Canada thistle surpasses leafy spurge as N.D.'s top noxious weed

Canada thistle may look beautiful and innocent with its bright yellow and purple flowers towering above North Dakota's crops and prairie grasses, but looks may be deceiving.

Canada thistle may look beautiful and innocent with its bright yellow and purple flowers towering above North Dakota's crops and prairie grasses, but looks may be deceiving.

The plant is an invasive species, believed to have arrived in North America in the 1900s by dirt used as ballast on ships.

According to, invasive species tend to leap into a void, quickly colonizing and crowding out native species. This is why Canada thistle, native to Eurasia, is now the top noxious weed in North Dakota and considered to be naturalized worldwide.

Canada thistle has been a North Dakota resident for more than 300 years, but the recent span of wet years has caused a sudden burst in the weed's growth.

"Around 1994, we went into a wet cycle with the crop land," says Rich Dollinger, weed specialist at North Dakota State University in Fargo. "And now Canada thistle is the worst perennial weed we have."


Canada thistle recently overtook leafy spurge as North Dakota's top noxious weed-- infesting more than 1 million acres within the state, while leafy spurge infests 861,000 acres.

"It is clear with just over 1 million acres of Canada thistle and 860,000 of leafy spurge this year that Canada thistle is the top weed and a larger problem," says Roger Johnson, agriculture commissioner for North Dakota. "In 2006, there was just over a 20,000-acre difference between leafy spurge with 1.16 million acres and Canada thistle with 1.14 million acres. In 2007, we saw a 140,000-acre difference between the two weeds."

The weed

Canada thistle is a member of the Asteraceae, or sunflower family, often referred to as the creeping or California thistle, according to the North Dakota Department of Agriculture. The thistle is an herbaceous perennial that reproduces through a spreading root system and seeds that can lay dormant several years for ideal growing weather.

Thistles can grow nearly 4 feet tall, with stems that are erect, grooved, hairy and nearly smooth, spiny mature leaves, branching root systems and colorful flowers. The leaves are alternate and oblong, with crinkled edges and spiny-toothed margins that form spines.

Canada thistle blossoms in late June through August. The flowers vary in color, from pale blue to shades of purple with the occasional white ecotype.

Thistle seeds are about an eighth of an inch long, flat and brownish in color, with a tuft of hair at the top. Each plant can produce as many as 5,300 seeds, with the average plant producing 1,530 seeds. Seedlings start to emerge in May, followed by spring rosette formations and rapid vertical growth in June. Canada thistle seeds can remain fertile within the soil for up to 21 years and can cover 5,000 feet.

Both horizontal and vertical Canada thistle roots are able to produce new shoots and entirely new plants from root segments.


Thistle growth is limited by temperatures that exceed 86 degrees Fahrenheit and short day lengths.

The effects

"It is a weed that grows more in wet weather, but can be found growing all over," Johnson says. "Canada thistle spreads through underground ribosomes and the seeds grow well in lowlands. Pasture and hay land with decaying matter such as old buildings also provide good growing conditions."

According to an article in the Minot (N.D.) Daily News, recent studies on cow-calf pairs indicated that nutrients and moisture is being lost from the beneficial native plants in pasture and CRP lands because of noxious weeds, like the Canada thistle.

Livestock dislike Canada thistle's prickly leaves and stems, leaving it to spread and take over pasture land.

"Historically, farmers were more concerned with the thistles in their crop lands as a weed problem," Johnson says. "Now it is an ever-growing problem in pasture and hay lands as well as everywhere else."

Canada thistle grows mostly in patches, but some thistles have spread out over a much wider range in pasture, hay and CRP lands.

"Canada thistle will reduce fields' yields down to almost nothing," Johnson says. "They affect the yields and cause a huge drag on the economy and intrude on production capacity and livestock feeding abilities. Livestock can't eat the thistles and crops lose space from the weeds. The weeds have also become extremely expensive to control."


Elimination and control

North Dakota counties participate in road side spraying, cost share programs and bio-control programs to help control noxious weed populations.

"We have started a bio-control program with stem-mining thistle weevils out of Montana. This program has only been in process for four years," Johnson says. "Leafy spurge was controlled with flea beetles, which is one reason for the thistle outbreak. We have been working with the spurge flea beetle for about 20 years and are just now seeing the effects. It takes a number of years to see the impacts from these insects."

Spurge flea beetles, used to control leafy spurge, are collected with sweep nets. The Canada thistle's natural predator, the stem-mining thistle weevil, has proven more difficult and expensive to collect and transport.

"Leafy spurge acreage has dramatically declined because of chemical and biological control, leaving Canada thistle as the top noxious weed," says Rod Lyn, noxious weed specialist at NDSU. "Canada thistle's acreage increase is not a dramatic increase like leafy spurge was. We have a lot more tools to deal with the Canada thistle outbreak that we did with leafy spurge."

Biological control of Canada thistle has been an ongoing process since the 1960s. Although successful biological control has occurred in North Dakota during the past four years -- mainly in Bowman County near the Bowman Haley Dam with stem gall flies and the stem-mining thistle weevil -- the program has seen limited success rates.

"The program has been around for awhile, it works occasionally but is not as effective as flea beetles were on leafy spurge," Lyn says. "The program is worth a try, but I wouldn't put my sprayer away."

North Dakota adopted no-till and minimum-till systems, which help eliminate the spread of Canada thistle seeds.


"It is easier to control the weeds in tilled land than it is in natural grass or pasture lands because of the chemicals that can be used on tilled farm land verses natural CRP or pasture lands," Johnson says.

A number of chemicals can be used to control Canada thistle, although 2,4-D and Wide-Match, used with row crops, do little to help control the weed.

"There are several chemicals for eliminating Canada thistle," Lyn says. "Milestone, Tordon, Transline, Curtail and Overdrive are just a few."

Herbicides must be labeled safe for CRP and pasture lands to be used for Canada thistle control. Lyn says Milestone, Tordon, Transline, Curtail and Overdrive are labeled for CRP and pasture land.

Roundup, or glyphosate, is another chemical that does a good job of controlling Canada thistle.

Prairie Ag Services Inc., owned and operated by Mandy Hagen, is a local crop scouting company located near Fordville, N.D. The company provides crop scouting and consulting, and soil sampling and analyzing and also provides pre-harvest Roundup in the fall to kill the weeds main source of growth: the roots.

"Applying pre-harvest Roundup in the fall seems to minimize the weed's population, especially in specialty crops," says Hagen, who also is a crop consultant and agronomist.

"We've seen a gradual increase of the problem throughout the state," Hagen says. "Moreso in CRP land this year and in row crops because of the increase in acres. I don't think it's that huge of a problem yet, but it does spread rapidly. If we can nip it in the bud, literally, then we can control the problem."



"It's an expensive weed to control because it is so spotty," Hagen says. "Farmers don't always want to spend the money to spray a whole field, so they spot spray, and this doesn't eliminate the thistle problem. Unlike wild mustard, thistles spread through roots and just keep growing. It's a yield-robbing weed."

Several counties provide refunds for noxious weed elimination costs, also called the Land Owners Assistance Program. The program helps landowners, producers and operators with the costs associated with noxious weed elimination.

Cass County is one of many counties in North Dakota offering the program. Cass County's refunds include 80 percent of chemical use and 50 percent of application processes, or as much as $3.50 an acre with a maximum refund of $1,000 per applicant per year for noxious weed control.

"They can go to and look under weed department. The applications and requirements for the application process and receipts, along with a list of chemicals will be online," says Stan Wolf, weed control officer for Cass County. "The lands that Cass County holds eligible for reimbursement are noncrop or CRP lands, such as driveways, lawns, pastures, hay lands and yards."

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