By Dr. Aaron Patton, Turfgrass Extension Specialist, Purdue University
Herbicides are ineffective on drought-stressed weeds and can be damaging to drought-stressed turf, especially when temperatures are warm. My rule-of-thumb is that if the lawn is more than 50 percent green, then herbicides can be applied.
Avoid the temptation to apply herbicides in a dormant (brown) lawn even though weedy species may be obvious as many are drought tolerant perennials, summer annuals, or sedges that grow well even in times of drought.
For yellow nutsedge and summer annual weeds (crabgrass, purslane, spurge, etc.), it's better to hold off on applying herbicides until rain returns and the turf begins growing again.
For perennial broadleaves, it might be best to wait until fall as these applications are better than spring applications for broadleaf control.
Here are some thoughts on summer weed control in irrigated areas during summer:
The question I most typically get in this irrigated scenario or during a summer when turf is not drought stressed is, Can I apply an herbicide when temperatures are 85°F or above?
First, you will need to follow the herbicide label as each herbicide acts differently.
Herbicide applications when daily high temperatures are above 85°F generally increase the risk of turf injury. Most herbicide labels do not prohibit applications at temperatures greater than 85°F or 90°F (each herbicide has a different maximum temperature), but they do caution against their use at these temperatures due to the increased risk for turf injury.
A general recommendation is to make the application if the site is more weeds than turf and you are willing to assume the risk of injuring the turf. In most cases, if there are fewer weeds than turf, then it is best to delay making the application until better conditions exist.
Information courtesy Purdue Turf Tips.