To Plant or Not to Plant
Consider the Roots
- Betsy Emes, Trees for Georgetown
Almost every day, I am asked the same question: “What is OK to plant in the tree box in front of my house?” My answer is simple and always the same: “Nothing.” The response is usually exactly the same: “You’ve got to be kidding — there’s got to be something we can plant that wouldn’t harm the tree.…” And I repeat the same simple answer: “No, nothing.”
And here are the reasons why:
- Most tree roots are located in the top 6 to 24 inches of soil.
- Roots get water, oxygen, and minerals from soil.
- Change in soil depth, improper watering, and digging in the root zone can injure roots, increasing stress and susceptibility to disease and insects.
- It takes many years for a tree to die.
By planting anything in a tree box, you are disturbing/damaging tree roots. Annuals are particularly harmful — every season they must be replaced, resulting in nearly constant digging, which cuts tree roots to make room for the offending plants. In the summer, annuals often need daily watering to survive. Trees need a slow watering once a week, with time in-between to dry out. The soil level is also increased, resulting in a tree that is planted too deeply, causing suffocation of tree roots.
Perennials, especially woody ones, have big root systems that compete with tree roots. Liriope, the most egregious offender, creates an impenetrable mass of roots. Trees need all the water, nourishment, and air that they can get from a tiny tree box, and they need room for their own roots to spread to anchor the tree. By planting trees in street tree boxes, we are already asking them to survive in an unnatural situation — they would much prefer a large garden or, even better, a field or forest, with plenty of room to spread their roots beyond their dripline.
Groundcovers, such as ivy, are equally undesirable: ivy is a favorite habitat for rats, which more often than not chew through tree roots. Vines climbing on trees will strangle them, ending in the tree’s demise.
To avoid root injury and disease, maintaining a healthy, vigorous environment around trees is vital to their survival. The best solution is to keep the tree box free of weeds (they are very thirsty) and to cover with only 2–3 inches (no more!) of mulch, making sure that the mulch doesn’t touch the trunk.
Because trees are generally slow to die, most of us don’t realize damage done until it is too late.
PLEASE BE KIND TO OUR TREES!
Water is the single most important component of a young tree’s survival. Water twice a week, slowly and thoroughly, soaking the ground with 20-25 gallons of water a week. Continue watering, as needed, until the leaves drop off in the fall. Resume watering when new leaves appear in the spring. But do not over water: pull the mulch away and test the soil with your finger -- if it is wet, do not water. The best water for trees comes from rainfall that is light, but lasts several hours. The heavy rain experienced in summer thunderstorms is not particularly beneficial because it mostly runs off into storm drains.
Dog urine is very caustic to both the bark and roots and as it accumulates in a tree box, it will often kill a tree. Please keep your dog from urinating on the trees.
Pedestrian traffic over the soil will compact soil, making it impenetrable to water and air. When roots are starved of these crucial elements the trees become highly stressed and are often colonized by insect or disease pathogens.
Cars and trucks often damage trees by breaking limbs or wounding the tree’s bark.
Salts act to draw water out of the soils which keeps trees from absorbing water. Be careful when applying ice melting salts near our street trees.
Fencing helps trees by discouraging dogs and pedestrians. Fencing should be at least 18 inches high and installed on only three sides (no fencing is allowed on the curb side). Fencing should be minimal and not have an impermeable border at sidewalk level that would keep storm-water from entering the tree box. Railroad ties and brick borders are undesirable because they encourage the addition of excessive dirt which can kill newly transplanted trees.
Room to Breathe and Grow
Most of our beautifully planted tree boxes are actually harmful to their host trees. Digging in the tree box severs tree roots. Adding excessive dirt interferes with the absorption of air and water into the tree well and creates an environment for future rooting problems called “girdling roots.” All this creates stress and jeopardizes the development of the tree. Recent surveys within Georgetown also found that trees in boxes planted with annuals were over watered. This can lead to many problems including root rot that will eventually kill the tree.
Mulching to a depth of two to four inches will retain moisture for roots in the summer and keep them insulated during the winter. But don’t over-mulch. Adding more than four inches of mulch inhibits the flow of air and water to the roots. Two to four inches of mulch is good, but more than this leads to long-term problems. Also make sure your mulch is pulled away from the trunk by an inch or two – otherwise it traps moisture which leads to cracking and sloughing of the bark allowing fungus, insects and rodents to invade.