Weed control is important for profitable asparagus production

Herbicides with different modes of action are registered for asparagus.

Asparagus harvest season will be upon us shortly, and growers should be applying herbicides to prepare for the harvest season. Weed-free fields result in more vigorous asparagus growth, easier harvest, and less habitat for insect pests. Recent herbicide registrations for asparagus give growers additional choices for preemergence and post-emergence weed control. Using several herbicides with different modes of action will improve overall weed control and reduce potential for weeds to develop resistance to one or more herbicide modes of action.

The photosystem (PS) II inhibitors Karmex, Sinbar, Sencor and Lorox have been used widely in Michigan asparagus production. These are excellent, inexpensive herbicides that provide good control of most annual and some perennial weeds for six to eight weeks. However, repeated use of PS II inhibitors has resulted in development of herbicide resistance in pigweed and amaranth species as well as other weeds. These herbicides still control the majority of target weeds, so they can be used successfully in asparagus as part of a rotational weed control plan.

Although they all are PSII inhibitors, they represent three different chemical families. Each group has a slightly different binding site in the photosynthetic system. Therefore, even within this group of herbicides, there should be annual rotation of the herbicides. Sinbar (terbacil) is a uracil, Karmex (diuron) and Lorox (linuron) are ureas, and Sencor or Tricor (metribuzin) is a triazinone, closely related to the triazines. Only one of these herbicides should be used in any application and no more than two of them should be used in a year preemergence and after harvest. In addition, Sinbar and Sencor are very soluble and excess use may result in movement of the herbicide down into the asparagus root zone, which may weaken the plants and cause early field decline. This is a greater potential problem on sandy soil with less than 1 percent organic matter.

Chateau and Spartan are known as PPO inhibitors which interfere with cell membrane structure. Herbicides with this mode of action are active at low to moderate rates and have both preemergence and post-emergence activity against weeds. They have broad weed control spectrums including many grasses and broadleaves. Chateau is a good tank-mix partner with a PS II inhibitor because it controls many of the same weeds, but with a different mode of action. It should be used only once in two years to avoid asparagus suppression. Spartan does not last as long as Chateau, so it can be used more frequently without fear of asparagus injury. It is very active against pigweeds and amaranths, so it also is a good tank-mix partner with PS II inhibitors.

Sandea is an ALS inhibitor that inhibits amino acid synthesis. The ALS inhibitors are very active at very low rates of active ingredient per acre. Their great activity has resulted in development of resistance in many weed species to multiple ALS herbicides. Sandea is active against yellow nutsedge, pigweeds and many other broadleaves. It has minimal grass activity and is weak against common lambsquarters and eastern black nightshade. It can be used preemergence, but probably is most effective post-emergence to kill nutsedge or pigweeds that emerge during harvest. It can be applied safely in the crop with a one-day preharvest interval.

Callisto and Solicam are pigment inhibitors that cause injured plants to turn white and then expire. Callisto (HPPD inhibitor) is used at very low rates to suppress most broadleaves. It also is active against large crabgrass. Callisto may cause some spears to bleach, but the effect disappears quickly and the weed control lasts through the season. Solicam is unrelated chemically to Callisto, but has a similar mode of action in plants. Asparagus is very tolerant of Solicam. Both of these herbicides are good tank-mix partners with the PS II inhibitors and the seedling inhibitors Prowl H2O and Dual Magnum.

Prowl H2O is a dinitroanaline that inhibits microtubule formation in mitosis (cell multiplication). It is an excellent preemergence grass herbicide and also gives good control of common lambsquarters and several other broadleaves. It is weak against mustards and composite weeds. Dual Magnum is a chloroacetamide that inhibits long-chain fatty acid production in new shoots of emerging seedlings. It also is a good preemergence grass herbicide. Prowl H2O and Dual Magnum have a shorter residual life than many of the other asparagus herbicides, but should provide four to six weeks of grass control. They may be good choices for application after harvest for growers who want to establish a rye cover crop in September.

With many labeled herbicides, asparagus growers have the tools to maintain fields almost weed-free. Perennial weeds continue to be problems and must be treated with post-emergence herbicides for some level of control. Please see Michigan State University Extension Bulletin E-433, “Weed Control Guide for Vegetable Crops,” for specific rates and recommendations for all asparagus herbicides.

Dr. Zandstra’s work is funded in part by MSU’s AgBioResearch.

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