Should I Spray Around a Mature Tree to Kill Grass & Weeds?

Low-hanging branches make mowing under trees a hassle.

Low-hanging branches make mowing under trees a hassle.

Grass and weeds under your favorite tree's canopy can create an overgrown look, but what is more important, they compete with the tree for moisture and nutrients. Finding the right herbicide to spray under your tree saves you the hassle of hand-pulling weeds and grass without harming the tree, but apply the herbicide properly to prevent accidental damage to the tree.

Post-Emergent Herbicides

Many pre-emergent herbicides, or those that keep weed and grass seeds from germinating, are granules you spread and mix into the soil around your tree. These only work before seedlings appear, and they aren't effective against grass that creeps in using rhizomes. Post-emergent herbicides, however, often appear in spray form, allowing you to dose large areas under the tree or pick a single plant to target with the liquid. Many of these travel into the plants through the leaves or blades, killing them from within.

Pick the Right One

Mature trees have thick bark that protects them from many post-emergent foliar herbicides -- which are those absorbed through plant leaves -- but pick the herbicide carefully to ensure it won't soak into the tree's roots. Those containing glyphosate, for example, typically are safe to spray around mature trees. The chemical doesn't leach through the soil, so it shouldn't reach the roots. Sethoxydim, another herbicide active ingredient, also is deemed safe to spray under trees to eliminate grass and weeds.

Keeping the Tree Safe

Even safe herbicides can hurt your tree if not applied carefully. The spray saves you time and labor when you want to rid the drip line of weeds and grass, but without the proper precautions, you can inadvertently harm your tree as well. Cover the trunk base with plastic or cardboard to prevent herbicide drift, which is where droplets blow in the wind to hit unwanted areas. The bark typically protects the tree, but sprays can sneak into small chinks in the bark, such as cuts from a weed trimmer. Covering suckers, or immature trees that shoot up from the trunk base, prevents the young bark from absorbing the chemical into the main tree's system. Choose a day with no wind so droplets don't float over to the tree's leaves; a few drops won't harm the tree, but a significant amount can stress or kill it.

Preventing Regrowth

Instead of constantly monitoring the area under the tree with herbicide spray in hand, take steps to prevent weeds and grass from getting a foothold. Mulch under the tree to stop seeds from germinating. For extra protection, place landscape fabric under the mulch. The landscape fabric prevents seeds and sunlight from reaching the soil while letting water pass through to the tree's roots.

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