Seed germination occurs when it has completed its dormancy requirements and when the environmental conditions are favorable for subsequent growth of seedlings upon germination. Most weeds that compete with crops during the growing season or interfere with lawns and landscape plantings germinate in spring (summer annuals) as the ambient air temperature and soil temperature start to rise.
Growing Degree Days
Apart from temperature, adequate soil moisture, oxygen and light are other requirements for seed germination. Typically, these are met under normal circumstances so the factor that primarily determines seed germination is air/soil temperature. Each weed has a biological clock (the seed) that keeps track of warming trends in its immediate microenvironment. Once a cumulative threshold is achieved, it commits to germination as long as other factors are suitable. This innate record-keeping mechanism of a seed, referred to as Growing Degree Days (GDD), can be considered as heat units stored in its memory.
Typically, GDD is calculated as the difference of the mean temperature and base temperature, usually 50° F (referred to as GDD 50), for a given day. GDD 50 accrued from the beginning of Julian calendar (January 1) can be used to predict germination of certain common weeds. For example, smooth crabgrass (Digitaria ischaemum) begins to germinate when the GDD 50 accumulates to about 150; however, large crabgrass (Digitaria sanguinalis) requires 150 to 200 more GDD 50 units to germinate. Certain phenological indicators, such as forsythia bloom drop, are used to estimate GDD accumulation that coincide with smooth crabgrass germination.
Determining Growing Degree Days
Not all populations of weeds belonging to the same species germinate at the same time; a small percentage may germinate early and be followed by a peak germination period. Some species have a narrow window of GDD requirements, while others have a broad range.
Online tools are available to determine GDD for a given location. A tool on the Climate Smart Farming website at Cornell University determines GDD for a given location once the address of the location is entered. By keeping track of GDD using this tool, one can maintain a record of weed germination in the backyard, especially during early- to mid-spring for future reference.
The GDD model can be used to time a management method to control a given pest if its GDD requirements are known. For example, the application of mulch or a pre-emergent herbicide can be timed when the GDD50 approaches 150 to manage smooth crabgrass in lawns. Similarly, the invasive weed shown here, joint-head Arthraxon (Arthraxon hispidus), emerges when GDD 50 approaches 175.
Rakesh Chandran, WVU Extension Weed Science Specialist
Last Reviewed: April 2018