Mark VanGessel, Extension Weed Specialist; mjv@udel.edu
Seems like all we have talked about this spring is rain. But, as we all know it’s not how much rain we get, but when does it come. Our soil-applied/residual herbicides (like Bicep Magnum, Harness Xtra, Lexar) are applied to the soil surface but they need to be moved into the top inch of soil where they can be absorbed by weed seedlings’ roots and shoots. Left on the soil surface they are not nearly as effective.
The weather station at the Carvel Research Station last registered rain on April 12. Our weather forecast is not showing a high probability of rain for the next 10 days.
Keep an eye on the forecast and if rain is not predicted for 5 to 10 days after your herbicide was applied you should consider irrigation to ensure the maximum effectiveness from that preemergence herbicide application.
How long is the time between herbicide application and rainfall/irrigation? It depends on the soil temperature. The time depends on how long it takes for weed seedlings to emerge. This time of year it is in the 10 to 14 day timeframe, but as weather warms up that could be as short as 5 days.
How much rainfall/irrigation is needed? This is not an exact science and it depends on the water solubility of the herbicides that were applied. Some herbicide are more water soluble than others. Those that require more water include atrazine and simazine.
It also depends on the soil moisture levels at time of application. The less soluble herbicides (atrazine and simazine) need may need 0.5 to 0.6 inches if the soil is dry, but less when soil moisture is high. More soluble herbicides like Dual, may need only need a third of an inch if the soil moisture level is high.
Water solubility is listed in the Virginia Tech Extension, Weed Management for Field Crops (ENTO-566)- https://www.pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/456/456-016/ENTO-566-E.pdf. (This publication is a joint effort with Virginia Tech, Univ MD, Univ. of WV, Univ. DE, Rutgers Univ. and Penn State. This is the same information in the Penn State Publication, but this version is free.)