Home Gardening Center Tip Sheet: Caring for Urban Street Trees

By Sonia Uyterhoeven

·· Urban Gardening ··

Caring for Urban Street Trees

Trees are an important part of the urban environment, making our cities more comfortable and more beautiful places to live and work. Trees soften the look of our neighborhoods, making them more welcoming and helping to block unattractive views. Studies have shown that street trees have a calming effect: Neighborhoods lined with trees tend to have a stronger sense of community and reduced crime rates. Trees can also increase property values by up to 30%.

Air pollution is an ongoing problem in the urban environment. Like all green plants, trees take in carbon dioxide and release oxygen, helping reduce the buildup of greenhouse gases that are a cause of global climate change. Trees also clean the air by trapping dust and other particles. They provide much-needed shade in summer, cooling streets, sidewalks, and houses and perhaps reducing air conditioning costs.

Trees can also help reduce rainwater runoff. Urban trees are always hungry for water. After a heavy rainstorm, rainwater will drain into the tree pit (the area around the tree), watering the tree and at the same time preventing water from accumulating around gutters and curbs.

Street trees contribute so much to cities that they deserve the best care we can give them. The life of street trees is hazardous: vandalism, bikes being chained to them, car doors slamming against trunks, trucks hitting lower branches, garbage being dumped into tree pits, and pets using tree pits as lavatories are just a few of the dangers. Perhaps the biggest problem street trees face is drought. Trees need 8 to 10 gallons of water a week to stay healthy, with young trees benefiting from 10 to 20 gallons while they are getting established.

Sidewalks encroach on the growing area, leaving trees with little room to grow and little soil to absorb water. Tree pits should be as large as possible. Although there must be 5 feet of sidewalk between a building and a tree pit, tree pits can be expanded along the street with the permission of the Department of Transportation and the Parks Department.

Sometimes tree guards designed to protect trees from cars, pets, and other hazards can actually do more harm than good. Many tree guards have raised edges or a lip that prevents rain water (often the tree’s only water source) from running into the soil. Good tree guards should allow water to get to the tree, not be placed too close to the trunk, decrease the size of the planting area as little as possible, and be removable.

Another challenge street trees face is poor soil. Urban soil is often compacted from construction and foot traffic. Compacted soil suffers from poor drainage and makes it difficult for oxygen and nutrients to get to the roots of the plant.

Nina Bassuk at the Urban Horticulture Institute of Cornell University has done comprehensive studies on structural soil--soil mixed with larger stone particles to prevent compaction. If possible, structural soil should be used in street tree pits and the surrounding area. Trees are a long-term investment--you will incur fewer costs in the long run and have healthier trees if you create the best possible soil and select your trees wisely.

Salt used to clear ice and snow from roads and sidewalks in winter is also a problem for city trees. It is important to flush out street tree pits at the end of the winter. One solution is to place mulch in the tree pit before the onset of winter. The mulch will absorb much of the salt and can be scraped off and thrown away in March or April. Remember when mulching not to pile the mulch up against the base of the tree trunk. Too much mulch may cause the trunk to rot and encourage pests and diseases.

One enjoyable way to improve soil and growing conditions around your street tree is to plant annuals or perennials. The roots of most annuals and perennials will not compete with the tree for growing space and water, and you can enjoy the flowers! Lightly cultivating around the base of the tree is fine as long as you observe a few rules:

1. Do not raise the soil level around the tree. You should still be able to see the root flare of the tree (the base).
2. Do not over-cultivate soil around young trees, give them a chance to get established.
3. Use annuals as an indicator of when the tree needs water. Wilting annuals are a good sign that your street tree also needs a drink.
4. Be careful not to plant annuals or perennials that are greedy and will compete for food, space, and water with the tree. Most perennials make good additions to tree pit planting, their roots break up the soil and allow more oxygen in.
5. Do not plant ivy. It is a thirsty plant and a favorite habitat for rats.
6. Improve soil by adding 1/2 to 1 inches of organic matter.
7. Do not fertilize annuals as frequently as you would in a container or garden situation. You do not want to over-fertilize the tree. Remember you are working in a small, restricted area.

When planting many street trees it is important not to plant just one type of tree. A mix of trees will be much less susceptible to large-scale attacks from pests and diseases. Learn from the case of the American elm (Ulmus americana), once the most widely planted street tree, now nearly wiped out by Dutch elm disease. Currently, emerald ash borer is threatening ash trees (Fraxinus) in the Midwest, and the Asian longhorn beetle is attacking maples (Acer) and other trees in the New York area.

Trees need water, sunlight, good soil conditions, space to grow, and proper maintenance such as pruning. Many trees prefer acid soil (maples are an exception). Concrete on the sidewalk is filled with lime, so the soil in cities tends to be alkaline. Height is also an issue. Small, understory trees tolerate shade, but trucks and other large vehicles may destroy their lower branches.

What, then, are the desirable characteristics of a street tree? Look for a tree that recovers quickly from transplanting; adapts to a wide range of soils; tolerates drought, salt, shade, and street lighting; and resists pests and diseases. Here are several common suggestions:

Ash (Fraxinus, e.g., European ash, F. excelsior; and green ash, F. pennsylvanica)
Callery pear (Pyrus calleryana ‘Chanticleer’)
Ginkgo (Ginkgo)
Golden rain tree (Koelreuteria)
Honey locust (Gleditsia)
Japanese tree lilac (Syringa reticulata)
Japanese zelkova (Zelkova)
Linden (Tilia, e.g., little leaf linden, T. cordata; and silver linden, T.tomentosa)
London plane tree (Platanus × acerfolia)
Maple (Acer, e.g., red maple, Acer rubrum)
Oak (Quercus, e.g., pin oak, Q. palustris; and willow oak, Q. phellos)
Dawn red wood (Metasequoia)
Sweet gum (Liquidambar styraciflua ‘Rotundiloba’)

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·· Urban Gardening ··

Generous support for the Home Gardening Center has been provided by Kenneth and Ellen Roman.