Weeds: Which Ones Should We Worry About?

Lewis and Clark's month at Kamiah in May 1806 coincided with one of the loveliest wildlflower blooms of the West. The canyon grasslands and forested river breaklands support species of orchids, lilies, columbines, paintbrush, roses–and dozens of others. This diversity of colors and shapes is remarked on by today's visitor, too, but a modern pest–the noxious weed–is now in place as a competitor.

During the summer of 2003 a weed inventory team from the University of Idaho searched the Lolo Motorway for weeds. The Motorway, a primitive and winding ridge top route that parallels K'useyneiskit for nearly 75 miles, is not the type of habitat that weed scientists usually associate with weeds because it is mostly forested and shady. But there are reasons for concern about the Motorway. Noxious weeds flourish nearby along roads and in clearings in the foothills below. Increased traffic is expected during the Lewis and Clark Bicentennial Observance: car tires, horse hooves, and boot soles are known for spreading hitchhiking weed seeds. And each summer more acres of susceptible habitat for weed invasion are created as large forest fires burn.

The team recorded its findings with GPS technology and produced a map of the 5 weed species they found between Rocky Ridge Lake and Thirteenmile Camp. They saw spotted knapweed four times, orange hawkweed twice, yellow hawkweed twice, henbane once, and goatweed 28 times:1 37 widely-scattered weed sightings in about 50 miles.

Is this good news or bad?


The answer requires looking at the world from a plant's point of view. Every plant has a "lifestyle." It may live a long time, like a century plant. It may reproduce by seeds that fly on the wind, like a dandelion, or fruits that fall under the tree, like an apple. It may co-exist with its neighbors, or spread out like an iris clump. And–this is part of its lifestyle too–people may think it's pretty and pick it, or shun it like poison ivy.

Every plant has a place it likes to grow. It may get what it needs from sunny, exposed hillsides, or perhaps its roots need to form partnerships with a type of underground fungi found in a shady cedar forest.

Knowing these traits makes it possible for weed ecologists to predict whether a weed might do serious damage to the native flora and urgent action is required. Since there are 460 known exotic (non-native) plants in Idaho and 4,000 species in the world that are potential invaders to temperate regions, and only a handful of weed scientists, county extension agents, volunteer weed pullers and licensed sprayers, "making the call" is critical.


weed with a pretty orange flower

Photo © Hugh Wilson, Digital Flora of Texas Vascular Plant Image Library

Looking down on the colorful, low-growing orange hawkweed, a very competitive species that has recently shown up on the Clearwater National Forest.


Hawkweeds have pretty flowers and were brought to the U.S. from Europe as an ornamental in 1875. Orange hawkweed is still grown in household gardens. Hawkweeds are tricky, because there are weedy ones and there are natives (see list of plant names at the bottom of this page), and sometimes it's difficult to tell them apart. But they behave very differently: while native hawkweeds reproduce from seed, weedy hawkweeds can also reproduce "vegetatively" from aboveground runners called stolons, or from wandering underground roots called rhizomes.

With this advantage, non-native hawkweeds like orange hawkweed can out-compete natives and dominate indigenous plant communities to the point where age-old ecological balances are irreparably damaged.

Some weedy hawkweeds are starting to adapt to unexpected habitats. Instead of occurring reliably in sunny openings, they are appearing in shady forest settings. This makes looking for them more difficult.

Hawkweeds are difficult to control, and the only known deterrent is herbicide. There has been little success with hand-pulling or biological controls.


Spotted Knapweed

weed with a bushy purple flower

Photo © Jim Stasz, USDA-NRCS PLANTS Database.

Unfortunately, the super-competitive spotted knapweed now covers hillsides that were once home to bright spring flowers and grasses.

Spotted knapweed

Spotted knapweed grows in every state in the U.S. In Idaho, it is found in every county. It first reached North America from its native Eurasia in alfalfa seed, in the early 1800s. Pink knapweed flowers line many western roads and fill sunny clearcuts and campgrounds. Knapweed honey is a big seller.

Knapweed plants grow to as much as three feet in height. Each plant can produce 5,000 to 40,000 seeds every growing season. As seedlings germinate, they form low "rosettes" the first year, then bolt up to their full height. This is the 2-year life cycle of biennials, which is how knapweed was described in weed identification manuals just a few years ago. But now plants 10 years old have been found, so it is called a perennial.

Small patches of knapweed expand into large monocultures where previously a blend of grasses and forbs co-existed. Outlying patches get their start when animals and people transport seeds.

Spotted knapweed produces its own herbicide that injures surrounding plants. Areas dominated by spotted knapweed also are more prone to erosion. Spotted knapweed is bad for elk in the winter. In the low elevation areas where elk go to escape deep snow, knapweed out-competes grasses.

Knapweed stands can be reduced by setting out insects whose larvae eat seeds or root parts. This has been an ongoing and promising program on the Clearwater National Forest since 1996.

There are several species of knapweed in the Northwest, all of which typically grow at low elevations and up into foothills, but are now moving into higher elevations.

What to do?

So, are the 37 weed sightings found on the Lolo Motorway in 2003 good news because the numbers are low, or bad news because they include two plants with proven bad records in the northwest?

At the University of Idaho Department of Plant Sciences, weed ecologist Tim Prather is sure: Of the five weeds discovered, urgent attention should go to the two hawkweeds. They are just getting started in the setting where they grow best. Forest visitors are likely to inadvertently spread them by picking a few. Hawkweeds can hybridize, too, making them genetic "loose cannons" with unknown abilities for adapting, spreading, or even cross-breeding with native hawkweeds. Weed eradication of smaller populations is cheaper and causes less environmental damage.

Ann Schwaller, of the Powell Ranger Station weed program, is worried. "The Motorway is perfect for hawkweeds," she says, recalling her experience with this weed in northeast Washington, where she saw it spread quickly through the remote backcountry. So, If you see any of them, please don't walk through them, pick them, or drive over them.

Plant names

Henbane—Hyoscyamus niger (high-oh-SIGH-a-mus NYE-jer), "pig" and "bean."

Goatweed—Hypericum perforatum (hie-PEAR-a-kum per-fore-AY-tum), "placed above shrines" and "pierced."

Native hawkweeds in north Idaho:

Western hawkweed—Hieracium albertinum (high-er-RAY-cee-um), "hawk," named for Dr. Albert Regel, a Russian plant collector.

White-flowered hawkweed—H. albiflorum (al-bih-FLORE-um), "white flowered."

Non-native hawkweeds in north Idaho

Orange hawkweed—H. aurantiacum (ore-ant-ee-AH-kum), "orange."

Meadow hawkweed—H. pratense (PRAY-tense), "of the meadows."

Tall hawkweed—H. piloselloides (pile-oh-sell-OYE-deez), "soft hairs."

Yellow devil—H. glomeratum (glahm-er-AY-tum), "grouped in a head."

Kingdevil hawkweed—H. floribundum (flore-uh-BUN-dum), "abounding in flowers."

Non-native hawkweeds in adjacent British Columbia

Mouse-ear hawkweed—Hieracium pilosella (pill-oh-SELL-uh), "soft hairs."

Common hawkweed—H. lachenalii (lack-enn-AHL-ee-eye), for W. de Lachenal, a Swiss botanist.

Whiplash hawkweed—H. flagellare (fladj-eh-LARE-ee) "whip-like."

New England hawkweed—H. sabaudum (sah-BAUD-um), "from Savoy."

Spotted hawkweed—H. maculatum (mack-yew-LAY-tum), "spotted."

Wall hawkweed—H. murorum (mew-ROAR-um), "of walls."


Timothy Prather. Weed Ecologist, University of Idaho. Moscow, Idaho. Personal interview. March 26, 2004.

Timothy S. Prather and S. Robins, et al. Idaho's Noxious Weeds. Bulletin 816. Moscow, ID: Agricultural Communications, University of Idaho, 2003.

Roger Sheley and Janet Petroff, eds. Biology and Management of Noxious Rangeland Weeds. Corvallis: Oregon State University Press, 1999.

1. Goatweed, also known as St. Johnswort or Klamath weed, is from Europe and Asia, and has been introduced around the world because of its medicinal and ornamental values. Unfortunately, it is also a noxious weed in rangelands, and has spread rapidly throughout the western U.S. Sixty years ago it occupied nearly four million acres in California, Oregon, Washington, and Idaho. During the 1940s two foliage-eating beetles were imported from foreign countries—North America's first use of biocontrol agents—and they successfully reduced goatweed in California. Since then, a root-boring moth, a leaf-eating midge, and a foliage- and flower-eating moth, have also been brought in. Today, we see goatweed come and go, depending on population fluctuations of those predatory insects. It is no longer considered the threat it once was.