War of the weeds: Invasive plants are transforming landscapes, harming crops and animals


Russian knapweed is one of the most problematic noxious weeds in Montezuma County. Credit: © Stan Shebs

This is the second in an occasional series about weeds and weed management in the Four Corners. The first article ran in the April 2018 issue.

Since the invention of agriculture, farmers and gardeners have cursed weeds – any plants that grow where they aren’t wanted.

But weeds today have grown into much more than a nuisance. Spread by hikers, livestock, motor vehicles, pets, birds, and the wind, weeds are everywhere, and battling them has become a major endeavor.

What are “noxious” weeds and why is it so important to get rid of them? The word “noxious” means physically harmful or destructive to living beings. The word also includes the idea of having “a harmful influence on mind or behavior, especially morally corrupting” according to Merriam Webster.

We could argue about how morally corrupting a thistle is, but a noxious weed is defined as a non-native plant that has one or more of the following characteristics:

may be poisonous to wildlife and livestock;

aggressively invades and/or damages agricultural or native plant communities;

may carry destructive insects, diseases or parasites;

is considered to be detrimental to ‘sound’ land management practices.

Bonnie Loving, director of the Montezuma County Noxious Weed Program, sums it up by saying that a noxious plant “is injurious to agricultural or horticultural crops, natural habitats or ecosystems, or humans or livestock.”

Loss of habitat

The county’s weed department was created under the 1996 Colorado Noxious Weed Act, which states that “certain undesirable plants constitute a present threat to the continued economic and environmental value of the lands of the state and if present in any area of the state must be managed.”

The Colorado Department of Agriculture places noxious weeds into three categories, but local governments can move plants from one list onto a higher-priority one, said Loving.

“List A requires eradication, List B requires control and suppression, and List C highly recommends control and suppression. Montezuma County has many that are on the state List B that we have moved onto our List A, such as cutleaf teasel.”

Eddy Lewis, a member of the Weed Advisory Board for the weed program and owner of Southwest Weed Control, told the Free Press he thinks  hoary cress (Cardaria draba), also called whitetop, is one of the most troublesome weeds locally.

“It’s a deep-rooted mustard perennial. It’s really hard to control – you can spray it, but if I treat it this year it will be back the next year. Nobody really knows if it is coming back from the root, or the seed. It’s one of those things that you have to keep re-treating,” he explained.

One of the concerns about noxious weeds is the harm they can do to livestock. For example, leafy spurge (Euphorbia esula), which is on the county’s A list, is toxic to cattle and elk.

Spotted knapweed, also on the county’s A list, can take over and alter a habitat so much that it reduces forage for wildlife. It invades ecosystems above 7,000 feet and was responsible for loss of prime elk habitat in Montana and Idaho – a disaster, according to Lewis, since the elk moved out due to the loss of the natural forage. In those states, managers had to spray herbicide in the national forest with helicopters to get rid of the weed – an expensive endeavor.

“We’re really concerned about it taking over the forest – we’re really trying to stay on top of it,” Lewis said.

Russian knapweed (Acroptilon repens) has the same potential at lower elevations. Not only will it take over, but it is very toxic to horses. “The plant has a chemical that affects the animal’s brain, essentially paralyzing the jaw muscles. This is non-reversible,” said Loving.

Russian knapweed is currently found on 1 out of 20 properties in Montezuma County, she said.

“It is so widespread it is hard to know where to start solving the problem and reclaiming the land. It is also very hard to manage. If we let Russian knapweed completely take over, our deer and elk will move out, we won’t have hay for our horses or cows, and our farmers will give up on their crops, since it’s too expensive to be constantly fighting the weeds,”

Lewis agrees Russian knapweed is one of his biggest concerns. “It will invade into alfalfa, roadsides, waste areas, into anything.”

“Thousands of acres of crops, rangeland, and habitat for wildlife and native plant communities are being destroyed by noxious weeds each year,” states the Colorado Noxious Weed Act – and that was in 1996.

“They are taking over,” said Loving.

50-year seeds

The fact that these invasives spread so quickly and are so hardy is a huge problem. Many are drought-resistant and will crowd out native and agricultural plants.

Lewis said that some species’ seeds can last up to 50 years – bindweed (Convolvulaceae sp.) has a hard shell that will not break down easily, and so does spotted knapweed.

Cheatgrass (Bromus tectorum) is shallow-rooted, and Lewis has seen some patches of it fail this year due to the drought. But, he continued, “It takes five years of actually getting rid of the cheatgrass seed for it to go actually go away.”

Another problem is that weed seeds are blowing from one property to another. Sometimes birds spread the seeds, and wildlife also pick the seeds up and carry them from place to place.

Some of these plants are also “allelopathic,” meaning they are “monoculture species” that emit biochemicals that impact other plants – usually harmfully. Allelopathic inhibition is when one plant kills or destroys another by diminishing seed production, germination and growth, or altering the soil composition.

Loving used the example of Canada thistle to explain this situation: “In our area we are well known for our quality alfalfa. It is a problem when neighbors to alfalfa fields have Canada thistle problems. Canada thistle is a monoculture species that will form dense stands and continue to grow outward, displacing the native vegetation or crops that were once there.”

Loving said most herbicides utilized to control Canada thistle also kill alfalfa, meaning that ranchers can lose product whether or not the thistle is treated. Ranchers may have to rotate their crops to get rid of the thistle after applying herbicides.

“Overall, the outcome is poor for alfalfa. The rancher will lose significant crop yield to the Canada thistle and most people purchasing the alfalfa don’t want it contaminated with thistle,” explained Loving.  “When you have a farmer growing a nice alfalfa field next to a crop of Canada thistle it is a big problem.”

Russian knapweed is also an allelopathic plant. Lewis said what is especially insidious about Russian knapweed is that “it puts out its own chemical that kills everything around it – but it doesn’t affect whitetop.” Thus, Russian knapweed and whitetop are commonly found together, making it difficult to control either one.

Another concern about noxious weeds is their impact on recreation. “A lot of people go into the backcountry, and they don’t want to be walking through musk thistle,” said Lewis. “We try to educate as many types of people that are going into the backcountry as we can,” he said. If citizens know what the noxious weeds are, and how seriously they can impact the environment, then they can help with the control of these invasives.

A variety of tools

Loving said her department has all kinds of programs to help residents learn about and control noxious weeds. “We are constantly putting on public seminars and promoting other natural-resource organizations who put on seminars. We strive to help our community become stewards of the land.”

The county offers a cost-share program that reimburses landowners for some of the costs of weed management. “Our program does property visits for free. We have a lot of people calling in asking for our help in identifying what they have growing on their properties. We get phone calls about animals dying and the landowners aren’t sure what is killing them, so we will come out and help identify poisonous plants both noxious and native,” said Loving.

How are these plants eradicated or suppressed? Loving said the best strategy is to use a combination of methods, with identification and education of the problem weeds required before taking any other steps.

“Our program recommends using an integrated management approach to dealing with noxious weeds. This means, use more than one tool in your toolbox. The categories of these tools are biological, mechanical, chemical, and cultural. We use all of these methods. You have to deal with noxious weeds by a case by case scenario.”

Herbicides used are primarily 2,4-D, and Escort, according to Lewis. These will break down in the soil quickly and thus not harm nearby native plants.

Lewis said he has been in business and on the Weed Advisory Board for over 21 years, and in that time has contracted with the county, Forest Service and private property owners to spray herbicides and remove noxious weeds. He said that 2,4-D is the standard because it degrades relatively in a few weeks and is efficient.

According to Tom Hooten of the Colorado State University Extension Office for Montezuma County, the time it takes for an herbicide to break down depends upon soil pH, microbial activity, moisture and temperature. Basically, depending upon the chemical content of the herbicide, the lower the soil’s pH and the higher the moisture and temperature, the more quickly the herbicide will degrade. “2,4-D is probably one of the safer ones,” he said.

Late fall is the best time to treat with herbicide because the plant will absorb the herbicide into its root system, said Loving. She said some “herbaphobic” people want to till instead of applying herbicides, but when it comes to Russian knapweed tilling may actually exacerbate the problem since it breaks up the roots, which leads to the weed spreading.

Hooten acknowledged that “pesticides and herbicides are prevalent throughout the environment globally – residues are found in Antarctica and in the ocean…You are never going to totally eliminate your exposure, but you can try to minimize your exposure and minimize the risk of exposure.”

Lewis, who makes a living spraying noxious weeds, said the important thing is to be educated about the plants and the herbicides. “Know when to treat the weeds, what time of year is best to treat which plant, and learn which chemicals are the most effective.”

He is a strong proponent of knowing where the nasty plants are, in order to monitor them and keep them under control.

“Be wise,” he said, “Don’t waste your time. With the problem weeds like whitetop and Russian knapweed, you have to retreat and watch.”

Keeping an eye out

The Montezuma County Noxious Weed Program works to do just that, locating weeds throughout the county. “Every year we drive public roads and map the noxious weeds we can see on private and public land. This way we can get an understanding of which species we have where and the rate they are spreading,” Loving said.

Landowners are notified when they have noxious weeds on their lands, and need to file a weed-management plan with the county. The Noxious Weed Act requires eradication of all plants on the A list, so if one of those species is found, either the county or the landowner must have an eradication strategy in place.

Loving said the county has a legal obligation to enforce weed management on all county properties. In 2017 they had to take enforcement action on several properties, and so far in 2018 they have had a similar number. If the landowner does nothing after the county contacts the owner and requests a management plan, the county has the authority to eradicate the weeds and charge the owner for the costs through a lien on the property.

Lewis urges vigilance about noxious weeds. “The more eyes we have out there, the better. Early detection is the key,” he said.

Hooten said his office is available for information and advice as well. He recommends that people read all labels closely. “It’s required that we control these weeds – it’s a legal issue. It’s also a part of the law to follow the herbicide directions,” he said.

If you find plants on your property or neighborhood that might be noxious weeds, call the Montezuma County Noxious Weeds Program or the Extension Office for help. You can also attend the next Noxious Weed Field ID seminar offered by the county.

“We strive to help our community become stewards of the land,” said Loving. “Land is a scarce resource and we should do all we can to protect it and make it healthy.”




From June 2018. Read similar stories about .