Are Weeds Popping UP in Your Lawn?
Michelle Le Strange , UC Master Gardener Advisor

Weeds occur in every lawn, but they seldom become problems in well-managed, vigorously growing turfgrass. Proper site preparation and turfgrass selection before planting are essential to give a new lawn a healthy start. Once a lawn is established, poor maintenance practices that weaken it-such as improper irrigation, fertilization, or mowing-are the primary factors likely to predispose it to weed invasion. Activities that lead to compaction also contribute significantly to turfgrass stress, making it easier for weeds to invade.

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; however, with good management practices a lawn can be practically weed-free without the extensive use of chemicals.

Weed Identification: Identifying weeds and knowing their life cycles are essential to management. Three general categories of weeds may be found in lawns: broadleaves, grasses, and sedges. Take care to distinguish weedy grasses from similar-looking lawn grass species.
The life cycle of weeds may be annual, biennial, or perennial. Annual weeds are most commonly identified as either winter/cool-season or summer/warm-season and survive only one season. If not controlled before they flower, they can produce seed that will sprout the following year. Biennial weeds survive for two growing seasons, reproducing vegetatively or by seed; however, seed is not produced until the second year. Perennial weeds survive many years, and though some may produce seed, many primarily reproduce vegetatively by creeping stems (stolons and rhizomes), tubers, or fleshy roots. Perennial weeds are the hardest to control once established.

Weed Invasion: 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. Most weed invasions can be prevented with good lawn care.

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.

The herbicide you choose will depend on the types of weeds you are trying to control and the tolerance of your turfgrass species.

Herbicide Success Tips. Lawn and garden companies market their own brand names of herbicides. Shop for herbicides by looking for the common name or active ingredient that appears on the label in small print under the title "Ingredients." Unlike brand names, common names for active ingredients do not change from company to company. Different products will vary in the percentage of active ingredient they contain. Some products are formulated as ready-to-use to allow for the convenience of no mixing; others are formulated as granules; and many others as higher concentration liquid sprays that require mixing. Follow all label directions carefully and only apply herbicides at the time of year and at the rates recommended.

February 26, 2004