Weed Management

 Too often, the term "sustainable lawn" has been associated with lawns allowed to become neighborhood weed patches. While lawns maintained on very low inputs may have some weedy plants in them, having well adapted grass species and varieties will help keep significant weed infestation to a minimum. It's also important to remember that a few weeds in a lawn are not an impending threat to lawn health and function. Monitoring the situation and adjusting cultural practices to sustain plant health is an appropriate and effective course of action when attempting to manage or control weeds with reduced or no herbicide inputs.

 It has been stated many times that a healthy, vigorous lawn is the best defense against weed invasion. Indeed, weakened lawns and bare areas in the lawn are the most common reasons for increased weed presence. Knowing and understanding a lawn's weaknesses is the first step to managing the encroachment of weeds into the lawn. Weakened and bare areas can be caused by many reasons. Some of the more common ones are listed below:

1. The grass species and/or varieties are not adapted to the particular site conditions or imposed maintenance program Damage caused by animals, diseases, insects or people Damage due to environmental stresses such as drought, shade, heat, cold and poor drainage Misuse of fertilizers and pesticides Improper mowing height and/or frequency
2. Overuse of the lawn area resulting in severely compacted and poorly aerated soils

It is important to determine the reasons behind increased weed encroachment into the lawn before reaching for an herbicide to kill the weeds. Killing the weeds without correcting the problem only invites continued and often increased weed problems. Sustainable lawn care practices revolve around good cultural practices that promote plant health. This helps create a vigorous lawn successfully able to prevent any serious weed problems from becoming established.

 In addition to having a working knowledge about sustainable lawn care practices and how they relate to weed management, it is good to know something about the weeds that may potentially invade a lawn. This becomes important when determining whether or not control measures are needed and when they need to be carried out. It is also important in determining the most appropriate herbicide product to use, should one be needed. Following is general information about the growth and habits of potential lawn weeds.

 Weed Identification and Characteristics

Lawn weeds may be conveniently divided into two classes based on the way in which they emerge from the seed. Monocots emerge with a single seed leaf whereas dicots emerge with two seed leaves. Most monocot weeds found in turf are termed weedy grasses. Examples include crabgrass, annual bluegrass, tall fescue, and quackgrass. Dicots, on the other hand, are termed broadleaf weeds and include such plants as dandelion, clover, ground ivy (creeping Charlie), knotweed, and plantain.

Grassy weeds and broadleaf weeds are further divided into groups according to how long they live. Perennial weeds have a life of more than two years, though new seeds may be produced every year. Biennial weeds have a life span of two years, generally storing up food reserves in the leaves and roots the first year and producing seed the second. Annual weeds germinate from seed, grow, flower, and produce seed in less than one year. Summer annuals germinate in the spring and mature in the fall, whereas winter annuals germinate in fall or late winter and mature in late spring.

Effective control of weeds in turf is based on correct identification. Many books and charts are available to help identify common lawn weeds. For additional help in weed identification you can contact your local county extension office.

If the lawn care practices of mowing, watering, fertilizing and herbicide application have failed to reduce weed populations to less than 40 percent of the lawn surface, renovation may be the answer to restoring a healthy lawn. When starting over, the old lawn must be removed or destroyed. This can be done by scraping off the lawn with a sod cutter or rototilling the lawn to develop a good seed bed. If you have perennial weeds such as quackgrass, the lawn should be completely sprayed with a nonselective herbicide such as glyphosate. This will kill all existing plants to give you a fresh start.

The most effective method of controlling lawn weeds is to maintain a dense and vigorously growing turf cover. Weeds are often an indication of problems in the grass plant environment. Killing the weeds without correcting the underlying problem will lead to unsatisfactory results. For example, a problem with knotweed is usually an indication of severe soil compaction. Control of knotweed without correction of the soil compaction will only lead to sparse grass cover until the area is again invaded by weeds that grow in compacted soil.

Often, turf weeds can be controlled simply by altering the cultural practices to favor the grass plants rather than the weeds. Cultural controls may include raising (or lowering) the mowing height, changing the frequency of mowing, lengthening (or shortening) the period between irrigations, increasing (or decreasing) the application of fertilizer, or aerifying the soil.

Hand weeding or pulling weeds is also an effective way to eradicate weeds from small lawn areas. The best time to pull weeds is after a good rain or thorough irrigation. This control is very economical (but labor intensive). It avoids needlessly applying herbicides over the whole lawn for only a few weeds. There are several tools on the market that will aid in hand pulling weeds.

A combination of proper cultural practices plus prudent use of herbicides is sometimes necessary to control weeds effectively in turf. When herbicides are used, container labels should ALWAYS be read and carefully followed.

The first step in using any pesticide product responsibly, including herbicides, is to follow the label directions exactly as stated on the product container. The label provides necessary information regarding proper product application and container disposal procedures. Labels are legal documents which are enforceable by law should the product be used in a manner inconsistent with directions.

Weeds should be identified to determine whether there is a need for an herbicide. You may find that only cultural practices need to be corrected. As with fertilizers, extreme care must be taken to prevent the direct application of herbicides into surface water areas.

Herbicides are subject to a number of different fates once they are applied to a turfgrass area. Some of them may be broken down by sunlight, the action of soil microbes, chemical action, or a combination of these processes. Others may evaporate back to the atmosphere, be absorbed by the plants, or stick tightly to soil particles limiting further soil movement. Herbicides may be subject to several of these different processes but some may be more important than others. Break down processes and plant absorption are particularly important as they account for removal of pesticides from the environment.

Types of Herbicides

Preemergence herbicides affect germinating seeds. To be effective apply the herbicide two weeks before you expect weed seeds to germinate. Consequently, preemergence herbicides are most effective against annual weeds. To control summer annuals such as crabgrass, apply preemergence herbicides between May 1 and May 15 in a typical year or once soil temperatures are consistently in the 55-60^o F range.

Postemergence herbicides are used to kill weeds after the weed plants are up and growing. To be effective, most postemergence herbicides must be absorbed through the leaves; consequently, liquid sprays generally work better than dry, granular materials. Postemergence herbicides are most effective if applied when weeds are young and growing vigorously, or in the fall when perennial weeds are storing food in their roots for the next year.

Selective postemergence herbicides are usually used to control annual, biennial, and perennial broad-leaved weeds because they will kill many broadleaf plants without damaging grass plants. These herbicides can severely damage or kill trees, shrubs, flowers, and vegetables; thus, they must be used with great care around these plants. When applying 2,4-D, a common postemergence herbicide, only the amine formulations should be used. Fumes from the ester formulations are more likely to injure ornamentals and garden plants.

Postemergence herbicides may be applied any time the weeds are actively growing, but most effective control is obtained when applied in early fall (August 15 - October 15) or in spring (late April - early June). Fall is preferred over spring due to the food storage process of plants and the reduced possibility of damage to flowers and other vegetation. Postemergence herbicides should be applied when the air temperature is 60-80^o degrees F, when there are no winds, and when there is no rain in the forecast for 48 hours. For some weeds, repeated applications at 10 to 20 day intervals may be required for control.

In the case of postemergent broadleaf herbicides, it is usually unnecessary to thoroughly drench an area with the herbicide solution to achieve satisfactory weed control. This may be wasteful of both water and herbicides as well as move the herbicide beyond the plants and into the soil where it may be more prone to leaching or affect non-target plants. Spraying only to wet the foliage is usually sufficient to be effective.

Where only a few scattered weeds exist, or where weeds are confined to small areas, hand removing or "spot" treating with an appropriate herbicide may be the most appropriate control measure. Application of a preemergent herbicide may only need to be done in those areas where crabgrass was found last year. These might include areas bordering sidewalks, driveways or curbs. Crabgrass may not be a problem in the remaining lawn area where it is cooler and the other lawn grasses are more competitive.

It is also important to remember that an occasional weed is not uncommon in lawns. Hand removal and tolerance of a few "weedy" plants, while maintaining an otherwise healthy lawn can significantly reduce weed control inputs.

Nonselective postemergence herbicides kill most green plants, both desirable and undesirable. These herbicides are used to control perennial grass weeds that are not affected by selective herbicides. Spot treat infested areas using only enough product to wet the foliage. It is not necessary to thoroughly drench the area to achieve satisfactory control.

The following herbicide summary lists several common lawn weeds and their recommended chemical controls. The chemicals are listed by their common names which can be found on product labels under active ingredients.

 Summary of Chemical Weed Control Methods for Lawns and Turf

Group 1-Dandelion, Plantain, Thistle (Perennial)

These weeds can be controlled with 2,4-D. The best times to apply are late April-early June or mid September or early October when broadleaf weeds have emerged and are actively growing. Both spring and fall applications are necessary to control some of the weeds. Apply when soil is moist, temperature is 60-80^o F, there is no wind, and weeds are growing vigorously. Don't forget, these plants may be hand-pulled or dug with a weed digger after a heavy rain or watering.

Group 2 - (Annual-A or Perennial - P) Black Medic (A and P), Chickweed (A), Clover (P), Ground Ivy or Creeping Charlie (P), Henbit (winter annual), Knotweed (A), Mallow (A or P), Spurge (A), Yarrow (P)

These weeds are best controlled by mixtures containing the phenoxy type herbicides 2,4-D and MCPP plus dicamba. Repeated applications are frequently required. Early fall is the best time to apply for winter annuals and perennial (P) weeds. Spring is the best time for summer annuals (A). Apply when soil is moist and weeds are young and actively growing.

Dicamba is potentially dangerous to trees and shrubs because it can move in the soil and be taken up by tree and shrub roots. Therefore, use extreme caution when applying any herbicide mixtures containing dicamba near root systems of trees, shrubs and other landscape plantings. Also, avoid application where mulches have been used around trees and shrubs. The tiny feeder roots will be actively growing into the mulch and can readily take up dicamba and/or be directly killed by the herbicide. Compacted soils, where there is extensive shallow root growth of trees as well as grass is another area where unintended injury from the use of dicamba can result to trees, shrubs, and other landscape plantings.

Use preemergent herbicides such as benefin (Balan), benefin + trifluralin (Team), bensulide (Betasan), pendamethalin, prodiamine (Barricade), dithiopyr (Dimension) or siduron (Tupersan). Apply two weeks prior to expected germination of weed seeds. Some of these herbicides may injure fine fescues or bentgrasses. Reseeding can follow siduron application immediately but cannot follow use of other preemergents for six to eight 8 weeks or longer, depending on the chemical used and soil type.

Post-emergent herbicides for annual grasses are usually not recommended because they're only effective when plants are very small. Typically these weeds are too large for good control by the time they're obvious in the lawn. If there are only a few grassy annual weeds they may be pulled when the soil is moist. Dithiopyr also has some post emergence activity against very small, newly emerged, grass seedlings. This can provide some additional control for plants that may have already emerged just before a pre-emergent herbicide was applied.

These weeds can be controlled only with non-selective herbicides such as glyphosate (Kleenup, Roundup and other brands). Spot treat only because this herbicide also will kill desirable grass plants. Apply when weeds are actively growing. Temperatures should range from the mid-50s through the mid-80s.

To minimize adverse environmental impacts while effectively eliminating weeds, proper timing of herbicide application is crucial. When weeds are quite large and mature, greater amounts of herbicide are usually needed and may not be effective. For example, it is relatively easy to control small, first-year dandelion rosettes with a minimal amount of herbicide. Larger, older dandelion plants have a greater capability to outgrow, and hence recover, from small doses of herbicides. Where appropriate, small dandelion rosettes can be effectively hand removed and eliminated if most of the root system is removed.

Fall is the best time to control perennial broadleaf weeds. At this time of year (mid-September through mid-October), these plants are storing carbohydrates for winter, actively growing and will readily take up the herbicide. Often, a one-time, relatively low rate of application of an appropriate herbicide will be effective. Since much of the other landscape plant material is either going dormant for the winter or has been removed from the garden and flower beds, there is less chance for off-target plant injury. However, that does not give one license to be careless.

Preemergent herbicides, typically used for controlling crabgrass and other annual weedy plants, must be watered into the soil to be effective. They affect the seed as it begins to germinate, before the plant emerges from the ground. Depending on the soil type, 1/4 to 1/2 inch of moisture should be applied following application of these products. This not only puts the product where it will be the most effective, but moves the material far enough into the soil so it will not be carried away in runoff. Do not water for several days after post emergent applications. It will wash the herbicide off the weeds and into the soil usually reducing its effectiveness.

In closing, remember after weed problems have been reduced, proper lawn care practices will encourage vigorous, healthy grass plants, making future herbicide applications minimal or even unnecessary.