Weed Control Services (209) 204-5512
Weeds often invade turfgrass that is over- or under-watered, improperly fertilized, improperly mowed, or highly compacted. Lawns that have been weakened by plant pathogens or insect pests are also likely to become weedy because there is more open space for a weed to establish.
An integrated weed management program can reduce most weed populations to tolerable levels and prevent large, unsightly weed patches. Total eradication of weeds is not a realistic or necessary goal for most lawns and park turfgrass; however, with good management practices a lawn can be practically weed-free without the extensive use of chemicals.
Weed Management in Established Lawns
- Irrigation Many lawns are watered incorrectly. Poor irrigation practices can weaken turfgrass growth, allowing weeds to invade. Annual bluegrass, crabgrass, dallisgrass, and nutsedge are just a few weed species that thrive in poorly irrigated lawns. To maintain a healthy lawn, uniform coverage is needed. Sprinkler heads that are broken, obstructed, or set too low or too high may not reach all areas of the lawn and can result in dry or dead spots in an otherwise healthy turfgrass. In general, deep, infrequent irrigation will encourage healthy root growth. Light, frequent watering is only required when the turfgrass has just been planted and the root system is developing. Watering established turfgrass lightly and frequently creates a shallow-rooted lawn, making it less durable and allowing shallow-rooted weeds such as crabgrass to get the competitive edge. Ideally, turfgrass should be irrigated deep enough to penetrate the soil to a depth of 6 to 8 inches. Allow the soil to partially dry out between waterings. The top 1 to 2 inches of soil should be fairly dry before water is applied again. Water requirements vary among turfgrass species and also vary depending on climate, time of year, and growing conditions. As a general rule, warm-season grasses only need to be watered once or twice a week. Cool-season grasses require more frequent watering, up to three times a week in hot weather. Turfgrass species growing in shade require less water than the same species growing in full sun. Turfgrass growing on clay soils does not need to be watered as frequently as turfgrass growing on sandy soils. Clay soils retain water longer than sandy soils; sandy soils dry out quickly. For specific irrigation requirements for warm- and cool-season turfgrasses in various regions of California and at different sprinkler outputs, see Lawn Watering Guide for California listed in References.
- Mowing Each turfgrass species has specific mowing height requirements. Mowing some grasses too short can weaken the turfgrass and increase weed invasions. Alternatively, if some grasses are not mowed short enough, the thatch layer can build up, reducing water penetration and weakening the turfgrass. Mow grasses more frequently when they are actively growing. A standard guide is to remove no more than one-third of the leaf blade at each mowing. If too much is removed at one time, it can take some time for the grass to recover, giving weeds a chance to invade. Lawns with weed invasions often appear uneven. Mow weedy lawns frequently enough to avoid this patchy appearance and prevent flower and weed seed formation. Be sure that mower blades are sharp enough so that the turfgrass is not damaged. If mowers are moved from weedy lawns to other lawns, be sure to wash off the blades to avoid transport of weed seeds and propagules. Avoid mowing lawns when the soil is wet, such as after rain or irrigation; moving a mower over wet soil can lead to compaction.
- Fertilizing To maintain a healthy lawn, follow fertilizing guidelines carefully. Begin a regular fertilization program approximately 6 weeks after planting. In general, lawns need to be fertilized about four times a year with no more than 1 pound of actual nitrogen per 1,000 square feet per application.
- Thatch Regular thatch removal will help keep your turfgrass healthy and competitive with weeds. Thatch is a layer of organic matter (stems, stolons, roots) that develops between the turfgrass blades and the soil surface. A thin layer of thatch is normal and even beneficial; it can help limit weed germination. Some turfgrass species, particularly creeping grasses, develop thick thatch layers that can prevent the circulation of air, water, and nutrients in the soil. Generally, you should dethatch your lawn when the thatch layer is more than 1/2 inch thick. For some species, such as zoysiagrass, bermudagrass, or Kentucky bluegrass, this may mean dethatching every year. For other species, such as tall fescue, dethatching may only be needed every 5 years or not at all. Use a thatching rake on small lawns to loosen the layer of thatch. On creeping-type grass lawns, such as bermudagrass, use a verticutter, or dethatcher, to cut through the lawn to the soil surface.
- Aeration Heavy traffic can compact soil over time. Soil compaction restricts the flow of oxygen, water, and nutrients into the roots, causing the turfgrass to grow slowly and making it more susceptible to weed invasions. Alleviate soil compaction with regular aeration. Lawns on heavy clay soils or lawns with heavy foot or equipment traffic may need to be aerated several times a year while lawns with little activity may only need to be aerated once a year or less. Aerators, sometimes called aerifiers, remove small cores of soil or create pores or channels in the rooting zone. A hand-held aerifier may be adequate on small lawns. For larger lawns, machine-driven aerifiers are more practical and can be gotten from equipment rental businesses. Aerate when the turfgrass is actively growing.
- Hand-weeding Controlling occasional weeds by hand-pulling may be all that is necessary if you practice regular and proper maintenance procedures. Hand-weeding is particularly important for preventing infestations of creeping woodsorrel, nutsedge, dandelion, spurge, dallisgrass, and bermudagrass. Remove weeds while they are still young and before they set seed or produce rhizomes or tubers. Remove small patches before they get large. Making this a regular habit will greatly reduce the number of weeds in your lawn. Be sure to remove the entire weed, including the root. A dandelion fork, or fishtail weeder, is useful for removing weeds with a thick taproot.
- Herbicides If your lawn is properly maintained, herbicides will generally not be necessary. When they are needed, use them as part of an integrated management program that includes good cultural practices. No single herbicide will control all lawn weeds, and not all herbicides can be used on all lawn species. You must identify your weed problem(s) and turfgrass species before choosing an herbicide. A few of the most serious lawn weeds, such as some perennial grasses, cannot be effectively controlled with herbicides without killing the turfgrass as well. Herbicides are classified in several ways: preemergent or postemergent, contact or systemic, selective or nonselective. Preemergent herbicides are applied before weeds emerge from the soil; they kill weed seedlings as they germinate and try to emerge. In lawns they are primarily used against annual grass weeds such as annual bluegrass and crabgrass, but there are also preemergent herbicides that are effective against many broadleaf weeds. Postemergent herbicides are applied after weeds have emerged from the soil; they control actively growing weeds. Postemergent herbicides may have either contact or systemic activity. Contact herbicides cause localized injury where the chemical comes in contact with the plant. In contrast, systemic herbicides move within the plant causing injury at additional sites in the plant and can control older weeds. Examples include glyphosate (Roundup), triclopyr (Turflon), or 2,4-D/dicamba/mecoprop mixtures. Selective herbicides kill target weeds without damaging desirable turfgrass species. They are toxic to only certain plants or weeds. For example, 2,4-D selectively kills only broadleaf plants and not grasses, and pendimethalin controls crabgrass as it germinates but does not injure established turfgrass. Nonselective herbicides kill all or most vegetation including turfgrass; use them only prior to planting a lawn, during renovation, or as spot treatments.