Peter H. Dernoeden, Ph.D.
University of Maryland Extension Turfgrass Specialist

Hot and dry summers are not uncommon to Maryland, but 2010 was one of the toughest in recent years. Summer-like conditions began in spring with record breaking high temperatures in April, which continued almost unabated to the current period. Most of Maryland experienced a hot and dry period during the latter part of June, which resulted in many lawns turning brown and becoming drought dormant. Crabgrass allowed to go unchecked for even one or two summers can dominate and push turf out of the stand thus degrading lawn function and aesthetic quality.

Crabgrass is probably the most common and invasive weed in Maryland lawns. Crabgrass is an annual plant that emerges from seed in April, but the highest populations usually emerge in May and June. Normally hot and dry soils in July and August do not allow for much additional emergence of crabgrass, even in occasionally irrigated sites. Crabgrass plants die following a few autumn frosts leaving brown and sometimes muddy voids in winter. Crabgrass has a special physiology that enables it to grow more rapidly and to become more competitive when air temperatures are high in summer. In College Park, crabgrass seedlings were first observed on 7 April 2010, which was 7 to 10 days earlier than usual. Normally, few crabgrass plants emerge in April and those that do only very slowly mature to produce new shoots called tillers. Crabgrass plants generally go unnoticed until early July, when they become tall enough to be seen in the lawn. Tillering normally begins in late June when air temperatures are routinely in the 80’s F., but this was accelerated in 2010. What really made crabgrass a much greater problem than usual was the June drought that turned non-irrigated lawns brown or at least stopped them growing and resulted in some thinning. This lack of turfgrass vigor, and ability to compete with the crabgrass, was a large part of the problem. The thunderstorms rolled through the central counties and stimulated more crabgrass seed germination in July and August. These thunderstorms were accompanied by very hot days, which stimulated both emergence and growth of weeds. Environmentally stressed –out lawns are not able to compete with the more heat-loving crabgrass. The result was major crabgrass breakthrough as well as other weeds including yellow foxtail, yellow nutsedge, spotted spurge, yellow wood sorrel and many others. Homeowners who had taken the precaution of managing crabgrass with a spring applied herbicide, or who have a lawn service that provides for pest control, probably are seeing much more crabgrass than in the past.

The best type of herbicide for controlling crabgrass is called a preemergence herbicide since it is applied before crabgrass seed germinate. These herbicides are applied in late March and April (some can be effectively used in early May).There are several chemically different preemergence herbicides that are commercially available. These herbicides do not kill seed in soil. Instead, the germinating crabgrass plant emerges and the very young roots take-up the herbicide, which prevents root development and the young seedlings die before they can be seen. Prememergence herbicides are degraded in soil by microorganisms. In general, an April applied preemergence herbicide will have broken down to ineffective levels by late July. This is exactly as planned since dissipation by microbes is desired so that overseeding of a lawn (and more importantly sports fields) may be safely performed by mid- August without worry that the herbicide exists to prevent newly planted turfgrass seed from developing properly. When soils become very warm and are kept moist by either irrigation or rain, preemergence herbicides breakdown much more rapidly than normal. The early breakdown of herbicides combined with thunderstorms in July resulted in much more crabgrass than would normally emerge and compete. Hence, by late July or early August it was apparent that a major breakdown of preemergence herbicides had occurred for many. It was environmental stress damage to lawns combined with thunderstorms that also encouraged the many other weed problems, which are now apparent in most Maryland lawns.