Issue: September 1, 2007
Amending soil under a tree
I have an area in my yard, about 18' by 25', around an old large elm tree. This area has been covered with landscape fabric and gravel for at least the past five years. I'd like to remove the gravel and fabric and plant the area with ground covers and grasses. Will the soil under the gravel need amending before I plant there? What is the best process for that? Also, I'm concerned about the impact on the elm tree when I remove the gravel and dig into the dirt underneath.
Cathleen T.
Albuquerque
Answer:
While it is possible to remove the rock mulch and landscape fabric from around the elm tree, there will be some risk to the tree. Digging to amend the soil and establish plants greatly increases this risk.
The roots of trees that have been growing under landscape fabric and rock mulch often grow just below the fabric. In this setting, they are protected by the fabric and mulch covering. Removing the covering material exposes the roots to light and drying. If you remove the fabric, do a small area at a time, keep the exposed roots moist and cover them with a light covering of good soil. Even if you carefully follow these instructions, the tree may be harmed. Many trees are not tolerant of changes to the root zone after the trees are established. This includes putting soil over the roots, especially if the soil is applied too deeply. Our common Siberian elms are more tolerant than many other trees, and may not be injured. Nevertheless, the possibility of injury to the tree exists.
Amending the soil under the tree will surely damage the roots and cause harm to the tree. Further from the tree, at the outer limits of the area you described will be the safest area to amend the soil over a large area. (If the tree is large enough, there may be no safe place to amend the soil.) However, the greater the distance from the trunk of the tree, the better the tree will be able to recover from injury to small roots. Avoid injuring large roots, they will often not recover. Grasses will not do well in the shade of the tree and should be as far from the trunk as possible in a sunny location. The grasses will benefit from amended soil. If the tree roots are not too severely injured, the additional water provided for the grass will also benefit the tree.
An option that minimizes the potential damage to the roots of the tree is to leave the fabric and rock mulch in place. You can move the rocks aside temporarily, cut a small hole in the fabric, and plant some types of plants (xeric and shade adapted) in the hole, then put the rocks back to mulch the new plants. The soil in the planting hole may be amended if necessary. As you prepare the small planting sites, you can relocate slightly if you find a large root. Be sure to provide irrigation for these new plants. Irrigation applied in the vicinity of the drip line will benefit both the new plants and the tree. Lawn-type grasses may not be appropriate in this kind of planting, but certain ornamental bunch grasses will be excellent choices and will not need to be mowed.
Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.
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