Soil Applied Herbicides And Rainfall For Activation

Fieldwork has progressed slowly in the past couple of weeks due to frequent rain showers and cool soil temperatures.  At this point, there is still uncertainty about how the weather will develop in the next couple of weeks.  As we approach May, growers are ready to start planting as soon as conditions allow.  Many of these acres will receive soil-applied, residual herbicides for control of germinating weed seedlings.  Soil-applied preemergence herbicides require moisture for activation.  What this really means is that we want the herbicide to be dissolved in the soil water (aka “solution”) and moved down into the upper inch or two of the soil profile, so it can be taken up by the germinating weed seedling roots or shoots.  When soil conditions are dry, herbicide molecules will remain closely associated with soil particles and are not able to move into weed seedlings via mass flow processes.  As a result, weed control with soil-applied herbicides under dry conditions can be less than desirable.

We are asked quite often how much rainfall it takes to activate a soil-applied, residual herbicide. The answer depends on many factors, which include:

1) How water-soluble the herbicide is;
2) how sensitive the weed specie is to the specific active ingredient;
3) what stage is the weed seedling at when exposed to the herbicide;
4) did the weed seedling receive a high enough dose to overcome any natural herbicide tolerance or metabolism mechanisms;
5) how moist was the soil when the herbicide was applied.

As you can see, the answer to the question “how much moisture is required to activate my herbicide” requires consideration of several factors.

A quick review of several herbicide labels (but not all) shows the following edited comments with regards to precipitation and herbicide activation:

Single-Active Ingredient Products Precipitation Required for Activation
Metolachlor (Dual) 0.5 inches on coarse soils, 1 inch on fine-textured within 2 days after application
Dimethenamid-P (Outlook) Nothing about precipitation amounts mentioned.
Acetochlor (Harness/Degree) 0.25 to 0.75 inches within 7 days after application
Pyroxasulfone (Zidua) 0.5 inches before weed emergence
Flumioxazin (Valor) When adequate moisture is not received after application, weed control may be improved by irrigation with at least 0.25-acre inch of water.
Sulfentrazone (Spartan) If adequate moisture (0.5 to 1 inch) from rainfall or irrigation is not received within 7 to 10 days after application, a shallow incorporation may be needed to obtain desired weed control.
Atrazine (Aatrex) Nothing about precipitation amounts mentioned.
Metribuzin (Tricor) In areas of low rainfall, preemergence applications to dry soils should be followed with light irrigation of 0.25-acre inch of water.
Isoxaflutole (Balance) Most effective in controlling weeds when adequate rainfall is received within 14 days after application.


Single-Active Ingredient Products Precipitation Required for Activation
Acuron Nothing about precipitation amounts mentioned.
Degree Xtra 0.25 to 0.75 inches within 7 days after application
Corvus Most effective weed control when applied and subsequently moved into the soil by rainfall, sprinkler irrigation or mechanical tillage prior to weed emergence within 14 days after application.
Sonic The amount of rainfall or irrigation required for activation following application depends on existing soil moisture, organic matter content and soil texture. If adequate moisture (0.5 to 1 inch) is not received within 7 to 10 days after the treatment with Sonic, a shallow cultivation may be needed to obtain desired weed control.
SureStart/TripleFlex Precipitation or sprinkler irrigation of at least 0.25 inch is required to bring SureStart into contact with germinating seeds.
Verdict Must be activated by at least 0.5 inch before weed seedling emergence.

As you can see, the answer varies a bit by herbicide product. As a rule of thumb for most soil-applied herbicides, we would like to see about 0.75 to 1 inch of precipitation within the first week. Also, we would like to see approximately 2 inches of precipitation spread out over the first two weeks after the herbicide was applied for optimal herbicide performance.

If 10-14 days have passed without rainfall following a soil residual application and weeds are starting to break, consider the following:

  • Start planning for a post herbicide application.
  • Use a rotary hoe to dislodge small seedlings and buy some time for a precipitation event to activate the herbicides.
  • Some herbicides can “reach back” or “recharge” on small annual weeds when rainfall occurs, although depending on this may be a little like buying a lottery ticket. The HPPD (Group 27) herbicides (Accuron, Balance, Corvus, Lumax, Lexar, Instigate, Prequel, etc.) tend to have better “reach back” potential than some other herbicides, and escaped grass control is probably of greater concern. The Group 5 herbicides (Photosystem II inhibitors) like atrazine, simazine, and metribuzin will also control small emerged susceptible broadleaves via root uptake.
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