Successfully spraying around an established tree requires care and timing.

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

by Daniel Thompson

Herbicides can stunt and deform the trees that provide your family with shade and a place to play. Using the right methods, you can spray the grass and weeds around your established trees without damaging them. You'll need to consider several factors when you use herbicides around trees, but in most cases the most effective way to prevent damage is to keep herbicide away from your trees and their roots.

1. Spray Drift

Even relatively mild winds can cause herbicidal sprays to drift onto the foliage of your tree and cause severe damage. The best time to apply herbicide is when wind speeds are below 10 miles per hour. In most cases, small amounts of herbicide on the bark of a mature tree is only a problem if it has thin, papery bark. You can reduce the effect of the wind on your spray by adjusting the nozzle of your sprayer to deliver a coarse spray of heavy droplets instead of a fine mist.

2. Inclement Weather

High temperatures and rainy weather can bring herbicidal sprays into contact with your trees. Temperatures exceeding 85 degrees Fahrenheit can cause sprayed herbicide to evaporate and spread to the foliage of your trees, resulting in damage. An untimely rainfall can wash your herbicide into the soil where it will damage the roots of your tree or move away from the weeds you are trying to control. Check the weather before you start spraying, and wait for at least a day of clear weather before you spray.

3. Beneficial Herbicides

Selective foliar herbicides are designed to kill certain species of plants. These herbicides can cause serious damage to trees when they come into contact with the foliage or roots of the tree. This type of herbicide often combines several types of herbicide to provide a wider range of control. Some of the chemicals commonly used in selective herbicides include 2,4-D, MCPP, triclopyr, dicamba and mecoprop. Non-selective foliar herbicides can also cause serious damage to your tree if they come in contact with its leaves, but are less likely to damage your tree after they have been sprayed. Non-selective herbicides include the chemicals paraquat, diquat, glyphosphate and glufosinate ammonium.

4. Harmful Herbicides

Non-selective residual herbicides are absorbed through the roots of plants, and they can persist in the soil for up to a year. Over time rainfall can wash this type of herbicide down to the expansive root system of your mature tree. These herbicides can cause serious damage and may kill your tree within two to three years. Non-selective herbicides are made from a variety of chemicals including bromacil, imazapyr, prometon, tebuthiuron, sulfometuron and diuron. You can check the ingredient label of your herbicide to find out what its active ingredients are.

5. Alternative to Spraying

Hand-pulling weeds and grass under a tree is a labor-intensive process, but it eliminates any chance of herbicide damage to the tree. If you need to remove a thick mat of grass or individual weeds beneath the tree, use a garden hoe to sever the plant's roots just below the foliage. Severing the roots will allow you to pull it off the ground in mat-like chunks. Avoid digging deeply since this could severely injure the roots of the tree. After you have removed the grass and weeds beneath your tree, lay down a shallow layer of mulch to prevent the grass and weeds from returning. Avoid adding more than 5 inches of mulch over the roots of your tree since a thicker layer can prevent water and oxygen from reaching the tree's roots.

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