The three R’s of good weed control

Getting good weed control with herbicides can be a frustrating experience. Reducing that frustration is possible by using the 3-R principle – the right product, the right rate and the right time.

Good chemical weed control can be a frustrating, nerve-wracking process causing sleepless nights for many applicators. A lot of this worry can be relieved by reading product labels and then deciding on the right material applied at the right rate and the right time.

The right material depends on the crop, the crop growth stage, and the growth stage of the weeds. First, make sure the product you choose is labeled for the crop. If it is, that means an extensive amount of private and public research has gone into the efficacy and safety of the product for use on that crop and in the manner in which it is to be used. Another aspect of selecting the right material depends on what you expect the product to do. Do you want it to control pre-emergent or post-emergent weeds? Few products do both. A combination of two products is often best; one as a pre-emergent and the other as a post-emergent or one for grasses and the other for broadleaf weeds.

The right rate is often a function of soil type, growth stage of the economic plant and of the weeds. Higher than recommended rates can cause economic plant damage while lower than suggested rates leads to resistance in the weed population. Application rates of pre-emergent products vary depending on the sand/clay/silt/organic matter content of the soil. Lower rates are recommended for sand-based soils and higher rates for clay, silt and organic-based soils. Labels on some products simply indicate to not use them at all on organic soils.

This rate change is due to the soil Cation Exchange Capacity (CEC). CEC values are an indication of how tightly these products are bound to soil particles. Sand-based soils have CEC values of 6 or less. As clay, silt and organic matter content increase, so does CEC. If pre-emergent products are applied to higher CEC value soils, more product is bound to soil particles and not available to control weeds, therefore more must be applied.

Rates of many post-emergent products vary with growth stage of the economic plant or of the weeds to be controlled. Higher rates are often needed to control larger weeds. This can lead to a bit of a conflict since the economic crop is also exposed to a higher rate, potentially causing damage. To eliminate this concern, it is always best, if possible, to control weeds when they are small.

The right time is often the most difficult decision or hard to coordinate due to other activities requiring your time. Many products have a fairly narrow application window and any time outside that window will potentially cause damage to the economic crop or not control the weeds of concern. It is pretty easy to make a call on those products indicating an application time of “immediately after transplanting,” “immediately after seeding,” “before crop emergence” or “after the two-leaf stage.”

What about a product that indicates “apply to three- to six-leaf stage” and you have some plants with two and some with seven leaves? Or a product indicating some water is required for activation but too much may potentially cause crop damage? You will have to look at these as a risk, but a management call that still needs to be made since the alternative – mechanical weeding or no weed control – is probably going to cost even more in labor or reduced yields.

Another aspect of timing was mentioned earlier and that is it is always best to control weeds when they are small. Lower herbicide rates can often be used and early control minimizes competition with the economic crop.

More information by Michigan State University Extension on controlling weeds in Michigan-grown crops can be found at the MSU Extension Bookstore. For more on controlling weeds in vegetables, see Extension Bulletin E-433 “2015 Weed Control Guide for Vegetable Crops,” available for $6.50.

If you have further questions concerning commercial vegetable production issues, contact your local MSU Extension office or Ron Goldy at

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