For example, he said, if the top inch of soil warms to about 90 degrees Fahrenheit for only a single day in April, giant foxtail seeds go into dormancy, refusing to sprout for the rest of the season.
Giant foxtail is the major grass weed in the Corn Belt.
Dry soil in the spring also causes a similar reaction in pigweed seed. Pigweed is the most common weed growing in the Corn Belt. Minn. Forcella has put the weather triggers for various weed species into a weed- forecasting model, called WeedCast. He will place the model on the Internet early next year. He said the forecasts can be used with another computer model that helps farmers reduce herbicide use by advising them when it's not needed.