Plan now for season-long weed control in asparagus

Cool, wet spring may increase weed control challenges in asparagus.

Weeds are emerging in asparagus fields even if asparagus is delayed by cool weather. Asparagus growers have an opportunity to prepare and implement a season-long weed control program to kill emerged weeds and suppress weed germination. Many persistent weeds require several herbicide applications to maintain control throughout the asparagus growing season (April through September).

Winter annual and perennial weeds have emerged and are growing well. Quackgrass, annual bluegrass, yellow rocket, dandelion, horseweed (marestail), mouseear cress, henbit, purple deadnettle, white campion and wild carrot are common early-season weeds. These weeds should be killed with foliar-active post-emergence herbicides either now or when applying the pre-emergence herbicides. Glyphosate (Roundup), paraquat (Gramoxone), 2,4-D (Formula 40) and Fusilade are potential choices for killing emerged weeds early in the season.

Quackgrass is not killed easily with glyphosate any time of the year. In early spring, quackgrass is suppressed by glyphosate application but reemerges soon. If quackgrass is the primary spring weed, consider using a high rate of Fusilade DX (1.5 pints per acre) plus crop oil concentrate (COC). A nitrogen (N) source, such as 28 percent liquid N, may enhance fluazifop activity. Reapply Fusilade 14 days later. A combination of glyphosate and Fusilade may improve quackgrass control.

Many overwintering annual broadleaves, winter annuals and seedling perennial broadleaf weeds will be killed with glyphosate applied in the spring. If only very small seedlings have emerged, paraquat should kill most of them. If the field is known to have glyphosate or PS II-resistant broadleaves (e.g., horseweed, pigweeds, lambsquarters), and those species have emerged, consider adding 2,4-D to the glyphosate. If horseweed is the primary broadleaf weed present, include 5-10 fluid ounces of clopyralid (Spur) in the mix. Clopyralid is very active against all composites, such as horseweed, ragweed, dandelion, mayweed and skeletonweed.

Pre-emergence herbicides should be rotated for spring and summer application and year to year. Diuron (Karmex) is a good photosynthesis (PS II) inhibitor with a broad range of activity. Because of the presence of PS II-resistant Powell amaranth (and other resistant pigweeds) in most Michigan asparagus areas, always add another pre-emergence herbicide with a mode of action different from diuron. Good choices are Solicam, Spartan, Dual Magnum, Prowl H2O or Callisto. During the growing season, note the weeds that emerge and survive. For the residual application after harvest, select two herbicides with modes of action different from those used in the spring.

During harvest, growers may apply halosulfuron (Sandea), linuron (Lorox), 2,4-D (Formula 40), dicamba (Clarity) or clopyralid (Spur) to control broadleaves, and fluazifop (Fusilade), sethoxydim (Poast) or clethodim (Select Max) to control grasses. With correct use of these herbicides, few weeds should survive. Check labels for pre-harvest intervals for all herbicides.

Growers have many options for weed control in asparagus. With knowledge of weeds present and herbicides registered for use in asparagus, growers should be able to control most weeds in asparagus throughout the growing season.

Current vegetable herbicide recommendations are available in Michigan State University Extension Bulletin E0433, “Weed Control Guide for Vegetable Crops.”

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