There are currently 36 officially designated "Noxious Weeds" in Idaho. These five are our choice for being the worst of the bunch, although all 36 have the potential to seriously damage Idaho's economy, landscape and wildlife.

Rush Skeletonweed

Description:
This perennial from Eurasia can grow four feet tall, with a taproot twice that length! In the early summer it produces yellow flowers. Mature plants can produce 1,500 flower heads.

This plant is built to reproduce. One plant can produce 20,000 seeds, which have a parachute of fine hairs which allow them to travel along wind currents up to twenty miles. Its leaves are inconspicuous, which gives this plant its skeleton-like appearance. This also gives it a leg-up in drought conditions, as does its mammoth taproot.

The Concern:
This noxious weed was discovered in Idaho in the 1960’s and has been spreading rapidly throughout Idaho. In places around Garden Valley, Idaho, this weed has already taken over the hillsides!

It displaces native species grazed by wildlife and livestock. It spreads from rangeland to croplands by seed. Its incredibly large root system makes it virtually impossible to pull by hand. In fact, attempting to pull it or cut it actually spreads the weed, since shoots can form from the lateral roots and from root fragments buried four feet deep.

Control:
It is difficult to kill this deep-rooted, rhizomatous perennial, since it seems to be tolerant to herbicides. For example, field trials suggest that a late fall application of Transline has a 95% control rate, although plants did show up three to five years later!  

Perhaps the best way to control rush skeletonweed is with biological control agents, like the gall mite (Eriophyes chondrillae), which affects the flowering buds, preventing seed production. It also affects the roots. But the likelihood of ever eradicating this noxious weed is a long ways off! The best we can probably do is slow its progress.

What you can do:
Learn to identify rush skeletonweed . Prevent the plant from going to seed by cutting or isolating plants before they flower. If flowering has occurred, bag and remove plants for burning. Use selective herbicides on already established plants. Control small infestations immediately. Clean equipment, tools vehicles and footwear before leaving infestations.

More Information:
http://www.oneplan.org/Crop/noxWeeds/nxWeed24.htm
http://www.nwcb.wa.gov/weed_info/skeleton.html
http://www.weedsbc.ca/weed_desc/rush.html

Eurasian Water-milfoil

Description:
This plant is Idaho’s first aquatic weed. It was discovered in Idaho in the late 1990’s, but has been spreading rapidly since then. It lives in lakes, ponds, and slow moving rivers. It is a slender, tan-stemmed plant with finely dissected feather-like leaves, arranged in whorls of four around the stem at each node. It doesn’t mind cold water and can reproduce in many different ways. It produces flowers and seeds that appear above the water, and it can spread by roots or runners in the ground.

It forms very dense mats of vegetation on the surface of the water. Needless to say, swimming, boating and water skiing are a problem when this fast-growing weed takes over!

The Concern:
This aggressive weed creates a dense canopy that shades out native plants. What is left is a poor substitute for waterfowl, fish and other wildlife.  When the plant’s leaves decompose at the end of the growing season, they increase phosphorus and nitrogen in the water. The plant also raises pH of the water, decreasing oxygen and increasing water temperature.

This extremely adaptable plant thrives in a variety of conditions and can easily be spread through boating activity and by anglers. It doesn’t take much to create a big, expensive problem in a short amount of time! This weed is already in McCall’s Payette Lake, as well as in some of the northern Idaho lakes.

Control:
Some places have pulled or cut the weed with underwater roto-tillers, but it almost always comes right back. Herbicides, like fluridone, have been used successfully in some places, although the results have been spotty in other places. In reservoirs, draw downs have sometimes helped keep the weed under control.  

A natural predator, the milfoil weevil, has also been effective in controlling the growth of this weed. The weevils lay eggs in the stems of the milfoil; when they hatch, the larvae eat the milfoil.

What you can do:
Learn to identify the plant. If you spot it, notify your local weed manager. Remove all plant material from your shoes, your boat, your trailer before you leave the boat access area. Make sure your bait bucket doesn’t have plant material in it; dump it on the land. Wash down your boat when you get home, as well as your trailer, your tackle and anything that could transport this plant to another body of water.  

More Information:
http://www.wapms.org/plants/milfoil.html
http://www.invasivespecies.gov/profiles/watermilfoil.shtml
http://nas.er.usgs.gov/plants/docs/my_spica.html
http://www.twinfallscounty.org/Weeds/eurasian_water-milfoil.htm    

Yellow Starthistle  

Description:
This member of the sunflower family emigrated from Europe. Its long taproot allows it to continue growing long after other annual plants have died. It has stiff, upright stems that branch from the base. The flower is yellow.  

A horse that eats this plant develops “chewing disease,” whose first signs are an inability to eat or drink. Permanent brain damage results, and horses can die of thirst and starvation. Strangely, cows and sheep are not affected, and can eat the plant until the sharp spines on the flower head make it inedible.

A single plant can produce thousands of seeds, which can last ten years or more in the ground. They act like dandelion seeds, floating in the weed. This weed will take over land from Idaho fescue, blue bunch wheat grass, and cheat grass, but not from sagebrush.

The Concern:
This noxious weed is a threat to all western rangeland, because it can spread rapidly on disturbed soils. Its root system allows it to grow faster and deeper than annual grasses, so its infestations can smother a large area and displace other vegetation.  

Control:
This is not a weed that you can pull out, so don’t try! Herbicides, like Tordon 22K, work, but are expensive. An insect, called the Yellow star hairy weevil, is making a sizable dent in the flowering heads of this weed. The weevil comes from yellow star thistle’s home land, near the Mediterranean, and after much research, have been released in areas like Hells Canyon.  

Goats have also been used successfully in Hells Canyon, to keep the weed population under control. The goats actually love to eat the seed heads off the thistle, and thus control the weeds’ numbers. 

What you can do:
Learn to identify the weed. Diligence can go a long way here! If you see this weed, notify your local weed manager. Once yellow starthistle colonizes an area, it is very difficult ­ and expensive ­ to eradicate.  

More Information:
http://www.invasivespecies.gov/profiles/yellowstar.shtml
http://plants.usda.gov/cgi_bin/plant_profile.cgi?symbol=CESO3

Leafy Spurge  

Description:
This deep-rooted, aggressive perennial has a lot of weed managers concerned! It came to America more than 100 years ago as an ornamental that some folks grew in their yards. It can grow more than three feet tall; but its roots can grow to more than twenty feet! The ability to maintain high root reserves means that this plant can recover quickly from chemical and physical damage.  

The small flowers are green and inconspicuous, but are surrounded by a pair of yellow-green, heart-shaped leaves that are often mistaken for flowers. When dry, the seed capsules shatter, ejecting seeds fifteen feet away from the plant. The seeds can live up to eight years in the soil.

Leafy spurge produces a milky latex that can cause blistering and death to some animals. It can cause blindness in humans if the milky substance gets in one’s eyes.

The concern:
This perennial noxious weed readily adapts to most environments; and has doubled in acreage every ten years since the early 1900’s. This plant is an economic and environmental catastrophe; it reduces rangeland productivity and plant diversity wherever it gets a foothold.  

It reproduces from spreading roots and by the production of enormous amounts of seeds, making it almost impossible to eradicate.

Control:
This is a difficult plant to control. Chemicals are only partially successful, because of the plant’s extensive root system. Goats and sheep have been used to “keep the yellow out” and to retard its spread. Several flea beetles are also being tested for leafy spurge control.    

What you can do:
Notify your local weed manager immediately, if you identify this weed. Once it’s been spotted in your area, persistence is imperative in gaining control of this weed.  

More Information:
http://www.oneplan.org/Crop/noxWeeds/nxWeed12.htm
http://invasivespecies.gov/profiles/leafspurg.shtml
http://www.mtweed.org/Identification/spurge/spurge.html  

Spotted Knapweed  

Description:
The pinkish-purple flower on this noxious weed is sometimes confused with a beautiful wildflower, but don’t be deceived; this plant can dominate an area if left to its own devices!  The weed is a prolific seed producer, which releases a toxin that hinders growth of plants around it.  

The plant grows two to four feet tall, with leaves that are one to three inches long. The purplish flowers are surrounded by stiff black-tipped bracts; thus the spotted appearance.

The Concern:
This Eurasian transplant tends to dominate an area, reducing plant diversity. A knapweed infestation can also increase surface run-off and sedimentation. This weed spreads readily in hay and on undercarriages of cars.  

Control:
Spotted knapweed reproduces solely by seed. The trouble is, each plant can release a thousand seeds, and they can live in the ground for seven years. This is one weed that can be removed by digging or pulling, when the soil is moist. Chemicals can also be effective, but the chemicals needed to control this weed are quite potent. Some experimentation has been done with root mining moths, a flower moth, and a root mining beetle.  

What you can do:
Notify your local weed manager. All visible spotted knapweed plants should be removed and destroyed. Treating the area with a herbicide to prevent reinfestation from seedlings is recommended.  

More Information:
http://www.oneplan.org/Crop/noxWeeds/nxWeed30.htm
http://www.invasivespecies.gov/profiles/spotknwd.shtml