Careful Use of Weed Control Chemicals in the Lawn Protects Trees

Careful Use of Weed Control Chemicals in the Lawn Protects Trees

Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent  

"Weed and Feed" or plain "weed" fighting chemicals can cause serious problems for trees and shrubs.  These materials are generally considered safe to humans, pets, and wildlife, if label directions are closely followed.  However, many weed control chemicals are not as safe to the desirable trees, shrubs, and ornamentals growing in the lawn or in surrounding planting beds.  The problem is not really with the weed killing products, but with us... those applying the products.  We frequently fail to read and fully understand the precautions given on the label.

Many products, especially those containing the active ingredient dicamba, advise against use in the root zone of desirable trees, shrubs, and other ornamental plants.  Damage often occurs when gardeners don't realize just where the root zones of these plants are located. 

Trees and shrubs are not tap-rooted (with a few exceptions). Roots expand laterally out from the trunk where there are larger "pipeline" roots that divide and subdivide, becoming smaller with each division until the very ends where the fine-feeder roots are located. The fine-feeder roots are responsible for absorbing water and nutrients from the soil.  The roots of most trees are found in the top 18 inches of soil, with over 50 per cent of their roots found in the top six inches of soil.  They're situated in this top layer of soil because that's where they can get the air, nutrients, and water they need for growth and survival.

Roots go out much further from the tree trunk than once believed.  It was once thought that most of the fine-feeder roots were located primarily in the drip-line zone. This is the area beneath the reach of a tree's side branches... the place where rain "drips" off a tree.  However, research has revealed that the root systems, especially the fine feeder roots, extend out laterally much further from the trunk.  In fact, tree roots often extend out from the tree trunk as far as the tree is tall... or even further.

Before we apply "weed and feed" or other weed controlling products to our lawns, we should step back and assess where tree, shrub, and ornamental plant roots may extend.  In most cases, it would be best to apply the chemicals only to the patches of lawn with weeds, not to the entire lawn where roots of desirable plants may be located.  If you have a large lawn with lots of trees and shrubs, these spot applications may be annoying, but they're important in protecting your plants from damage.

Of course if you don't have many weeds, it's much healthier for your trees and shrubs to either dig out the weeds by hand or spot treat individual weeds.  It's overkill to repeatedly apply herbicides to your entire lawn when you don't have many weeds.  The very best form of weed control is a healthy, dense lawn.  Proper watering, fertilizing, and mowing go a long way in helping you avoid the use of weed control chemicals... and protecting your trees, shrubs and gardens. 


If you chose to use weed control chemicals on your lawn. here are more precautions you should follow to protect your trees and shrubs:

Calibrate—  If you're applying a "weed and feed" or other weed controlling products, be sure to only apply the right amount. If it's a granular product and you're applying it with a drop spreader, be sure to calibrate your spreader correctly and only apply the amount needed for the square footage of lawn.  If you're applying the material as a liquid spray, you should also check the calibration of your sprayer or applicator.  Damage to desirable plants in the lawn or nearby plants is often due to over-application.

Do Your Math—  Do you know the square area of lawn that you're planning to treat?  Don't guess. Actually measure your lawn area and determine the square footage being treated. If your lawn is not an easy rectangle or square, determine the footage by breaking the area into rectangular, square, triangular, and circular blocks.

When Using a Granular—  Many of the granular materials will direct you to apply them when there is dew on the grass or when the grass is wet from a recent rain or irrigation. This helps the material stick to the grass and work more effectively.  If applied to dry grass, they will be much less effective.  There will also be directions on how long to refrain from watering and mowing, whether the material is applied as a spray or a granular.

Sensitive Plants—  Some plants are particularly sensitive to the application of dicamba and phenoxy (2,4 D, MCPP, and MCPA) herbicides in their roots zone.  Damage may occur even when you closely observe all the label precautions.  Trees and shrubs that are particularly sensitive to damage from either dicamba or phenoxy herbicides or both include apple, birch, box elder, catalpa, dogwood, forsythia, grape, honey locust, Norway maple, redbud, Siberian elm, sycamore, and walnut.  Extreme caution should be employed if you have any of these plants in your yard and you use these weed killing chemicals in your lawns. 

Avoid Drift—  Many broadleaf weed controls are applied to lawns as liquid sprays.  Extreme caution must be utilized to prevent drift to nearby areas.  Don't spray when it's windy.  The lower to the ground the application and the larger the spray droplets, the less likely drift will be a problem.  It's worth repeating...don't spray when it's windy or breezy.

What Weed Is It?  —  Get your weed identified first, BEFORE applying any chemicals for control.  If grassy weeds are the problem in you lawn, the typical "weed and feed" products for controlling broadleaf weed chemicals will not touch them.  If they're perennial grassy weeds, they will have to be spot- treated with a non-selective herbicide, such as Roundup or other glyphosate containing product.

If they're annual grasses, you will need to apply a preemergent herbicide, such as we discussed in a recent column. Effective herbicide applications for both perennial and annual weeds in the lawn depend on the type of grass and the appropriate timing of the application.  Get your weeds positively identified before attempting control.

Picking An Herbicide—  Not every weed is controlled by every herbicide chemical.  Most broadleaf weed control products for home gardeners contain more than one active ingredient.  2,4 D has been a popular ingredient for many years and provides good control of a number of common broadleaf weeds, such as dandelions and plantain.  However, there are other weeds for which it really doesn't do a good job of control.  Mixes of 2,4 D andMCPP or MCPA, are typically utilized.  These broaden the number of common broadleaf weeds the material will control, including chickweed, ground ivy, black medic, and clover. 

Many mixes now also contain dicamba to get at some of the more difficult-to-control weeds, such as bindweed, prostrate spurge, mallow, and oxalis.   Mixes that contain dicamba, 2,4 D, and MCPP are often referred to as "Trimec" when listed on the active ingredients of the label.

There are even tougher weeds, that the Trimec combination sometimes won't control.. A newer material, triclopyr, is now available to home gardeners to use on their lawns for control of some of the toughest lawn weeds, such as oxalis, violets, and spurge.  The only home garden product for use on lawns in Washington that contains triclopyr is Monterey Spurge Power that also contains dicamba and MCPA