Controlling Broadleaf Weed in Lawns

Robert Bishop
Frederick County Master Gardener Program

Lawn weeds are divided into two major groups, grassy weeds (e.g. crabgrass) and broadleaf weeds (e.g. dandelion). Chemical control methods differ and were developed for each specific group. The best approach to weed control is keeping the lawn thick and healthy through proper fertilization and watering techniques. The thinned-out areas of poorly fertilized or drought-stressed lawns can leave open areas for weeds to gain a foothold, and eventually take over. It is too late in the season to control grassy weeds like crabgrass, but if dandelions and other broadleaf weeds are taking over your lawn now is the time to do something about it.

Most broadleaf weeds are easily identified by wide leaf blades as compared to the much narrower leaves of the turfgrass. Wild garlic & wild onion are two exceptions to the rule, both are considered broadleaf weeds even though they have narrow leaves. Some other examples of broadleaf weeds are: Dandelion, Clover, Chickweed, Thistle, Spurge, and wild violet.

Timing- Broadleaf weed control strategy calls for herbicides to be applied after the weeds are visible and actively growing, this is referred to as post-emergent control. Adequate soil moisture prior to herbicide application is vital for good control because weeds that are drought stressed will not absorb herbicides easily. The cool temperatures and abundant rain of spring and fall provide ideal conditions for broadleaf herbicide application, with May and September proving to be the best months. Heavily weeded lawns may require repeated herbicide applications for control. The first year it may also be necessary to treat the lawn in the spring and again the following fall. Once control is achieved you can usually reduce herbicide use to one annual spot treatment. A spot treatment is the application of a herbicide to the weeds only, instead of the entire lawn.

Product Selection- Broadleaf herbicide products are available in liquid (applied with a spray) or granular (applied with a spreader) forms. Because the herbicide is absorbed through the leaves I have found the liquid broadleaf herbicide formulation to work best. The spray is applied to a dry leaf and will remain there as is dries. Liquid herbicides are packaged and sold in three different containers: as a bottle of concentrated herbicide that is diluted with water first and then applied; in a pre-mixed spray bottle; or in an aerosol can. The herbicide should be applied to the weeds only, instead of spraying the entire lawn. Why waste time and product spraying everything when all that needs the herbicide application are the weeds? Spraying the weeds only reduces stress on the weed-free lawn areas as well. If mixed and applied according to label directions liquid broadleaf herbicides will not harm the lawn. Mixing too strong and or applying in hot, humid weather can burn the lawn. Always follow the label directions when using any pesticides.

Granular broadleaf herbicide formulations often do not produce the same results as liquids because they do not stay on the leaf. Before a granular herbicide is applied the lawn must be wet. This is accomplished naturally by applying the product early in the morning before the dew evaporates or by first watering the lawn lightly with a sprinkler. Foot traffic, gravity and wind will often knock the herbicide granules off the leaves as the lawn dries, reducing its destructive effect on weeds. Granular broadleaf herbicides are most often combined with fertilizer and sold as Weed & Feed products. This is a combination that contradicts itself because fertilizer should be applied to the soil surface so the turfgrass roots can uptake the nutrients, and the herbicide must stay in contact with the foliage to control the weeds. May is not a good time of year to fertilize lawns because grass plants have a natural flush of growth and fertilizing them actually stresses them by forcing them to grow too much. The stress increases their susceptibility to insects and disease problems in the coming hot months of June, July and August. The best time to fertilize home lawns is in the fall and again lightly in early spring.

Weed & Feed herbicide products that control Grassy type weeds like crabgrass are great products. They must be applied in early spring (before May) when a light feeding is beneficial to the lawn and before grassy weeds appear. Watering the lawn after the product is applied is required to wash it from the leaves onto the soil surface. Both the fertilizer and herbicide need to contact the soil. The herbicide creates a chemical barrier in the soil which does not allow the weed seeds to push through as they germinate. This strategy should help to win the battle with the weeds.

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