There is a lot to think about when planting tree pits. Most important is the health of the tree already in the pit.

Remember that in New York City street trees are city property. Additional plants in your tree pit will compete with the tree for essential resources. When you consider the very small amount of open space a street tree has from which to take its rainwater, oxygen and nutrition, reducing the available resources can create a lot of stress on it. Naturally, the tree would send roots out about 2x the tree canopy with the fine roots that take up water largely in the top two feet below the surface, but the root zones for your trees are quite limited by hardscaping (sidewalk and pavement).  Avoid planting the tree pit densely or with greedy neighbors for the good health and longevity of your tree and to minimize the support the tree will need from you during the growing season. If it is a small tree pit relative to the size of the tree, it is better for the tree to leave it with just some mulch around its roots.

While you do see sizable shrubs planted in tree pits, that it is not what we recommend for the best health of the tree. (NYC Parks Department also asks you not to plant woody shrubs in a tree pit.) There is a further reason to avoid taller, dense, shrubby tree pit planting as well if you are in NYC - rats! Dense planting near an urban building will be a safe harbor to them. To deter rats, it is useful to cut back any tall, dense, or climbing plants around the trees and building, remove fallen fruits and nuts from a tree pit and remove fecal matter (dogs, rodents) and dropped food waste daily. (Sadly, if the population is high, even a patch of weeds will attract rats.)

Most perennial and annual plants that are adaptable to the conditions of the location are better tree pit companions. Plants that naturally grow under trees are the best choice if you have the space for companion planting. Densely growing plants with thick root systems (like grass or ivy) should be avoided as should any plants that need a lot of sun to flourish. You also need tougher plants that can withstand exhaust, a little road salt and wind. When you plant, disturb that area as little as possible. Space plants so they don't provide dense cover for rodents and so the tree gets the benefit of more water and soil nutrients. Keep a few rules in mind that will help protect the tree:

• Do not raise the soil level around the tree—you should still be able to see the root flare of the tree (the base).
• Do not over-cultivate soil around young trees—give them a chance to get established.
• Use annuals as an indicator of when the tree needs water—wilting annuals are a good sign that your street tree will also need a drink.
• Most perennials make good additions to tree pit planting—their roots break up the soil and allow more oxygen in.
• Improve soil by adding ½ to 1 inch of organic matter.
• 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.

In some cases, the type of tree growing in the tree pit may affect the success of the plants growing with it. Some NYC street trees have a strong (Quercus rubra - red oak) to medium (Crataegus - hawthorn) suppressive effect on competing plants. This phenomenon, called allelopathy, is the result of chemical inhibitors  released by the tree's roots and leaves. In these situations, annual plants that last for just one growing season are the best choice.

There are some good suggestions for Planting in Tree Pits and Care of Tree Pits available from the NYC Parks Department. If you have an idea for something more specialized, feel free to get in touch with us at plantinfo@NYBG.org for planting recommendations. Let us know what your planting objectives are, the size and type of tree (if you know) and the amount of sun available in the setting. If you are not in New York City, please share your location.

You can read more about selecting and supporting urban trees in our Guide to Street Trees.

Courtesy of NYBG Plant Information Service