Beth R. Jarvis
Small, bright yellow flowers each have five petals; they usually appear in early summer and bloom throughout the season. Each flower produces a five- sided seed pod which bursts when ripe, spraying seeds. With such an effective seed dispersal mechanism, it's no wonder that yellow woodsorrel spreads easily. It is common in pastures and waste places as well as lawns and gardens.
Hoe or pull these weakly tap-rooted weeds from flower and vegetable gardens. If you've had trouble controlling woodsorrel, you could also use a pre-emergent herbicide. Either trifluralin (Preen) or DCPA (Dacthal) may be used in gardens as a pre-emergent herbicide. Pre-emergent herbicides prevent most seeds from sprouting, so use them only after the desired plants are established unless the label suggests otherwise. Be sure to read the label carefully to check whether the product is approved for use with the plants in your garden.
A vigorously growing lawn will crowd out weeds. The most effective means of weed control is proper lawn care that encourages healthy, thick grass. If woodsorrel has been a problem in the past, herbicides will help you regain control when used in conjunction with cultural practices that will improve the health of your lawn.
Pendimenthalin is a pre-emergent herbicide which may be used to control both yellow woodsorrel and crabgrass in lawns. Apply it by mid-May and again in late June or early July.
Once woodsorrel plants are actively growing, you can use a contact herbicide to control them in lawns. Unless the plants are evenly scattered throughout your lawn, spot spray them with a liquid herbicide rather than apply a weed and feed over everything. If you'd rather not spray, you can paint diluted herbicide onto individual weeds. Use a sponge secured to a stick that you dip in the herbicide. Buy a ready-to-use formulation or mix a small amount of herbicide from concentrate. Use a broadleaf herbicide labeled for turf containing the following active ingredients:
Apply herbicide during late May or early June. Repeat applications, seven to ten days apart, may be necessary for good control as woodsorrel is a difficult weed to eliminate.
These herbicides are most effective when temperatures are between 60 and 80 degrees F. and should not be used when temperatures are forecast to exceed 85 degrees within the next 48 hours. Choose a time when no rain is forecast for at least 24 and preferably 48 hours. To avoid herbicide drift, spray only when the air is still. Herbicide drift can harm or kill desirable plants such as flowers, vegetables, trees and shrubs.
Reference for Illustrations
USDA. 1971. Common Weeds of the United States. Dover Publications, New York. p. 241.