Some weeds are strong enough to resist weed-killing products called herbicides. But researchers are finding natural ways to deal with unwanted plants. Steven Mirsky is a scientist with the US Department of Agriculture. In January, he went looking for weeds at the USDA's research stations, just north of Washington, DC. He found evidence of pigweed in a field of dry corn. One pigweed plant was more than two meters tall. The scientist says pigweed takes water nutrients and light from nearby crops. The weeds can quickly get out of control. Each plant can produce up to 500 thousand seeds.

Many farmers control pigweed and other problem plants by growing genetically modified crops. Such crops are not harmed by the weed-killing product called Roundup. Steven Mirsky, however, notes that some crops may be resisting the treatment. He says the repeated use of herbicide has caused weeds to grow resistant to it. He and others are studying another possibility, controlling weeds with other plants instead of chemical products.

In the fall, they cover the ground with a "cover crop", like rye. When it reaches a height of nearly two meters, they roll it falt. The rye crop keeps the ground cool and builds up the soil. The cover crop stops some weeds from reproducing. Later, a machine cuts through the protective mat to plat the crop which grows through the summer. Steven Mirsky says this could greatly increase production of organic crops without using herbicides. He says the method will not work everywhere. But where it succeeds, it offers farmers a new tool.