Monday, March 18, 2013

Use Growing Degree Days to Better Time Your Applications

Monitoring growing degree days can be an effective way to better time applications and increase your weed control. A helpful website ( is available to help you monitor growing degree days for timing herbicide and plant growth regulator applications. What can this website help me with?
  1. Need help determining when to apply your preemergence herbicide? Visit
  2. Need help estimating when crabgrass is going to germinate in your area? Visit
  3. Should I use an amine or an ester this spring to control broadleaf weeds? Visit (I’ll also cover this more on tips in the upcoming weeks).
  4. When should I apply Embark or Proxy/Primo to my putting greens to regulate annual bluegrass seedheads? Visit
What is a Growing Degree Day (GDD)? A growing degree day (GDD) is a method to track the heat units that have accumulated and are needed for plant growth and development. The formula for calculating GDD is:
GDD = ((max temperature °F + min temperature °F) ÷ 2) - base temperature °F, where the base temperature is normally either 22, 32 or 50 °F but varies based on the model.

For example, if the high today was 74°F and the low was 52°F and we used a base temperature of 50°F, our calculation would be, GDD= ((74°F + 52°F) ÷ 2) - 50°F = 13

In another example on a cooler day you might have a high of 58°F and a low of 40°F and with a base temperature of 50°F, our calculation would be, GDD= ((58°F + 40°F) ÷ 2) - 50°F = -1. When a GDD calculates to <0, we simply make it a zero and determine that no growth (or plant development) occurred on that day.

Models that help us predict plant development use accumulated GDD which is simply adding the GDD calculated each day and determining how many GDD units have accumulated thus far.
In some cases accumulated GDD can be used to monitor when weeds might germinate or flower or when grasses might produce a seedhead while In other cases accumulated GDD can be used to help optimize application timing such as with preemergence crabgrass applications or the selection of amines or esters for spring broadleaf applications. Research into plant development and optimal herbicide application timing has determined a window of accumulated GDD needed to best predict when to time these applications or when these events might occur.

While it is possible to track your own GDD using a local weather station, an easier way to do this is to use an online tool. The Purdue University Turf Science Program and the Midwest Regional Turf Foundation help provide financial support of Michigan State University Extension’s GDD Tracker, This website tracks GDD for the Midwest and provides updates for golf courses on timing annual bluegrass seedhead suppression applications, updates for all in the turf industry on crabgrass germination and preemergence herbicide application timing, and updates on broadleaf flowering and the timing of amines and esters for spring broadleaf applications.

Watch a video from Dr. Ron Calhoun (creator of to learn how to use this website.

Dr. Aaron Patton, Turfgrass Extension Specialist


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