Influence of Cultural Practices on Weed Encroachment

Poor turf culture is a major reason for weedy lawns. Any effort to control weeds in turf should start with improving cultural practices. One goal of cultural weed control is to maximize turf density and maintain healthy, disease-free turf for a major part of every year. Some of the most important cultural practices are discussed below.

Mowing Mowing practices have a larger impact on weed invasion in turf than any other cultural practice. Infrequent mowing, where turf is severely scalped, causes root dieback and forces regrowth from axillary buds, which consumes stored carbohydrates and results in thin turf that is slow to recover and less dense. In hot weather, turf may die in irregular patches after severe scalping. Reduced turf density allows weed invasion due to lack of competition. Once weeds invade, they often spread rapidly, since many (rosette types) are relatively unaffected by the infrequent scalping.

Regular mowing (i.e., about weekly) allows turf to achieve maximum density throughout the year. Under these conditions, turf will compete favorably with many common weed species, including common dandelion, Taraxacum officinale.

Proper mowing height is critical to maintaining turf density. In general, mowing below optimum height increases invasion of weedy grasses such as annual bluegrass (Poa annua). Some desirable grasses, such as Kentucky bluegrass (Poa pratensis) and red fescue (Festuca rubra) may not do well if continually mowed low. Acceptable mowing heights for commonly used turfgrasses are below.

Mowing Height Ranges for Northwest Turfgrasses


Optimum Height Range (inches)

Colonial bentgrass

0.5 to 1

Chewings fescue

1 to 2.5

Red fescue

1.5 to 2.5

Hard fescue

1 to 2.5

Tall fescue

1.5 to 3*

Perennial ryegrass

1 to 2.5

Kentucky bluegrass

1.5 to 2.5

*Assumes improved varieties.

Irrigation This ranks with mowing in terms of its impact on weed encroachment. Excess irrigation is a primary reason annual bluegrass invades many lawns. Surface wetness aids seed germination and also shifts the competitive edge toward existing annual bluegrass plants. Proper irrigation means thoroughly wetting the root zone, then allowing soil to dry until desirable grasses begin to wilt. Thus, turf density remains high, and surface conditions do not aid weed seed germination.

Lack of irrigation (e.g., prolonged summer drought) causes turf to go dormant and survive via crowns, rhizomes, and stolons; turf density decreases, which allows weeds to compete freely once fall rains come or irrigation begins. Once weeds are established, they often thrive under this drought cycle because many are exceptionally deep rooted. Lawns allowed to go dormant every summer require more intensive efforts to control weeds chemically than lawns that are irrigated enough to ensure optimum turf density.

Fertilization The primary goal of any long-term fertilization program should be to use the least amount of fertilizer necessary to maintain quality turf. Many home lawns can perform reasonably well with 2 to 4 lb of available nitrogen (N) per 1,000 sq ft of lawn annually, depending on soil type, the area's use, and grass species. Special-use areas require more inputs. Excess N may stimulate invasion of annual bluegrass or undesirable bentgrass species. It also may stimulate diseases such as Microdochium (Fusarium) patch or Drechslera leaf spot, which may severely thin turf. Inadequate N levels may result in gradual loss of turf density and more rust and red thread diseases.

Proper nutrient balance ensures long-term turf vigor. N-P-K balance should be about 4-1-3 or 6-1-4 annually. Too much phosphorus (P) may encourage annual bluegrass and could degrade water quality if it leaches out of the soil; too little may result in increased diseases such as Microdochium patch, and red thread, and could increase erosion of soil. On soils that are not pure sand, a safe rule of thumb is to apply no more than 1 lb P/1,000 sq ft per year if a soil test indicates it is needed. If you are using an organic fertilizer, they usually contain almost equal amounts of N and P, so if your soil has adequate P, to protect water quality, an organic fertilizer is not an appropriate option to use for an N source. East of the Cascades, P usually is adequate. Do not apply P near water sources in late fall or winter, to limit the risk of leaching. Always conduct a soil test before applying nutrients. It is now a best management practice in the state of Washington for homeowners and publicly owned turfgrass areas to conduct a soil test before they are allowed to purchase a turf fertilizer containing phosphorus. This must be done at a minimum of every three years. This is part of a program to try to further protect water quality.

Long-term research on putting-turf demonstrates the importance of adequate levels of sulfur (S) to minimize encroachment of annual bluegrass. Normally, 2 to 3 lb of elemental S/1,000 sq ft of turf per year is adequate, when used over time. The importance of sulfur nutrition on home lawns is still being researched.

Removal of clippings during mowing significantly increases fertilizer requirements. Tests indicate from 25% to 50% of applied N may be removed with clippings. Removing clippings may deplete nutrients in soil and ultimately lead to loss of turf density and increased weed invasion. It is good to return clippings to the site when possible. It can add back enough nitrogen to back off on one of your fertilizer applications if you have been fertilizing appropriately.

Thatch control Thatch is an accumulation of living and dead stems and roots between the soil surface and green vegetation. The accumulation is common and is a leading cause of turf failure. Because roots must penetrate thatch to reach the soil, excess thatch often results in poor drought tolerance and, once weather is dry, may lead to localized dry spots and dead turf. Excess thatch also decreases efficacy of fertilizer applications. Finally, thatchy lawns are often spongy and tend to scalp badly during mowing.

While excess thatch may lead directly to weed invasion, worse weed problems may result from attempts to remove thatch. Mechanical de-thatchers can strip away not only thatch but desirable turf as well. Countless examples exist where weeds severely invaded lawns that were destroyed by incorrect efforts to remove thatch.

The key to proper de-thatching is to remove thatch before it exceeds 0.75 to 1 inch. There is little danger of severely injuring turf in this case since most roots penetrate the thatch and are anchored in the soil. On lawns with heavy thatch (in excess of 1 inch), removal must be gradual over a period of perhaps 2 or 3 years. At any given de-thatching, avoid removing more than 0.5 inch of thatch. Also, if you de-thatch the area more than once, do so at approximately 45° angles to avoid tearing out large chunks of turf. For quickest recovery, remove thatch before a major growth period. In western Oregon and Washington, April may be slightly better and early fall (late August/early September) slightly less desirable. Spring or fall is equally appropriate east of the Cascades.

Caution Information in this handbook is not intended to be a complete guide to herbicide use. Before using any chemical, read the container label recommendations. Before a chemical can be recommended for a specific use, it must be thoroughly tested. Following the recommendation on the manufacturer's label can prevent many problems from arising through the wrong use of a chemical.