Kristen Obeid, M.Sc. OMAFRA Weed Management Program Lead – Horticulture
It has been very dry in many parts of the province (other than Essex County); therefore, it is time to evaluate your pre-plant incorporated (PPI) and preemergence (PRE) herbicide applications. Good weed control with PPI and PRE herbicides depends on many factors, including rainfall after application, soil moisture, soil temperature, soil type and weed species. Herbicides that are incorporated into the soil surface (PPI) usually require less rainfall after application for effective weed control than unincorporated herbicides (PRE). Scout your fields for weeds and determine if your herbicides have been activated. A general rule of thumb is 15-20 mm of rainfall within 7-10 days after application is required to activate preemergence treatments. If this has not happened, consider shallow cultivation, rotary hoeing or harrowing. This will control weed escapes and improve herbicide activity in the absence of rainfall. Another option, if available, is to irrigate. If there are too many weed escapes it will be time to apply a postemergence herbicide.
Knowing the length of time that a herbicide persists in the soil can help you reduce the risk of crop injury and determine the expected weed control period. Some herbicide labels list the product half-life, defined as the length of time it takes for 50% of the herbicide to break down to inactive compounds. Herbicide adsorption and breakdown vary with soil temperature, soil pH, and soil moisture.
Soil-applied herbicides are adsorbed and inactivated by soil constituents in the following order: organic matter > clay >silt >sand. Adjust herbicide rates for soil type and organic matter content. Most soil-applied herbicides require higher rates to be effective in high organic matter soils, but crop safety may be marginal on low organic matter soils.
The table below provides a guide to the residual activity of various vegetable herbicides.