Jan 8, 2015

Get the most from your pre-emergent vegetable herbicide

If used correctly, pre-emergent herbicides are an effective means of controlling early season weed problems, helping crops get a good start and potentially enabling them to out-compete mid- and late-season weed pressure. Herbicide applications are also less expensive than alternative methods. Understanding modes of action and other properties of these herbicides can help producers use them more effectively and safely.

Pre-emergent herbicides are applied to the site shortly before or after planting or transplanting the economic crop and prior to any weed emergence. They do not affect the weed seed itself, but create a chemical barrier at the soil surface. As germinating weeds grow through this barrier, the product affects the biological processes of the root, shoot or both. The longer this barrier is maintained, the longer the control period.

Some herbicides volatilize or degrade in sunlight. So establishing this barrier may require incorporation through lightly tilling the soil, or rain or irrigation water moving the product slightly below the soil surface. Tilling the soil too deeply will dilute the product or get it into the root zone of the crop, causing potential crop damage. Similarly, too much water can move the product into the root zone of the economic plant. Properly maintaining this barrier is important for good weed control. Once the barrier is established, it should not be disturbed through other activities such as cultivation. The effective life of the barrier varies with the product, soil type and the population of the soil microorganisms responsible for breaking down the product.

Pre-emergent herbicides differ in modes of action and other properties. Probably the most important chemical aspect of these products is their water solubility. The more soluble it is (higher number) the greater mobility it has in the soil. The higher the solubility, the less water is needed to move it into the soil and therefore into the root zone of the crop and potentially into water sources. To reduce resistance potential it is best to rotate herbicides when possible based on their mode of action.

More information on controlling weeds in Michigan-grown vegetables can be found in the 2015 Weed Control Guide for Vegetable Crops” (Extension Bulletin E-433).

– Ron Goldy, Michigan State University