The Ohio State University

Home TurfNotes Predicting Weed Emergence: A useful method of predicting weed emergence is phenology. Typically phenology refers to using temperature as a basis for predictive methods because plant growth and development depend on temperature. One method of prediction is "growing degree-days" (GDD).

How is GDD calculated? First, growing degree day accumulation is the sum of the numbers of degrees by which each days average temperature exceeds a base temperature. For most organisms, the base temperature is 50ºF. For example: if the average daily air temperature on March 12th was 58°F, the growing degree day (GDD) would be 8. Each day thereafter the GDD numbers are added together to give a total accumulative GDD figure. Negative growing degree days are considered to be zero. OSU, MSU and others are also evaluating a base temperature of 32ºF to better predict plant growth regulator applications for Poa annua in the Midwest US (see Table 1, which can be downloaded as a PDF at the bottom of this page).

By monitoring weed emergence patterns in conjunction with calculating degree-days accumulation, turfgrass managers can develop models as a guide for targeting preemergence (residual) herbicide applications in their region. It is important to understand that biological processes don't suddenly turn on when a specific degree-day total is reached but they occur within a range of degree days. For example, crabgrass (Digitaria spp.) germinates within a degree-day range of 42-78.

It is also important to recognize that weeds will continue to germinate & emerge for several weeks after initial emergence is observed. Peak germination of crabgrass in Ohio occurs around 150-225 growing degree days (GDD) and then for a considerable time thereafter. Therefore, preemergence herbicides applied after the initial germination period will still provide a significant benefit in preventing additional crabgrass emergence that occurs before the peak.

New Poa Research at OSU: GDD models are being researched for predicting annual bluegrass (Poa annua) seedhead development. The purpose of this research is to determine the best time to apply plant growth regulators, such as Proxy (ethephon), to control the development of the prolific seedheads that Poa produces each spring. There are models in place for some plant growth regulators, such as Embark (mefluidide) - for example, for the 50°F base the recommendation for Embark application is 50-75 GDD. However, there has been little GDD model research using the 32ºF base and no research to date on using the 32 GDD model for Proxy application.

The attached table provides GDD units per day and cumulative GDD units for both base temperatures of 50 and 32°F. It can be printed from this page or downloaded HERE as a PDF document.

Posted by John Street, Pam Sherratt & Aneta Studzinska