Vegetable weed control during cool growing conditions

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

The cool, dry growing conditions have complicated weed control programs in many crops. Preemergence herbicides do not work well when there has been little moisture after application. Consequently, weeds germinate and emerge when the crop is still too small for postemergence applications. In addition, small grain cover crops are tough and do not respond to graminicide applications. As soon as growing conditions improve, the weeds and cover crops will grow rapidly, but growers will have to wait to apply postemergence herbicides to avoid crop injury. Maximum weed control is important to obtain maximum yields, but stand thinning or severe crop injury from herbicides can reduce yields.

As a general rule, be conservative in herbicide applications during cool weather. If irrigation is available, apply one-quarter to a half inch of water within two to three days of herbicide application to activate and incorporate herbicides into the soil moisture zone. If weeds emerge before crop plants have grown to size required on herbicide labels, it is wise to wait until the crop is larger and able to withstand the herbicides, then apply a higher rate (if labeled) of the postemergence herbicides.

With small grain companion crops, it is important to kill them before they compete for moisture with the crop plants. During cool weather, increase the rate of the graminicides to improve kill of the cover crops. If the graminicides are applied alone, include crop oil concentrate (COC) or nonionic surfactant (NIS) in the mix. If other postemergence herbicides are included, it may be wise to avoid the adjuvant to reduce chances for crop injury.

Most crops continue to produce roots during cool weather, and as soon as it warms up, the foliage will grow rapidly. Be ready with your choice of herbicides to spray as soon as the weather improves.  When air temperature exceeds 70ºF for several days, most crops become very tolerant to labeled herbicides. If you are in doubt about a situation, please contact your county extension educator or me direct.

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