The Purdue Plant and Pest Diagnostic Laboratory

P&PDL Picture of the Week for
March 31, 2014

Crabgrass Control: Timing is Critical

John Orick Purdue Master Gardener State Coordinator Department of Horticulture and Landscape Architecture, Purdue University

“Will spring ever arrive?” The good news is there are signs of spring’s arrival and with the gradual increase in air and soil temperatures comes the time to apply a preemergence herbicide to control crabgrass on turf areas. However, the timing of that application is critical for effective control of crabgrass and other summer annual grassy weeds. Since the pre-emergence 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. So, 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”, or “optimum” categories. As of March 31, 2014, southern Indiana is in the “optimum” category, central Indiana is in the “early” category, and northern Indiana is in the “under” category. The “bottom-line” is that applying preemergence herbicides now in southern Indiana and within the next 1-2 weeks in the rest of Indiana would be optimum 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.

References:

Purdue Turf Tips: When will Crabgrass Germinate? March 16, 2012

Purdue Turf Tips: March’s Weed of the Month is Large Crabgrass; March 26, 2013

Purdue Turf Tips: When Should I Apply my Preemergence Herbicide for Crabgrass Control? April 3, 2013

Control of Crabgrass in Home Lawns, AY-10-W (pdf file)

 

Click image to enlarge

Screenshot of the Growing Degree Day Tracker website showing the current preemergence herbicide timing for Indiana for March 31, 2014.

Crabgrass
Photo by Doug Akers, Retired Purdue Extension Educator

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service