Noxious Weed List

General Notice to Destroy Weeds

Aerial view of a river with two bridges in Boundary County ID, surrounded by a small town, forested hills, open fields, and mountains in the background under a partly cloudy sky. | Boundary County ID

By Order of the Board of Boundary County Commissioners

Notice is hereby given that, pursuant to the Idaho Noxious Weeds Law, Section 22-2408, Idaho Code, to every person who owns or controls land in Boundary County, that noxious weeds standing, being or growing on such lands shall be destroyed or eradicated by effective cutting, tillage, pasturing or treating with herbicides or other effective methods, or combination thereof approved by the County Weed Superintendent, as often as may be required to prevent the weed from blooming and maturing seeds, or spreading by root, root stalks, or other means. Upon failure to observe this notice, the County Weed Control Superintendent is required to proceed pursuant to the law and have weeds destroyed by such method as he or she finds necessary, the expense of which shall constitute a lien and be entered as a tax against the land, and be collected as other real taxes are collected or by other means as provided by law.

The following weeds are hereby officially designated and published as noxious, as per the authority vested in the Commissioner of Agriculture as per Section 22-2404, Idaho Code, and the Board of Boundary County Commissioners as per section 22-2405, Idaho Code:

Idaho’s 67 Noxious Weeds designated noxious by state law. The spread of these weeds and the damage they do to Idaho agriculture can be lessened through proper identification and control.

* present in Boundary County

Noxious Weeds of Boundary County

St. John’s Wort (Klamathweed or goatweed)

A tall plant with slender reddish stems and clusters of small yellow flowers at the top, set against a plain blue background. Green oval leaves grow along the stems, reminiscent of flora found in Boundary County ID Public Info displays. | Boundary County ID

A European perennial discovered in the Clearwater and Salmon River drainages in the 1930s. In 1950, over 600,000 acres were known in the state of Idaho. In the mid-1990s, various companies contracted for acreages of St. John’s Wort to be utilized for the manufacture of Prozac … but the ensuing research led to stronger and more disease-resistant varieties. Thanks a lot! St. John’s Wort has numerous bright yellow flowers and grows to three feet in height. There are fair bio-controls, but control is best achieved with 2,4-D and Escort.

Common Tansy

A cluster of green plants with numerous small yellow flowers flourishes outdoors in Boundary County ID, set against a blurred background of sky and landscape. | Boundary County ID

Common tansy is another escaped ornamental and European perennial. It can grow to a height of six feet, choking out other native vegetation. It is poisonous and possibly causes abortion in livestock, although most animals typically avoid it. It’s medicinal properties have been touted since the middle ages, supposedly helping gout and jaundice, but even small doses may lead to hemorrhage, convulsions and death from paralysis of the heart and lungs. There are no known bio-controls. The best control is through the use of herbicides; clopyralids, chlorsulfuron, picloram or dicamba in conjunction with 2,4-D.

Houndstongue

A tall green plant with elongated leaves and clusters of small, dark buds or flowers at the top stands beneath the clear blue sky, symbolizing the natural beauty of Boundary County ID. | Boundary County ID

Houndstongue is an Asian biennial that is a serious problem to the livestock industry because animals transport its seeds in their hair. It grows an erect stem up to three feet tall and has an extensive root system up to four feet in depth. It is this root system which makes it so competitive and especially difficult to control during its second season. First year plants are easily controlled with 2,4-D, but second year plants require picloram or dicamba and 2,4-D.

Hawkweeds

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The ancient Greeks thought hawks used the sap of hawkweeds to strengthen their eyesight, but they are now considered one of the most serious noxious weeds in the Pacific Northwest, where about 20 of the hundreds of species in the Old World, where the plant originated, exist.
Hawkweeds resemble dandelions and spread prolifically, by seed, stolons or rhizomes, especially in poorer, uncultivated soils, most often in meadows or pastureland, which should be treated early in the season with picloram or dicamba mixed with 2,4-D. Left untreated, hawkweeds force out useful plant species, increasing the cost of livestock production.

Spotted Knapweed

A close-up of a thistle plant with a single purple flower in bloom and several buds, captured in scenic Boundary County ID, against a blurred green background. | Boundary County ID

A European native, Spotted Knapweed is one of the most widespread of the noxious weeds in Boundary County and throughout the state. Each plant produces approximately 25,000 seeds, which are spread by wind, animals and people. The weed grows to about three feet in height.

Several biological control agents are available and provide fair control, including root boring beetles and moths, 2 seed head gall flies and the seed head weevil. Effective herbicides are also available.

Dalmatian Toadflax

Close-up of several yellow snapdragon flowers on green stems, with a blurred background in Boundary County ID. | Boundary County ID

Often mistaken for a snapdragon, Dalmatian Toadflax is native to the Mediterranean and was probably introduced as an ornamental plant, but quickly spread to invade dry timber and rangelands, mostly in the intermountain west. A perennial that grows up to four feet tall, Dalmatian toadflax spreads both by seed and creeping roots.
A defoliating moth is available as a biological control agent, though its effectiveness has not yet been proven in Idaho. Herbicides are available for control of this noxious weed.

Canada Thistle

Several tall stems with spiky, purple-pink flowers and jagged green leaves stand against a dark background. The clustered blooms evoke the wild beauty found in Boundary County ID’s landscapes, offering a glimpse of local flora for public info displays. | Boundary County ID

Despite the name, Canada Thistle is native of Eurasia introduced to Canada as a crop seed contaminant in the late 1700s. This noxious weed is found throughout the state, growing up to five feet tall.

Biological control agents, including the crown/root weevil, stem gall fly, seed head fly, stem boring weevil and defoliating butterfly are available, but control is considered only poor to fair.

Herbicides are also available that provide much better results if applied consistently.

Absinth Wormwood

A close-up view of a single plant with narrow, pointed leaves and clusters of small, round, light-colored buds on multiple upright stems, set against a dark background in Boundary County ID. | Boundary County ID

Absinth wormwood is an escaped ornamental imported from Europe, reducing available forage and tainting the milk of dairy cattle. The plant is recognizable by its strong, sage-like odor.

This noxious weed is a perennial that grows to approximately three feet in height, its leaves and stems covered with fine, silky hairs, giving the plant a grayish appearance.

Control of absinth wormwood is relatively easy, compared to other noxious weeds. Application of clopyralid, dicamba, 2,4-D, picloram or glyphosate when the plant is about 12 inches tall and actively growing (late June through mid-August) usually gives the best results. The plants should be mowed in early to mid-summer to promote active regrowth prior to fall treatment.

Leafy Spurge

Dense green field in Boundary County ID with small yellow-green plants. Two insets show close-ups: one of thin, green plant stems and another of yellow flowers with green leaves—helpful for Public Info or Public Service materials. | Boundary County ID

Leafy Spurge was brought to the United States from Eurasia in about 1897. A perennial weed with roots that often reach a depth of 20 feet, this plant secretes a milky latex which can produce blisters in humans and animals and can cause blindness is rubbed into the eye. Found throughout most of the state, several biological control agents, including several stem/root boring beetles, a shoot tip gall midge and a stem-boring moth are being evaluated for effectiveness, and sheep and goats have also been used to help check the rate of spread.
Herbicides are also available for control. Those who handle leafy spurge are encouraged to wear gloves and eye protection.

Field Bindweed

A member of the morning glory family, Field Bindweed is a Eurasian import that has spread throughout the nation, forming climbing vines up to six feet in length that often form dense mats and choke native plants and agricultural crops. This weed spreads by both seed and creeping roots, and seeds can remain viable for as long as 50 years.

There are no biological control agents that have proven effective against field bindweed, but active herbicide control methods can be. Tillage can also be effective, beginning two weeks after the plant appears and continue every two weeks through the growing season for up to three years.

Hoary Cress

Introduced from Europe in the late 19th century, most likely in ballast used for ships, hoary cress rapidly spread inland from both the east and west coasts and is now common throughout the nation, including most of Idaho. A perennial that grows approximately three feet tall, hoary cress spreads by seed and creeping root, and is adaptable to a wide variety of environmental conditions.

There are no biological agents which have proven effective against hoary cress, though some herbicides are registered for and effective on this noxious weed.

Yellow Toadflax

Similar to Dalmatian Toadflax, Yellow Toadflax is often mistaken for a snapdragon, though yellow toadflax originated in Europe and contains a poison often harmful to livestock. The smooth, generally unbranched stems grow to about three feet in height, its yellow flowers are about one inch long with an orange throat.

A defoliating moth, seed head weevil and flower beetle provide fair to good biological controls of yellow toadflax, and herbicides are also effective.