Monday, September 1, 2014

Controlling Broadleaf Weeds in Grass

Controlling a grassy weed in a lawn (grass) is difficult since most chemicals cannot distinguish between good and bad grass species. Fortunately, certain herbicides can distinguish between grassy plants and broadleaves. It is the chemical composition of the herbicide and the morphology of the plant that will cause certain herbicides to affect broadleaf plants but not affect grasses. Therefore, controlling broadleaf weeds in a grassy environment is relatively easy. Purchase an herbicide with the active ingredient 2,4-D, MCPP (mecoprop), or dicamba (Banvel). These products are often sold in combination, giving better management and a wider spectrum of the broadleaf weeds controlled.

For light weed infestations, spot treating is more appropriate than treating the entire lawn. Apply just enough of the solution to wet the leaf, do not apply to the point that the herbicide is dripping off the leaf. To insure maximum absorption, stop mowing 2 or 3 days before treatment and allow 3 or 4 days before mowing again. This allows sufficient time for the weeds to absorb the herbicide and transport it to their roots.
Caution: these materials are volitle, espectially 2,4-D. The drift can damage other valued broadleaf plants; tomatoes and hibiscus are particularly sensitive.

Controlling weeds however, should begin long before chemical herbicide applications are considered. Cultural measures ought to be employed from the beginning. These include: 
  • Proper irrigation -  weeds often invade lawns that are either over-watered or under-watered. Perhaps the worst possible irrigation schedule, yet common, is daily watering for 5–10 minutes. Light, frequent irrigation creates a shallow-rooted lawn which is more susceptible to weed invasion, as well as being less tolerant to drought.  In dry areas, where irrigation is employed, water should penetrate to a depth of 6 to 8 inches. The best practice is to allow the soil to partially dry out between irrigation. The top 1 to 2 inches  should be fairly dry before watering again. For more information on irrigating lawns, see UH CTAHR publication, “Watering Lawns”, at

  • Proper fertilization – apply suitable amounts of fertilizer on a regular basis; nitrogen is the key nutrient for turf.
  • Proper mowing heights – each turfgrass species has an appropriate mowing height. Mowing some grasses too short can weaken the lawn and predispose it to weed invasion.  A typical guide is to remove no more than one-third of the leaf blade at each mowing. If too much is removed, it will take more time for the grass to recover, giving weeds a chance to invade.
  • Thatch is a layer of organic matter - stems, stolons, roots - that develops between the turfgrass blades and the soil surface. Regular thatch removal will help keep turfgrass healthy and competitive with weeds.  For Hawaii residents, see the UH CTAHR publication “Removing Thatch from your Lawn” for more information on this procedure.
  • Any activities that lead to soil compaction will contribute significantly to turfgrass stress. This in turn, will make it easier for weeds to invade. These activities may include heavy foot traffic as well as vehicles traveling over the area.

It is very impossible to get 100% weed control in your lawn. Weeds will occur, but the problems can be minimized with a well-managed, vigorously growing lawn. By combining cultural methods with herbicide applications when needed, weeds will  be minimized.