JoAnne Skelly
Although gardeners love plants, few of us love weeds. Although it seems absurd to talk about weeds while there is snow on the ground, the Boy Scouts say, “Be prepared!” If you want to do less pulling or hoeing of weeds in spring and summer, consider applying a preemergent herbicide relatively soon.
No single herbicide will kill every type of weed, so for maximum effectiveness, be sure your weeds are identified on the product label as ones that are controlled. Herbicides are chemicals that kill plants. Preemergent herbicides are applied directly to the soil before annual grass and broadleaf weeds germinate.
They come in granular or spray formulations. Before applying, remove any dead vegetation, existing weeds, trash or other debris from the site, so the chemical can contact the soil. A preemergent herbicide doesn’t kill seeds directly.
Instead, when the sprouting seeds encounter the chemical barrier on the soil, cell division in the young root system is inhibited, causing death in the young seedling. These products DO NOT work on existing vegetation.
Read the label carefully and follow the directions completely. The label will tell you when to apply the herbicide for the particular weed you want to control. It will designate if the product can be used with food crops or if it will damage adjacent vegetation.
The label defines the appropriate personal protective equipment that is necessary, particularly when mixing the product. The signal word “caution” indicates the least toxic product. Usually, you must water the herbicide into the soil after applying to activate it. Don’t use these products where you want to grow plants from seeds.
The chemical can’t distinguish between desirable and unwanted plants. Preemergent herbicides have low water solubility and stick to the soil particles. This means that they usually stay where applied unless wind, water or some outside activity moves the soil, which can then contaminate surface or ground water.
Movement with water is a greater risk in sandy soils than in clay soils. Products with a longer persistence have a greater potential for water contamination. Disturbing the soil after application will greatly reduce weed control, so keep animals, people and equipment out of the area.
A spring application will control summer annual weeds, such as puncturevine or tumbleweeds. A fall application will deter winter annuals, which include cheatgrass, mustards or redstem filaree. Since we are already seeing these weeds now, a preemergent won’t work on them at this time of year.
For information, read Sue Donaldson’s and my publication at https://naes.agnt.unr.edu/PMS/Pubs/2011-3172.pdf.