When do I apply pre-emergent herbicides?


When do I  apply pre-emergent herbicides?

asked February 5, 2014

1 Answer


This is more of a horticulture question than arboriculture question, but there is an important tree care point to be make here.

First, pre-emergent herbicides are designed to manage weed seeds as they germinate. The weed-killing products are generally granular. A late spring or early summer application helps to control warm season grassy weeds, such as grassburs and crabgrass. An early fall application will help manage cool-season weedy grasses. The application windows are weather and location specific, so consult you local nurseryman or extension office for the best dates for you. Do know that once you have a healthy lawn, and your grassy weeds under control, repeated annual applications of pre-emergent herbicides should not be necessary.

Now, the arboriculture angle: Do not confuse pre-emergent herbicides with post-emergent herbicides. Post-emergent herbicides are commonly found in “Weed and Feed” products. These products are absorbed by an existing plant’s root system to kill the plant. Home owners frequently apply them to their lawns, not realizing that the herbicides are absorbed by the root systems of trees and shrubs growing out into lawn areas. Remember that a tree’s root system may extend well beyond its dripline. These herbicides can have a long-lasting negative impact on your trees, and can kill your trees. Our arborists frequently diagnose “herbicide damage” because these products are so often used by unknowing home owners. A better approach to managing weeds in turf grass is a foliar spray herbicide application. Always follow the directions on the herbicide container, and do not allow any contact with desirable plants and trees–not even on bark tissue.