Crabgrass is the most common member of the annual grassy weed family, and one the main threats we face in caring for fine turf. Left untreated, it can grow and spread rapidly, competing with your turf for nutrients, sunlight, etc. Being an annual plant, it has one complete life cycle per growing season, similar to annual flowers like petunias, geraniums, etc.. Crabgrass is a prolific seed producer, with each plant capable of producing 10ís of thousands of seeds. Crabgrass seed left over from last seasonís plants can begin to germinate once soil temperatures reach 55 degrees F consistently, usually by mid- May. The plants continue to grow and mature through the early and mid-summer, peaking from mid- July to mid-August., especially in open, sunny lawn areas where turf cover is thin and is exacerbated by high temperatures and dry soil. As crabgrass plants reach maturity, they begin to produce seed spikes at the tips of the leaves. Crabgrass in general stops germinating in mid-August, and mature plants gradually turn a reddish-purple color as they begin to slowly die off and fade, especially after killing frosts.
Pre-emergent weed control products can effectively reduce crabgrass by preventing their seed from germinating, but they will not eliminate it entirely. Properly applied, these products can last up to four months. There are factors that can limit the effectiveness of pre-emergent control materials including weather conditions, soil temperature and moisture content, turf density and the amount of weed seed available in the soil.
To increase the effectiveness of the pre-emergent, rainfall or irrigation should occur within 20 days of application. This helps the pre-emergent to adsorb (or bond) to the surface of the soil particles that creates the barrier to help prevent crabgrass seeds from germinating. In the spring, natural rainfall is usually plentiful, but supplemental irrigation might be needed if there is an early extended dry period.
Once the product is in place, steps should be taken to minimize anything that might disturb the soil surface. Surface raking, mowing or sweeping the turf is fine, but if you de-thatch your lawn using a vertical mower, you risk effecting the level of control down the road. Many landscapers have switched to a spring-loaded tine rakes that attach to the front of their mower decks, which effectively break up snow mold and dormant top growth without disturbing the soil.
During July and August, warm and/or dry soil can contribute to pre-emergent control breaking down prematurely. This is why crabgrass can appear along drives, sidewalks and any thin/ bare areas of your lawn. To help minimize this, keep mowing height at 3-3Ĺ ď and water deeply every 2-3 days between rainfalls. This keeps soil cooler and helps reduce the chance of break- through, not to mention it is better for your lawn.
Post-emergent treatments are available to control crabgrass break through, but are only effective on actively growing weeds and cannot be applied to brown (dormant) turf caused by drought and/or heat stress.
Feel free to contact us if you have any questions or would like to learn more about crabgrass.
Dead crabgrass plants from previous year, with emerging new