Ready, Set, Go - The Time to Control Summer Annual Weeds in NOW!

Spring has appeared more quickly this year than it has in years.  With the arrival of warmer air, we also see the gradual increase of soil temperatures which means it is time for homeowners to apply a preemergence herbicide to control crabgrass on turf areas. The timing of that application is critical for effective control of crabgrass and other summer annual grassy weeds. Since the preemergence herbicides recommended for crabgrass control actually prevent germinating seedlings from emerging, these products must be applied before crabgrass seeds germinate. Researchers report that crabgrass seeds germinate when average daily soil temperatures reach 57 to 64 degrees Fahrenheit at a 1-inch depth. 

Some turf managers have monitored soil temperatures as a method of predicting crabgrass germination and determine the optimum time to apply preemergence herbicides. Likewise, the bloom period for forsythia has been used as an indicator for the best time to apply crabgrass control. Forsythia blooms just prior to crabgrass germination signaling the time to apply preemergence herbicides. 

In recent years, models using growing degree days (GDD) have been developed for predicting crabgrass germination and preemergence herbicide application timing. Homeowners and professional turf managers may find the GDD tracker website to be a useful tool to track crabgrass germination and determine the optimum timing for preemergence herbicide applications. Good control of crabgrass can be observed when preemergence herbicides are applied when the points on the GDD Tracker map reach the “under”, “early”, “optimum”  or "late" categories. As of March 30, 2016, central Indiana is heading into the “late” category, and northern Indiana is in the “optimum” category. The “bottom-line” is that applying preemergence herbicides now in central Indiana for effective preemergent control of crabgrass in turf areas.

The following tips are also important for minimizing crabgrass pressure and increasing the effectiveness of pre-emergent herbicide applications:

• Mow cool-season turfgrasses at a mowing height of 3 inches or higher
• Irrigate turf areas deeply and infrequently
• Apply 2 to 4 lbs. of nitrogen per year with the majority applied September through November each year
• Ensure an even distribution of preemergence herbicides, avoiding skips or misses on turf areas
• Always read the label for all pesticide products to determine the appropriate rate of application and spreader setting
• Pre-emergent herbicides must be watered in thoroughly to be effective, time application before rainfall or use irrigation to accomplish this important step.

Taken from Crabgrass Control: Timing is Critical
John Orick Purdue Master Gardener State Coordinator Department of Horticulture and Landscape Architecture, Purdue University