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Control of Broadleaf Weeds in Homelawns Zac Reicher and Clark Throssell Purdue University Turfgrass Specialists

AY-9

Dandelions and other broadleaf weeds are listed among the most troublesome turf pest problems in lawns. Even though these weeds are fairly easy to control, it is important to understand that you need not eliminate all weeds from your lawn, and a few weeds are acceptable. It is not economically or environmentally practical to eliminate all weeds from your lawn. The best way to minimize weeds in your lawn is through good cultural practices. On the other hand, the best way to encourage weeds in your lawn is by using poor management techniques such as low mowing, no or improperly timed fertilization, and over-watering. Weeds can also infest areas killed by disease or insects.

Where Do Broadleaf Weeds Come From?

• The seeds of broadleaf weeds occur naturally in all soils, and they persist for 30 or more years.
• Most broadleaf weeds are prolific seed producers. Many produce thousands of seeds per plant, and these seeds can disperse into your lawn from miles away.
• Boxes of poor quality, inexpensive grass seed may have many unwanted broadleaf weed seeds.
• Broadleaf weed seeds are often in any topsoil brought on the lawn.

Cultural Control Measures

Mowing

• Mow at 3.0 to 3.5 inches.
• Mow frequently so as not to remove more than 1/3 of the leaf blade at one time. This may mean mowing twice weekly in spring and every other week in summer.

Irrigation

• Irrigate deeply and infrequently. Water to wet the soil to the depth of rooting and then do not water again until you see the first sign of drought stress. The first sign of drought stress is a bluish-grey hue of the grass or footprints remaining in the turf after it is walked upon.

Fertilization

• Fertilize to create as dense of lawn as possible applying 2 to 4 pounds nitrogen per 1000 ft².
• Apply 60-100% of the nitrogen in two applications in fall: one in September and one in November after the final mowing.

Shade

• Many broadleaf weeds originate and thrive in the shade, eventually spreading to other parts of the lawn. Selectively prune tree branches to maximize the hours and amount of sun that reaches your lawn.

Herbicidal Control Measures
The best herbicide choice is a general-purpose mixture made of two or three of the following components: 2,4-D; MCPP (mecoprop); and dicamba (Banvel). Products with more than one herbicide component will control a wider spectrum of broadleaf weeds. Read and follow all directions on the herbicide label.

The best time to apply a general-purpose broadleaf herbicide is mid-September to mid-October. The fall represents the best time to control perennial broadleaf weeds such as dandelion, plantain, and clover. These weeds are storing energy reserves for the winter in their roots. Thus the herbicide will enter the plant and travel to the roots with the food reserves, thereby giving a complete kill of the weed. The second best time is in the late spring or early summer period after the weeds have flowered. If applying in the late spring, be extremely cautious with these herbicides near ornamentals, trees, flowers, and vegetable gardens because these plants can be damaged by these herbicides through direct application, drift, and/or volatilization. This is one of the reasons why we prefer to apply these herbicides in the fall.

• If you only have a few weeds in your lawn, simply spot-apply a herbicide rather than applying to the entire lawn. Apply just enough to wet the leaf and do not apply to the point that the herbicide is dripping off the leaf.
• Apply to actively growing preferably young weeds.
• Do not apply broadleaf herbicides when the soil moisture is low. Not only is effectiveness reduced but damage to the turfgrass could result.
• Apply on a calm, clear day when the air temperature is between 50 and 85^oF.
• If rain falls within 24 hours, consider reapplying the weed control if no results are obvious after 13 to 21 days.
• Remember that pre-emergent crabgrass controls will not prevent perennial broadleaf weeds (dandelions, etc.) from infesting a lawn.
• Do not apply to new seedlings until after the grass has been mowed at least three times.
• Delay seeding a bare spot caused by the removal of weeds for 3 to 4 weeks and after a good soaking rain or irrigation.
• Delay applying a broadleaf herbicide to newly sodded areas for 4 to 6 weeks.
• Delay mowing the treated area for at least 3 days after the treatment.

Summer Annual Weed Control
Summer annual weeds (spurge, knotweed, purslane, etc.) are very difficult to control for a number of reasons. Depending on the species, summer annual weeds germinate at different times during the summer and mature in a very short period of time. Thus, a single application of herbicide might only control a single species of weeds because other species haven't germinated yet or have grown too large to be controlled. Plus, summer weeds have a thick, waxy layer on their leaves to prevent water loss which also limits the herbicide from penetrating the leaf. Consider the following strategies for controlling summer annual weeds.

• Apply a single application of a broadleaf herbicide in late May, realizing you may not control all of the weeds.
• In April, apply a product containing isoxaben which is a herbicide that controls broadleaf weeds before they germinate. It does not control already germinated weeds so it must be applied with a postemergence herbicide containing 2,4-D, MCPP, and/or dicamba.
• Use cultural practices mentioned earlier to limit the weeds.

Difficult-to-Control Weeds
Weeds like creeping Charlie (ground ivy), thistles, and wild violets are difficult to control because they spread by underground stems or root stocks. Herbicides often control or burn back the top growth, but generally will not translocate to give a complete kill. Plus, these weeds often get started in the shade or other areas where the desirable grass is compromised. The non-pesticidal approaches are necessary to help minimize these weeds. Broadleaf herbicides containing 2,4-D, MCPP, and dicamba should be used. A herbicide containing triclopyr and chlopyralid is very effective on these weeds, but this product is currently available only to professionals. Either consider hiring a professional to control these difficult weeds, or learn to coexist with these weeds and not spend time and money trying to control them.

It is impossible to get 100% weed control in your lawn. By combining cultural methods and an application of broadleaf herbicide in the fall, you should be able to minimize the number of broadleaf weeds in your lawn.

Purdue University

Indiana 47907

3/98

Cooperative Extension work in Agriculture and Home Economics, state of Indiana, Purdue University, and U.S. Department of Agriculture cooperating;
H. A. Wadsworth, Director, West Lafayette, IN. Issued in furtherance of the acts of May 8 and June 30, 1914.
The Purdue University Cooperative Extension Service is an equal opportunity/equal access institution

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