BROADLEAF WEED CONTROL IN ESTABLISHED LAWNS
Agronomy Mimeo 79
Revised
February, 1999
BROADLEAF WEED CONTROL IN ESTABLISHED LAWNS
Agronomy Mimeo 79Revised
February, 1999
INTRODUCTION
The presence of weeds in a home lawn
mar the appearance of turf, but more importantly they compete with
the desired turfgrass for water, nutrients and light. Lack of control
of these weeds often results in a deterioration of the turfgrass
stand as the number of weeds increase. Control of weeds is not by
itself enough because a weed problem is often symptomatic of a more
basic cultural or soil problem. If these problems persist, weeds also
will be a continuous problem. Thus, weed control not only includes
removal of existing weeds, but also taking corrective management
measures for the factors causing the poor lawn so that weeds will not
become a recurring problem.
Principle Causes of Poor Lawns
Inspection of lawns throughout the state has shown that one or more of the following factors are typically responsible for poor lawns, and resulting weed problems:
1. Poor Quality Seed: Many lawn grass seed mixtures sold in Maryland contain types of grasses that are not well adapted to Maryland and/or may contain excessive weed seed. When seeding lawns, use cultivars recommended in Maryland (see Agronomy Mimeo 77, "Turfgrass Cultivar Recommendations For Maryland"). Buy certified seed to insure that you obtain quality seed.
2. Close Mowing: An insufficient leaf area for the development of a vigorous root system occurs when the lawn is mowed too closely or frequently. Kentucky bluegrass lawns should be maintained at a height of 2 to 2½ inches; whereas, tall fescue lawns should be maintained at a height of 3 to 3½ inches. The lawn should not be allowed to get taller than 1/3 the desired height before remowing. For example, if the desired height is 2 inches the lawn should be mowed before it exceeds 3 inches in height.
3. Too Little or Too Much Fertilizer and Lime: A variety of problems arise from improper fertilization and liming. Weeds are favored more than turf when fertilizer and lime are applied at an improper time, particularly in the summer. Too little fertilizer or lime results in poor turf vigor, which decreases competition against weeds. Too much nitrogen fertilizer can increase drought and disease injury, which results in decreased competition against weeds. Have your soil tested through your County Extension Office to determine the fertilizer and lime needs of your lawn.
4. Improper Watering: Frequent and shallow watering usually does more harm than good in maintaining a lawn by increasing chances for disease, restricting the depth of rooting, and encouraging germination of weed seed. When water is needed, apply with a sprinkler and wet the soil to a 4 to 6 inch depth. Never apply water so fast that it stands on or runs-off the surface. See Agronomy Mimeo 88, "Irrigation and Water Conservation on Home Lawns" for more information.
5. Droughty or Poorly Drained Soils: These soils may not be suited to growing a dense turf. Certain ornamentals or ground covers may be better adapted.
6. Lack of Sunshine: Many turfgrass species will not tolerate shade and even shade grasses grow slowly with insufficient light. If the area receives less than three hours of direct sunlight each day, then the use of certain ornamental ground covers should be considered.
7. Too Much Traffic: Heavy use of lawn areas or concentrated traffic across one section, particularly when the soil is wet, compacts the soil and weakens the turf. Under these conditions, weeds will be more competitive.
8. Insect and Disease Damage: Decreased vigor and thinning of lawns will occur where insect pests and/or diseases are left unchecked. As a result, weeds will rapidly invade damaged areas of the lawn.
Cultural Control of Broadleaf Weeds
The numbers and types of weeds found in lawns are greatly influenced by management and cultural practices. For example, close mowing and too little nitrogen favor white clover. Close mowing also favors weeds such as carpetweed, spurge, plantains, and dandelion. Poorly drained areas favor weeds such as ground ivy and chickweed, while compacted sites favor knotweed.
Correcting improper management practices so that a dense, vigorous turf develops is the best and most lasting method for broadleaf weed control. Of particular importance are proper fertilization, mowing, and watering. Although chemical control of most weeds is possible with the proper use of a labeled herbicide, weeds become a recurring problem if poor management and cultural practices are continued. A few broadleaf weed species cannot be controlled satisfactorily by herbicides, so proper management is necessary to reduce the opportunity for their establishment and spread. Herbicides should be considered an aid, but not a cure for broadleaf weed problems in lawns. Once large numbers of weeds have been controlled, vigilant digging or hand-pulling young weeds as they emerge can effectively keep lawns free of broadleaf weeds for long periods.
Chemical Control of Broadleaf Weeds
In lawns where broadleaf weeds have become a problem, an application of an approved herbicide may be necessary for their removal so that the turf can be improved through better management and cultural practices. Several materials are available for broadleaf weed control, but different weeds are susceptible to different products. Thus, proper identification of the weeds is essential before the most economical and effective herbicide can be chosen.
Frequently, two or more different herbicides are sold as pre-packaged mixtures. Most of the materials discussed are sold by several manufacturers, often under different trade names with different formulations and concentrations. Therefore, no rates are given. READ and FOLLOW the label directions on the herbicide container carefully. Applying rates too low may result in inadequate control, while applying rates too high also can reduce control and injure the turf. The following herbicides are available for the selective removal of broadleaf weeds from lawns:
2,4-D is the oldest, most widely used turfgrass herbicide and provides broadspectrum control. This chemical is particularly effective for control of taprooted weeds such as dandelion, broadleaf plantain, mustards, and shepherd's purse. Amine and salt forms are most commonly used, however, the low volatile ester form of 2,4-D is often recommended for control of wild garlic and wild onion. Some notable weeds not controlled well by 2,4-D are white clover, chickweed, purslane, and ground ivy.
MCPA is chemically-related to 2,4-D and may be used as a 2,4-D substitute in pre-packaged mixtures. MCPA is not as broadspectrum as 2,4-D, and its use alone (i.e., not mixed with another herbicide) is not usually recommended.
MCPP is most effective in controlling several perennial or winter annual turf weeds that often are viney in nature. Important weeds controlled include chickweed, clover, and young lespediza.
Dicamba controls many different weeds, several of which are not easily controlled with 2,4-D or MCPP. Of particular importance are the summer annual weeds that have a prostrate growth habit, including knotweed, purslane, and spurge.
Dichlorprop (2,4-DP) and Triclopyr are sold in pre-packaged mixtures with 2,4-D and provide broadspectrum weed control.
Triclopyr + Clopyralid is a non-phenoxy, prepackaged mixture only sold under the trade name of Confront®. This mix also provides broadspectrum control of many common broadleaf weeds including creeping oxalis. It also does not have a strong persistent odor like other broadleaf herbicides.
Bromoxynil will safely control broadleaf weed seedlings in newly established turf. This herbicide, however, does not control mature, broadleaf weeds and may only be used on sod farms. It is now difficult to find bromoxynil in the marketplace and it may soon be withdrawn for use on all turfgrasses.
Isoxaben is only sold under the trade name of Gallery®, and it is used primarily in the early fall for preemergence control of numerous winter annuals (especially henbit and common chickweed) and some perennial broadleaf weeds. It has no postemergence activity against emerged broadleaf weeds.
Quinclorac is only sold under the trade name of Drive®. It effectively controls only a few broadleaf weed species including white clover, corn speedwell, violet and dandelion. A primary use will be for postemergence crabgrass control.
COMBINATIONS of the above mentioned herbicides are very common. Combination products allow for the control of a broader range of weed problems than single herbicides. In addition, some combinations may allow for the control of certain weed species that cannot be controlled easily by the individual components alone. Some commonly used combinations are: 2,4-D + MCPP; 2,4-D + dicamba; 2,4-D (or MCPA) + MCPP + dicamba; 2,4-D + dichlorprop; and 2,4-D + triclopyr. A broadspectrum, non-phenoxy herbicide is a mix of clopyralid + triclopyr, which is only sold under the trade name of Confront®.
These herbicides will effectively control many broadleaf weeds found in lawns. Listed in the table are several weed species and their susceptibility to the most common turf herbicides. The best times of year to control most broadleaf weeds are fall (especially late September) and spring (especially May).
To use these herbicides effectively for broadleaf weed control in lawns, several points should be remembered:
1. READ and FOLLOW the label directions CAREFULLY.
2. Spray when the temperature is over 70oF and the weeds are actively growing. Do not spray when the temperature is over 85oF as turfgrass damage may result, and because some of these products (i.e., low volatile esters) are prone to volatilization.
3. Treat only when soil is moist and plants are growing vigorously. Do not apply herbicides during droughty periods when soil is dry.
4. Do not mow one day prior to and after spraying.
5. Spray formulations (i.e. liquids) are generally more effective than granular forms of broadleaf herbicides, but granular products or more easy to handle and apply for homeowners.
6. Granular formulations should be applied when the foliage is moist, such as during early morning hours when there is a heavy dew present. Newer formulations of some granular products are just as effective as sprayable materials for weeds such as dandelion, broadleaf plantain and white clover.
Precautions for Using Broadleaf Herbicides
1. Ornamental plants, trees, shrubs, and vegetables can be susceptible to these chemicals. Do not spray around homes and gardens when there is a wind. Even a slight breeze is likely to carry spray droplets to susceptible ornamental and garden plants. Ester formulations (even low volatile-type) are volatile and are therefore more likely to injure nearby ornamentals, vegetables, etc.
2. Dicamba is included in many herbicide combination products and also in some weed and feed (fertilizer-herbicide) combinations. This and other broadleaf herbicides move readily in some soil types and can be absorbed by plant roots. Therefore, products containing dicamba in particular should not be used near the drip-line of trees or near sensitive ornamentals where it can be absorbed by roots.
3. Do not use any of these herbicides on newly seeded lawns. Wait until the new lawn has been mowed at least three times before treating (usually about 6-8 weeks after seedling emergence).
4. The herbicides listed in this publication are safe to use on established tall fescue, Kentucky bluegrass, fine-leaf fescues (i.e., creeping red, hard, Chewings and blue sheep), bermudagrass and zoysiagrass lawns. All herbicides have the potential to cause some foliar yellowing. Do not use 2,4-D on lawns where bentgrasses or roughstalk bluegrass are considerable desirable species.
5. Thoroughly clean sprayer to include hose and boom after using these herbicides. It is advised that one sprayer be used for lawns and another for spraying ornamentals. Do not allow spray mixtures or sprayer wash solutions to spill or leak onto areas where they can be taken up by foliage or roots of trees or ornamentals.
6. Keep herbicide containers closed, properly labeled, and safely stored.
7. Always keep a pesticide in its original container.
CAUTION
Most herbicides are relatively safe to humans and pets when handled and applied carefully. Greatest care must be taken during the mixing of concentrates. Applicators should wear rubber gloves and boots, long sleeve shirts, long pants and eye protection. Avoid prolonged or repeated contact with skin, and be sure to wash thoroughly after using them. Store away from children, animals, fresh produce and other food products. It is best not to allow people or pets onto treated sites until the herbicide has dried on the leaves. Reference to trade names does not constitute an endorsement, guarantee, or warranty. No discrimination is intended against products not mentioned.
CHEMICAL CONTROL OF COMMON BROADLEAF WEEDS FOUND IN HOME LAWNS
RESPONSE:
R = Resistant (i.e. not susceptible to the herbicide),S = Susceptible,
I = Intermediate (retreatment may be necessary).
*Very young plants (2-3 leaf stage) can be controlled in early spring with 2,4-D.
**Some weed species can be controlled by a fall applied, preemergence herbicide such as isoxaben.
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RESPONSE: R = Resistant (i.e. not susceptible to the herbicide),
S = Susceptible,I = Intermediate (retreatment may be necessary).
*Very young plants (2-3 leaf stage) can be controlled in early spring with 2,4-D.
**Some weed species can be controlled by a fall applied, preemergence herbicide such as isoxaben.
Mimeo prepared by Peter H. Dernoeden, Extension Turf Specialist, Department of Natural Resources and Landscape Architecture