TechNotes Tips - Weeds Archive

Nimblewill, a perennial warm season weed, becomes a concern of many homeowners this time of year because it goes off color with the cool temperatures (thus becoming more conspicuous). There are no selective chemicals for the removal of this weed.

Although not commonly thought of by homeowners, one of the best times to control broadleaf weeds is during the autumn.

Yellow nutsedge sprouts in many homelawns and ornamental beds. This weed occurs in areas of thin turf where it is normally hot and dry. Yellow nutsedge is easily identified by its triangular stem and light yellow color. It is a perennial that reproduces by seed, rhizomes, and small tubers. These tubers, called nutlets, are a primary source for yearly infestations. These nutlets can survive in the soil for several years assuring the regeneration of plants from year to year. Yellow nutsedge is found throughout the northern U.S. In the southern U.S. purple nutsedge (similar to yellow nutsedge) is an additional nutsedge species to deal with.

Crabgrass generally reaches full maturity near the beginning of September. The plants are in multiple tiller stage and have "spread-out" causing an unsightly situation. Postemergent control of crabgrass at this time of year is often a difficult decision. With the arrival of cooler nights and frost conditions, crabgrass will die. Thus, if the first frost is almost upon you, treating for crabgrass may not be needed. However, if the first frost is still a long ways off, treatment may be necessary.

Crabgrass often germinates in mid to late May in open areas and sidewalk cracks throughout the Midwest. As soil temperatures increase, crabgrass will begin to germinate in turf areas.

As of mid-February, 2000, Crabgrass season was rapidly approaching and probably had already started for some areas of the southern U.S. Crabgrass emergence is oftentimes associated with phenological events like 1.) flower withering of Forsythia or 2.) the initiation of flowering by dogwoods. Other emergence keys are based on temperatures. Below are some key soil temperatures (1.5 to 2 inches) associated with smooth crabgrass emergence (based on reported research: Fidanza et al. 1996. Crop Science 36:990-996).

• Average minimum soil temperature at which initial appearance is observed = 54 F
• Average mean soil temperature at which initial appearance is observed = 60 to 64 F
• Average minimum soil temperature at which major emergence occurs = 68 to 70 F
• Average mean soil temperature at which major emergence occurs = 73 to 75 F

Quackgrass is a major weed contaminate in many home lawns. It is often identified by its dull green color (coarse leaf blade), strong rhizomatous growth habit and clasping auricles. Quackgrass is a perennial weed that is difficult to control. It’s rhizomatous growth habit allows it to spread quickly once it is in a home lawn. Control of this weed, short of nonselective removal, is difficult. The best control is to inspect the site before turf installation. Also if the lawn is to be sodded, check the sod for any quackgrass contamination before installing.

Henbit and chickweed are two winter annual weeds that continue to thrive in lawns throughout the northern U.S. Henbit is an upright plant that produces a characteristic small purple flower. Chickweed is a low growing weed that forms a dense mat. The leaves are bright green with a white flower. These weeds are quite obvious this time of year in thin areas of the lawn.

Winter annuals like henbit and chickweed are present in Kentucky bluegrass lawns and other turfs that are generally thin. Although these winter annuals are disruptive to the aesthetics of the turf, they soon die with the arrival of summer temperatures.

Dandelions are in full bloom through the transition zone and are just about ready to bloom through most of the northern U.S. Dandelions are best controlled in the fall with herbicides with spring treatments being second. If treating for dandelions this spring, control is best achieved after the dandelions have bloomed. Herbicide applications early in spring before bloom are not as effective. Maintaining a dense turf through proper fertilization, and mowing at the proper height of cut can reduce dandelion populations over time. Thin low-density turfs mowed at a low height of cut encourages large dandelion populations.

Kudzu is a major weed in the southeastern U.S. This weed is a vine that overgrows trees throughout the region. It is estimated that more than 7 million acres are infested with this weed. United States Department of Agriculture (USDA) researchers have found a fungus that is highly effective for controlling kudzu. In greenhouse and field studies control has been 100 percent. The fungus is Myrothecium verrucaria.

Broadleaf weed control is still advisable during fall. Autumn is the best time to control broadleaf weeds. As long as the weeds are actively growing, the herbicide can be absorbed and translocated in the plant.

Autumn is an excellent time to control broadleaf weeds in lawns. Why wait until spring when you can get a jump on them early.

Bur reed (Sparganium erectum), a noxious weed banned by the U.S. government, has been found in 35 states after Home Depot received it in a shipment of exotic reeds from Holland and sold it as a pond plant. The plant can choke waterways and interfere with recreation in shallow waters. Federal agriculture inspectors have scrambled to recover most of the plants from gardeners over the summer but fear the plant may be impossible to trace because some people may have paid cash for the reeds. The bur reed can grow to 6 feet producing a small yellow flower that contains a bur-like fruit.