One thing that makes weeds tough opponents is they basically practice a sort of guerrilla warfare, avoiding frontal attacks and saving resources to fight another day, says Steve Fennimore, University of California Extension weed specialist.
In his talk during the 53rd Annual Conference of the California Weed Science Society in Monterey, Fennimore said those guerrilla tactics are demonstrated in the way relatively few seeds in a weed population germinate at the same time.
"They may not germinate for ecological reasons. It may be too hot, as in the example of common chickweed. You won't find it in the southern end of the Salinas Valley or in Bakersfield in August."
Likewise, temperatures may be too cool, as in January when common purslane tends not to germinate except for a small amount on southern sides of beds.
Or the seed may be too deep. Small seeds, such as those of purslane or pigweed, need to be very close to or on the surface to germinate. If they germinate too deep in the soil, they do not have the carbohydrate resources to reach the surface.
Herbicides applied to the soil and soil tillage also affect the rate of germination.
So Fennimore, who is based at the USDA station in Salinas, developed a weed emergence calendar spanning from September 1999 to September 2000 to chart the fluctuations in weed emergence during the year.
Support for the research came from the California Department of Pesticide Regulation and the California Lettuce Research Board.
Results of the study showed the germination periods, allowing him to set up a system for when to treat for the weeds. "Our goal is to develop season-specific weed management recommendations. It all adds up to increased herbicide efficiency. Anything we can do to cut costs in the very expensive lettuce growing business benefits the grower's bottom line."
Fennimore, who received the society's award for excellence at the conference, said weed control in vegetable crops "is not a direct route."
Weed biology "We have only two new vegetable herbicides, Prism and Shadeout, available, so we don't have much to talk about in herbicides this year. But one way to make progress and preserve our older herbicides is to better understand weed biology."
He said weed scientists have focused mainly on how to kill weeds with herbicides for decades, while entomologists and plant pathologists have spent more time understanding the organisms they deal with.
"So we are really playing catch-up, trying to learn as much about weeds as possible. We are focused on the soil seed-bank, or the weed seeds in the soil or on it."
The winter germinating group includes annual bluegrass and southern brass buttons. The summer group is common purslane and hairy nightshade. Burning nettle, common chickweed, common groundsel, henbit and shepherdspurse make up the continuously germinating group.
Data was collected at Hartnell College near the Salinas Airport and at a slightly warmer site at the Spence Farm, about six miles to the south of the city.
He pulled soil cores eight times, each representing a "half-season," during the 12 months. The purpose was to determine the relative potential of weed seeds to germinate at different times of year. Samples were taken from the same spots across the 0.4-acre sites as determined by a grid on one-foot squares.
Unlike a seed of a commercial crop that would be expected to have a germination rate close to 100 percent, weeds manage with much less. Using the purslane example again, Fennimore said of a count of 100 seeds, perhaps only 10 germinate at any one time.
The plots were planted to lettuce, broccoli, cauliflower, and spinach. Weed control was entirely by cultivation or hand weeding. Fennimore said he did not want to use herbicides, which might influence the results.
Findings From the data, he learned that annual bluegrass has highest potential for germination in the fall and winter. Burning nettle may dip and rise in germinability during the year but never completely stops.
Common chickweed, common groundsel, and shepherds purse, the most common weed pest in the Salinas Valley, germinate year round, while common purslane prefers summertime conditions.
"With hairy nightshade, we saw very little emergence during the fall and winter, and then it emerges well in the spring and summer," Fennimore said.
The study data is valuable, he said, in selecting herbicides. Prefar, for example, is extremely active on purslane and pigweed.
"During the warmer times in the Salinas Valley, that's probably the herbicide you'll need if those weeds are in your lettuce.
"There's no reason to treat the entire bed top in January if you are planting lettuce. But if you are planting lettuce in July in a purslane-infested field, you can bet your bottom dollar there will be purslane there unless you deal with it."
On the other hand, Kerb is extremely active on chickweed during the winter.
Fennimore said the selective use of the herbicides would allow them to remain in use for as long as possible, perhaps indefinitely.
The study also can benefit organic crops or herbs such as cilantro or basil by guiding the timing of crops to avoid the high germinating seasons of weeds that are difficult to control by cultivation.