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Tall Whitetop's crowding out the natives.

The delicate, milky-white flowers of tall whitetop resemble baby's breath and seem the perfect addition to a dried flower arrangement.

But this fast-growing weed actually belongs in the garbage---or better yet, the incinerator.

Tall whitetop--also known as perennial pepperweed--is nothing to sneeze at. It's invaded tens of thousands of acres of pastures, marshes, and riverbanks throughout the West, crowding out desirable native plants. What's more, scientists fear the weed may be poisonous to farm animals.

"All of these people, from ranchers to environmentalists, have been calling their senators, asking them to do something about it," says ARS range scientist James Young.

That's why he and colleagues in ARS' Conservation Biology of Rangelands Research Unit at Reno, Nevada, are busy scrutinizing tall whitetop, hoping to uncover the secret of its success. Detailed knowledge of where it grows, how it spreads, and its other nasty habits should help scientists find a way to stop it.

Research by the University of Nevada's Extension Service and the Nevada Department of Agriculture has shown that 2,4-D helps control the weed. But concerns for fish and other wildlife prevent its use near waterways.

"And young seedlings of willow and cottonwood--the trees that will later shade rivers and provide homes for nesting birds--are sometimes more sensitive to herbicides than tall whitetop is," Young points out.

Grazing livestock may avoid the weed if other forage is available, but they are not so likely to discriminate when tall whitetop contaminates hay harvested from weed-infested meadows.

To find out whether the weed is indeed dangerous for animals to eat, scientists in the ARS Poisonous Plant Research Unit at Logan, Utah, have begun tests.

Other plants related to tall whitetop are suspected of being toxic to horses, according to Lynn James, who heads the Logan lab.

Given a choice, livestock don't prefer tall whitetop, says Young. But they'll eat it, if it's the only green thing around. Because of the recent 6 years of drought in the western United States, that was an increasingly common scenario.

The weed's true identity is a matter of some confusion among scientists, admits Young. Officially, tall whitetop is known as Lepidium latifolium. "But we're looking for a European weed expert to confirm the identification," says Young.

Scientists think the weed's seed came to North America accidentally, in a bag of sugar beet seed from Europe near the turn of the century. But tall whitetop has only been considered a pest in the past decade or so.

Today, pastures and prime riparian areas in Nevada and California, once prized for their diverse plant and animal life, are overrun with thick, woody stands of tall whitetop, which grows up to 3 feet high.

In some of the hardest hit areas along the Humboldt and lower Truckee Rivers in western Nevada, native grasses--sedges and rushes that feed grazing livestock and wildlife--were pushed out by the weed.

Alfalfa and sugar beet farmers battle it in California and Nevada.

"It's such an aggressive weed, we think it may produce a substance that repels or even kills nearby plants--a phenomenon known as allelopathy," says Young. Or perhaps tall whitetop somehow disrupts the nitrogen cycle in the soil, making it impossible for other plants to flourish, says Robert R. Blank, also based at Reno.

Because tall whitetop is in the Cruciferae family--which includes broccoli, cauliflower, and mustard--finding beneficial insects to control the weed will be difficult. That's because any insect that might feed on tall whitetop may also relish its crop relatives.--By Julie Corliss, formerly with ARS.

James A. Young and Robert R. Blank are in the USDA-ARS Conservation Biology of Rangelands Unit, Renewable Resource Center, Room 25, 920 Valley Rd., Reno, NV 89512. Phone (702) 784-6057, fax number (702) 784-1712. Lynn James is with the USDA-ARS Poisonous Plant Research Laboratory, 1150 E. 14th N., Logan, UT84321. Phone (801) 752-2941, fax number (801) 752-3075.
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Title Annotation:western US weed
Author:Corliss, Julie
Publication:Agricultural Research
Date:May 1, 1993
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