Mid-season weed control in vegetable crops

Dry weather makes weeds more tolerant of herbicides.

The warm, dry weather we have enjoyed for the past few weeks has made preemergence and post-emergence weed control in vegetable crops more difficult. While fewer weeds germinate in dry weather, those that do emerge and grow often become almost immune to post-emergence herbicides at normal use rates. Weeds develop thick cuticles and more extensive root systems during dry weather, which allows them to regrow after an initial burndown.

Very tenacious weeds such as ladysthumb smartweed, common purslane, prostrate knotweed and carpetweed may survive herbicide applications that would normally kill them. Common lambsquarters, pigweeds, ragweeds and nightshades also have become much tougher to kill. Even seedling foxtail and large crabgrass have survived application of graminicides.

To improve activity of post-emergence herbicides, use adjuvants as recommended on labels. Nonionic surfactants (NIS) are recommended with many post-emergence herbicides. Generally, petroleum crop oil concentrate (COC) is more active than NIS, and will improve post-emergence activity of many herbicides. However, crop injury may increase with use of additional adjuvant, so improved weed control activity needs to be considered with the potential for crop stunting. Methylated seed oils (MSO) are recommended with some post-emergence herbicides, but normally are not needed under Michigan conditions. MSO is used in drier areas of the country, so it may improve activity of some herbicides under current conditions.

It is always easier to control small weeds than larger weeds. Killing them in the cotyledon or one leaf stage is easier, and they are just as dead as if killed when they are much larger. The effect may not be as apparent and may not give as much satisfaction (seeing weeds curl up and die), but the results will be better. So monitor fields and spray when weeds have emerged.

When applying mid-season preemergence herbicides, try to apply before rain or irrigation. A preemergence herbicide on dry soil eventually volatizes or breaks down from solar radiation. If the herbicides can be watered in, they will suppress most weeds that would germinate in mid-summer. Herbicides applied in June will help maintain clean fields through harvest.

In a dry year, weed control is very important because of competition for moisture with the crop, and the subsequent potential loss of crop yield.

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

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