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.
Table 1. PPI and PRE Vegetable Herbicides and Their Residual Activity
| Herbicide | PPI and/or PRE | Residual Activity | Notes |
| Chateau | PRE | 4-6 weeks | Residual control is dependent on application. |
| Command 360 ME | PRE | Season-long control | Soil texture impacts residual and product efficacy. |
| Dacthal W-75 | PPI, PRE, POST or POST transplant | Average half-life is 100 days in most soil types. | Effective weed control may be maintained for as long as 2 months, depending on application rate and soil type. |
| Devrinol | PPI | Season-long control | Deep ploughing will minimize carryover effect. |
| Dual II Magnum | Early Pre, PPI and PRE | 10-14 weeks | Yellow nutsedge control requires PPI. Do not use on muck soils or coarse-textured soils low in organic matter. |
| Eptam | PPI | Season-long control | No soil residues the following year to prevent crop rotation. |
| Frontier Max | PPI and PRE | Season-long control | Length of residual depends upon soil moisture, rate and timing. Heavy rainfall following an incorporated treatment may reduce weed control. |
| Gesagard | PRE | 6-8 weeks | |
| Karmex or Diurex 80 WDG | PRE or Directed Early POST | Residues last about a year | |
| Kerb | PRE | Persistence is variable: 2-9 months | Persistence is greatest in sandy soils with low organic matter. |
| Nortron SC | PPI and PRE | less than 5 to greater than 14 weeks | Half-life depends on soil temperature and moisture conditions. Half-life increases in cooler, drier soils. |
| Prefar | PPI | Season-long control | Half-life of 4-6 months depending on soil type. Recropping restrictions. |
| Princep Nine-T, Simazine 480, Simadex | PPI and PRE | Soil residues may persist for more than 1 season | Needs sufficient moisture to be activated. |
| Prowl H2O | PRE and Early POST | Persistence depends on weather and soil conditions. More persistent under dry conditions. Strongly adsorbed to soil particles. Most effective when rain is received within 7 days of application. | |
| Pursuit | Early PRE, PPI, PRE and Early POST | Persistence depends on weather and soil conditions. More persistent under dry conditions. Rotational restrictions apply. | |
| Pyramin | PPI and PRE | 4-8 weeks | Length of residual depends on soil moisture and temperature. Residue carryover problems are unlikely. With PRE applications, at least 1.25 cm of rainfall is required for good weed control. |
| Reflex | PRE and Early POST | Persistence depends on weather and soil conditions | More persistent under dry conditions. Do not apply to any field more than once every two years. |
| Sandea | PPI, PRE and POST | 4-6 weeks | Length of residual control is dependent on soil and moisture factors, application rate and timing. Refer to label for rotation crop guidelines. |
| Sencor or Tricor | PPI and PRE | Varies with climate. At normal use rates the half-life is 1-2 months | Heavy rainfall following application may cause crop damage. Some varieties of potato and tomato are less tolerant than others. |
| Sinbar | PRE | May be up to 2 years | Moisture is necessary to activate the chemical within 2 weeks after application. |
| Treflan or Rival or Bonanza | PPI | Season-long control | Activation upon incorporation, rainfall is not required. |
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