In addition to strangling pretty flowers, weeds squeeze pocketbooks worldwide. Weed-related costs add up to more than $500 billion, according to John Masiunas, a weed scientist at the University of Illinois at Urbana-Champaign.
"They can reduce yields or crop quality, make harvest more difficult, reduce the beauty, utility or land value for an area, and poison livestock, pets, or humans," Masiunas said.
So, how did weeds get there, and why, even after several exterminations, do they keep haunting our gardens?
Poorly understood cycles
Weeds are defined as plants that grow out of place. They can be native to an area, brought in by a contaminant, or introduced as an ornamental plant. Like other plants, weeds sprout regularly; they come as annuals, biennials, and perennials.
However, some years the backbreaking work of weeding gets worse.
That's because some seeds can remain dormant in the soil for long periods of time until they're triggered to germinate, according to recent research by weed ecologists at the Spanish National Research Council (CSIC).
"Our research has shown that, in the specific case of flixweed (Descurania sophia) in winter cereal fields, these seeds have a tendency to wake up every four years," said study team member Cesar Fernandez. "Probably other species will have a different behavior, but we still do not know."
The findings are published in the September issue of the journal the American Naturalist.
By understanding the sleeping cycle of weeds, scientists hope to stop the guessing game farmers and gardeners play with weed killers.
"This may have significant agronomic implications," said Jose Gonzalez-Andujar, another co-author of the study and president of the Spanish Weed Society. "One major aspect of the farmer's decision-making process regarding weed management is the annual assessment of the efficacy of the control tools used."
A farmer's use of herbicides seesaws with the number of weeds he sees in his fields. If a farmer's fields are plagued with weeds, he's likely to start spraying more herbicides. In future seasons, when weeds go dormant, he would be needlessly overdosing his crops.
Since the 1940's, herbicides have been the go-to weapon for weed control. Selective herbicides work by killing off certain targets, while leaving desired crops relatively unharmed.
Costs beyond money
Weed-killing chemicals aren't cheap, nor are they light on the environment or human health.
Atrazine, the second most frequently detected pesticide in U.S. Environmental Protection Agency's National Survey of Pesticides in Drinking Water Wells, turns male frogs into hermaphrodites. Agent Orange, used by the United States during the Vietnam War to defoliate forests, has been determined the culprit of cancer and nerve diseases in veterans. In 1995 Du Pont phased out its production of cancer-linked cyanazine, which controlled broadleaf weeds and grasses in corn and cotton fields.
"Because of environmental concerns, interest is growing for the use of new approaches to weed control to avoid or reduce the herbicide use," Gonzalez-Andujar told LiveScience.
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