Forecasting Weed Seeds: One Day Can Make a Difference

MORRIS, Minn., Nov. 22--One hot spring day can avert a weed nightmare for a year in a farm field or backyard garden.

A U.S. Department of Agricultural agronomist, Frank Forcella, found in studies of weed species that each weed has a different "weather trigger" that either prevents or encourages the sprouting of seeds. "This trigger is an important reason why weed sprouting varies each spring, typically from 1- to 100-percent of the buried seed," he said.

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. "This is why pigweed was a major problem in areas that had a wet spring this year, such as west central Minnesota," said Forcella. Pigweed is the most common weed growing in the Corn Belt.

"While these two aren't typical garden weeds, the same principle applies to common garden weeds such as crabgrass and common purslane," said Forcella, who has his own garden and is a weed forecasting expert for USDA's Agricultural Research Service at the North Central Soil Conservation Research Laboratory in Morris, 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.

He said the two-year study of several weed species from Ohio to Colorado and Missouri and other work provides new hard data for farmers as they figure out ways to reduce the reliance on weed killing chemicals.

Typically, he noted, buried weed seeds can survive three to five years in numbers as high as a thousand per square foot. Weed seeds don't survive in the soil as long as once thought, he said, but seeds still find ways to spread across fields.

Weed seeds get into the soil, he said, through a variety of means: carried by farm equipment, planted with crop seeds, blown by the wind, carried in rainwater runoff and borne by birds.

Scientific contact: Frank Forcella, agronomist, North Central Soil Conservation Research Laboratory, Agricultural Research Service, USDA, Morris, Minn.; phone (320) 589- 3411.