Eurasian spotted knapweed, a thistle-studded thug that has overtaken grazing pastures, wages war by releasing toxin into soil to kill other plants, Colorado State University researchers report in today's issue of the journal Science.

Because the weed is immune to its own poison, the research project leader hopes those protective genes can be engineered into other plants so they're better armed for the plant warfare. Equipping native, beneficial plants for battle happens first in the laboratory, then will move to the greenhouse and, finally, to the field.

"Ultimately, they're going to go to the field," said Jorge Vivanco, a CSU assistant professor in the Department of Horticulture and Landscape Architecture.

The finding has immediate value for ranchers and land managers who try to control the weed with herbicide and immediately reseed with a beneficial plant, Vivanco said.

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"What they've seen is 99 percent of these plants don't grow. Now we know why," he said. "You have a toxic compound in the soil."

Conventional wisdom holds that weeds simply out-compete desired plants, hogging water and nutrients by sinking roots deeper and faster, and rising high to grab more sunlight.

When earlier researchers tried to argue that plants also waged chemical warfare against neighbors, their science was so sloppy that the notion was mocked.

The CSU work puts the topic "back on center stage," Alastair Fitter, a British biologist, wrote in a commentary also published in Science. "Their findings suggest that chemistry may play a large part in successful invasion by certain plant species."

A few dozen weed species, including spotted knapweed, cost Coloradans $100 million a year. Spotted knapweed, hardy enough to return stronger after some fires, has been called one of the nation's most economically destructive exotic plants. Spotted knapweed, unintentionally introduced to America from Europe in the late 1800s, now covers millions of acres.

And it reigns over that sprawling kingdom like the harshest human despot.

"This is coming from a far land. It has got to be inventive here," said Harsh Bais, a postdoctoral student in Vivanco's Fort Collins lab.

That extra something is releasing catechin to disrupt the chemistry of would-be neighbors. The toxin attacks vulnerable plants, like American grasses, along the roots. Ultimately, the root cells lose their structural integrity and the plant fails, much like toppling a high- rise by undermining its foundation.

"Nobody ever looked at what was going on in the soil. Now we know soil interactions are important," Vivanco added.

Coming to that realization meant harvesting the knapweed's barky roots to isolate key proteins.

"I have gone in very rough conditions - when it's been snowing out," Bais said. "Whenever I have seen such an invasion, the roots are still alive. ... This is a very, very tough plant."

CSU researchers found that spotted knapweed is among three related species that sow the soil with poison.

Denver Post science writer Diedtra Henderson can be reached at 303-820-1910.