Mark VanGessel, Extension Weed Specialist; mjv@udel.edu
Some areas have not received rainfall in two or more weeks and I have been asked what to expect from soil-applied herbicides that have been applied during that time.
There are a number of factors going on that will influence the results. First, although we lack the rain to move these herbicides into the soil, there is enough soil moisture for the weeds to germinate and emerge.
The effectiveness of soil applied herbicides (such as s-metolachlor [Dual Magnum], acetochlor [Harness], or pendimethalin [Prowl]) sitting on the soil surface is greatly reduced compared to herbicides moved into the soil by rain, irrigation, or mechanically incorporated. These types of herbicides are taken into the weed seedlings as they germinate and start to emerge from the soil; absorbed by the seedlings’ roots and/or shoots. Therefore, once the weeds have emerged, these herbicides are not able to control them. Most of these herbicide labels state that lack of rainfall or irrigation within 7 days after application will result in reduced weed control.
Atrazine applied to the soil surface is absorbed by the root of the weeds. Under these dry conditions, atrazine-susceptible weeds may take up the herbicide after emergence if atrazine is moved into the root zone by rainfall or irrigation and provide good control. Weeds with large seeds are capable of emerging deeper in the soil, may escape control if the rainfall is not sufficient to move the herbicide down to the roots.
Thus, many of these emerged weeds will not be controlled by the soil-applied herbicides. Keep an eye on these fields since they may need follow-up treatment sooner than you typically treat. If you are applying herbicides during a dry spell and have access to irrigation, running the irrigation within a week of application will move that herbicide into the soil and maximize their effectiveness.