos. They were planted more than 20 years ago because I vividly remember them, much smaller of course, when I was a horticulture student there.

Willow oak endures
Even now, so many years later, those willow oaks are only around 20 to 25 feet tall. Given the harsh environment they have had to endure over the years, I thought they looked great, especially for a tree that should have grown to be around 40 to 50 feet tall or so by this time. This is certainly a testament to the toughness of the willow oak. It is not uncommon to find street trees growing, on average, one-half their potential or normal size given the confined and stressful conditions.

The willow oak is an excellent choice for a street tree planting, the garden, or parks and office building landscapes. It is much easier to transplant than most oaks, and its simple, long and narrow leaves don’t make such a mess on the sidewalk and street, making fall clean up quite easy.

The leaf doesn’t look like the traditional oak leaf that most of us think of, and is often misidentified as a result. The leaves of willow oak are an average green in summer but turn a beautiful yellow to rust or red in the fall. It can be considered a shade tree, although it does not cast the density of shade that some people desire due to its leaf size and shape.

In most environments, willow oak will grow more than 60 feet tall and is generally about half as wide as it is tall. No serious disease or insect-related problems affect the willow oak, but if the soil pH is too high, you will see chlorosis, or yellowing of the leaves.

What not to choose
Some trees are not cut out to be a street tree, with only the toughest species surviving in the long term. I observed a street tree planting near my home where flowering dogwoods, Cornus florida, were planted more than 15 years ago. A beautiful tree in the right spot, but not for a narrow street median where it is dry and hot. Not one of them survived. In fact, most were in full decline within five years and they were all dead within seven or eight years.

It was very sad that, due to the improper species selection given the site, the dogwoods had no chance of surviving.

What to choose
Certainly willow oak, goldenrain tree, zelkova, black gum, Japanese tree lilac, or Chinese elm would have been a much more appropriate choice. When selecting a tree for planting, always consider the site and all it has to offer or, more importantly, does not offer. The more difficult the environment or situation, the more time and research you should put into the selection. As a general rule, the tougher the site the shorter the list is from which to choose.

Plants and trees can be so amazing, that even in the harshest of environments they make all attempts to grow. Without urban green spaces such as small parks and street tree plantings, your cities—no matter how large or small—will just get hotter and hotter and storm water runoff will become more of a problem.

As long as we continue to drive cars, build buildings, and pave parking lots, I hope we will also remember to plant lots of trees along the way.