
Forecasting Weed Seeds: One Day
Can Make a DifferenceBy Don Comis November 22, 1996 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. "Most farmers have a feeling, over the years, as to when or if weed
species will emerge in the spring," Forcella said. "You have to have
a sense of that when you're spending $20 an acre on herbicides each season, as
corn and soybean farmers do." 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. U.S. Department of Agriculture | |