Dry Weather and Weed Control
Posted: July 13, 2012
Some crops are still being managed for weeds. However, with much of the region in drought, there is some concern about the success in trying to control certain weeds under these dry conditions. Certainly, annual plants become much more “tolerant” with dry weather, maturity, and persistent warm temperatures. “Large-drought stricken annuals are harder to kill”. With perennial weeds, the effect of drought is less clear. Cool season perennials including Canada thistle, quackgrass, and dandelion will certainly go into a summer dormancy period when dry warm weather persists. If possible, they should not be treated with an herbicide until actively growing. Cool season perennials mimic the same growth cycle as your lawn; active in the late spring and early summer followed by a slow period and then a rebound in later summer and early fall. Once the “heat” of summer has passed and assuming they have relatively healthy green leaves, then an effective systemic herbicide should work well.
Some things to consider as we experience dry weather and the weeds continue to grow:
- Increase the herbicide rate if the label allows and make applications at the most favorable time for increased control. Make applications in the morning when the weeds are most active.
- Apply herbicides to smaller weeds or wait a few days to spray if rainfall is in the forecast.
- The post grass herbicides (Assure, Select, Poast, etc.) tend to be one of the most susceptible groups to decreased efficacy in dry conditions followed by the ALS-inhibitors (Sandea, Matrix, Raptor, etc.). Contact herbicides (Reflex, Liberty, Gramoxone, etc.) are generally less effected by drought stress, but be sure to increase carrier volume to achieve good coverage.
- Think about adjuvants. You may need to use a higher rate or switch to MSO (methylated seed oil) or COC (crop oil concentrate) if they are allowed which can increase herbicide uptake and improve control. However, remember oil-based adjuvants can also increase the potential for crop injury. Sometimes there is a fine-line between controlling the weeds and injuring the crop.