Why is there so much ragweed? Blame the worms.

For year scientists have puzzled over why giant ragweed spreads so rapidly when it produces few seeds. Now they know.

The Weed Society of America reports that "underground gardening" by night crawlers -- a favorite fishing bait; natives of Europe -- spread one of the nation's most irritating weeds. -- by dragging them to their burrows and planting them in the soil.

“Earthworms help ragweed thrive by systematically collecting and burying its seeds in their burrows,” said weed ecologist Emilie Regnier of Ohio State University. “In fact, we’ve found that more than two-thirds of all giant ragweed seedlings emerge from earthworm burrows.”

It turns out that night crawers are excellent gardeners. In fact, ragweed spread by the worms grew into the largest, healthiest plants.

Before the project began, earthworms had an unsullied "green reputation" because their burrows allow water to permeate soil faster and because they contribute to improving nutrients in the soil.

"Now, though, it appears there is a dark side to the earthworms' work," says Lee Van Wychen of the Weed Science Society.

They can affect the forest environment for the worse, and there's speculation that night crawlers might help to spread invasive plants.

What scientists hope to learn is why the worms cart the ragweed seeds off to their burrows and plant them. Dr. Regnier is quoted in a University of Illinois report as speculating that it might be the seed's size and shape: "Unique seed design might play a factor in an earthworm’s ability to grip and drag the seed back to its burrow. Giant ragweed seeds have a lot of variation in their shape and size, and they have ridges along the sides and a 'beak' at the tip of the seed, which might make it easier for the earthworm to grip it.”

But they don't know for sure. So far the implications are far beyond what the researchers initially envisioned, including the interaction between an alien species -- night crawlers -- and a native one, ragweed.