Montana Invasive Weed Species

It is worth noting that not all noxious weeds appear to be weeds at all. Many of the weeds mentioned on this list can actually be pleasant in appearance. Many have pretty flowers or look more like a garden plant than something that will poison your horse. Many of our most problematic weeds were actually brought here intentionally by early settlers of this land to be planted in gardens and flower beds. The State has designated these plants as noxious weeds because they are invasive, not native, and toxic. That being said, there are also many detrimental native species that you should also consider trying to control. It is because they are native though that they cannot be put officially on the designated weed list. This is not a complete list of all the Noxious Weeds in Montana. It is a list of the ones we find commonly in Southwest Montana. For a complete list you can go to your County Weed District for a booklet. Please use this list to identify possible problem plants on your property. If you are unsure on any species you might have or have identified a certain species and would like to discuss control options please don’t hesitate to call for free estimates and consultation.

Leafy Spurge

This aggressive weed has invaded every county in Montana. It is terribly invasive, it has the ability to reproduce by seed or root. Its roots can run as deep as 30 feet, and each plant can produce thousands of very viable seeds that can be thrown from the host plant more than ten feet when the seed pod opens. This is one of the most detrimental weed species in Southwest Montana. It grows in all different types of conditions. It can be found in hayfields, river bottoms, hillsides, etc. You can identify this plant by the unique neon green flower head and by the white milky sap that runs through the leaves and stems. This weed is very difficult to control. Herbicide is about the only means of control. Pulling the weed will do nothing to control it as it has such an extensive root system. Some people do use sheep and goats who like to eat the plant, but this method will never totally eradicate the weed. This weed is toxic to humans, cattle, and horses.

Houndstongue

Almost everyone is aware of this weed. It is a terribly invasive weed species that is scattered all across the state. It produces a head of hundreds of sticky burs that stick to everything from your socks to your dog. This weed derives its name from the large leaves the plant has that sort of resemble a hound’s tongue. This is a biennial plant, meaning it only grows for two years. It reproduces entirely by seed. In its first year of growth it doesn’t produce any seeds, it makes a highly concentrated rosette of leaves. In the second year it produces a rigid stem sometimes up to three or four feet high that has a seed head that has hundreds of burs. During its flowering stage it produces small scarlet colored flowers. The flowers of the plant smell like hot buttered popcorn. After its second year the plant is dead but it leaves behind a stem with very sticky burs on it. Because seeds can be transferred so far away from the host plant it is a difficult weed to entirely control. It is also highly tolerant to most herbicide. If the weed population is small enough, you can pull the weed. It has a large tap root. Make sure to get it all. This weed can be anywhere but look for it especially in shaded areas, such as along fencelines, beside buildings, and inside willow clusters. This plant is toxic to all foraging animals.

Canada Thistle

This weed is easily recognizable by its thorny leaves and stems. Canada thistle is prominent across the entire state. It is a perennial plant with a creeping root system, so it reproduces by root and by seed. In its flowering stage it produces dull purple flowers which turn into silky seeds that blow away in the wind similar to the seeds of dandelions. Usually, you find this thistle growing in clumps, and it can get as high as four feet tall. This plant will easily choke out native forage of all types if left unchecked. Although not toxic to animals, studies suggest that the roots secret a toxin that harms native vegetation.

Field Bindweed

This weed occurs in varying landscapes across Southwest Montana. Many people are unfamiliar with this State listed noxious weed. You will often find it growing in people’s yards and fields or up vines and fencelines. This weed grows along the ground and as its name suggests it literally wraps itself around other plants and binds them, effectively “choking” other plants. It is a member of the Morning Glory family, so the flowers close at night. The plant produces small, bell shaped half dollar sized smooth flowers that are white to pink in color. This weed is very difficult to control once it is established. Toxic to horses.

Spotted Knapweed

Spotted Knapweed occurs everywhere across the state. Its leaves are small and stems are thin. The leaves are a dull greyish green. This weed is very invasive and can take over vast areas of land, especially in drier grasslands. The roots secrete toxins that inhibit growth of neighboring plants. In its blooming stage it makes small pinkish purple flowers similar to that of Canada thistle. At the base of the flower is a serious of black spots, which is where the plant derives its name. The plant has two cousins that are similar in appearance: Diffuse Knapweed and Russian Knapweed, that both occur in our area. This weed reproduces from the root and seed, but the root does not creep like most other perennials. It is not known to be toxic to animals, but it is inedible so when pastures are grazed it gets left with no competition, further allowing it to take over. Knapweed is somewhat difficult to control, but with the proper application and timing it can be controlled.

Whitetop

This weed occurs in isolated areas of Southwest Montana and is extremely difficult to get rid of. It is one of the earliest noxious to come out in the spring. It has gray green leaves that grow in an alternating pattern around stems. The top of the plant has white or cream-colored flower clusters that have a flat top. Look for this plant on ditch banks, inside of willows, in hayfields, and areas that get extensively grazed. This is another weed that seems to prefer growing in shaded areas. Whitetop can be difficult to identify because it has many look a likes, the most notable being Field Pennycress, or Fanweed. In their early stages, they are very similar in appearance. It is only until after the flowering stage that some easy distinctions can be made. Field Pennycress has a series of heart shaped seed pods below the flowering buds while Whitetop does not. If you see a plant that looks like Whitetop, it is probably Field Pennycress. This weed is toxic to cattle.

Oxeye Daisy

Here is a good example of a weed that looks like a pretty wildflower. Oxeye Daisy is scattered all across the state in concentrated populations. The plant looks like its name suggests… a daisy. It has showy white pedals and a yellow center. It grows from one to three feet tall. This weed closely resembles some garden variety daisies (if they aren’t in fact the same thing). Oxeye daisy has an extensive root system that makes control kind of difficult. Look for this weed to be in riparian areas or sub or flood irrigated fields.

Common Tansy

Common Tansy is most commonly found on ditch and river banks. It has leaves that resemble that of a fern. The plant later in the season makes tall shoots that produce clusters of bright yellow button-shaped flowers. They can get up to six feet tall. They can be identified before flowering stage by looking for last year’s stems… There will be many of them and they stick up into the air stemming from the current years leaf system. Common Tansy is another weed introduced by people as an ornamental. This plant is very common in the Bozeman area.

Tall Buttercup

This weed is found in irrigated hayfields or pastures most commonly. It has a small palm-like leaf that remains close to the ground. It creates several stems that can grow up to three feet tall. It has a bright yellow flower that is about a quarter in size if not a little smaller that almost appear to be “buttered” or glossy. It is very invasive in irrigated wild hay fields. Cultivation does nothing to control this weed, so regular haying is ineffective against this noxious weed. It is very competitive and in heavy populations can make up a good portion of the tonnage of a crop yield. It is thought to be toxic when grazed but not toxic in cured hay, although some people may argue that. Along with the fact that it is resistant to a lot of herbicides, it is also a difficult weed to get rid of because of the side results of spraying a hayfield that may also house other beneficial broadleaves such as alf-alfa or clover. This plant closely resembles cinquefoil plants in leaf structure and flower so professional identification is encouraged. Tall buttercup is in very isolated areas but if you think it may exist on your property you should take immediate action.

Diffuse Knapweed

Very similar to spotted knapweed other than the fact that the flowers are white instead of purple and does not have the black spots that spotted knapweed has. Also, diffuse knapweed doesn’t typically grow as high. This knapweed likes to grow in sandy soil in full sun and spreads only by seed. It is very difficult to identify until the flowering stage, but then is very obvious.

Russian Knapweed

Leaves of this knapweed aren’t quite as large as either spotted or diffuse knapweed. The stems are more prominent. The stems all separate close to the ground and shoot out, then bending back towards the center towards the top. The plants typically appear to have alternating pink and white flowers. It has an extensive root system that makes it difficult to kill. Like it’s cousin spotted knapweed, Russian knapweed secretes a poison that prohibits growth of neighboring plants.

Cheatgrass

This is the only grass species designated in Montana to be an invasive species. While not yet listed as a noxious weed, it is certainly a plant to worry about. Cheat grass has a unique set of characteristics that make it extremely competitive and detrimental to native species. For one, it is what is referred to as a winter annual, meaning it germinates in the fall, similar to winter wheat. By spring, this grass is ready to quickly go to seed and cure out by as early as mid-June. The seeds are extremely viable and have barbs on their structure which stick to clothing and animal hair. This grass is very tolerant to weather and very competitive. It is capable of creating a canopy over native grasses before they can get a good start. Cheat grass can be difficult to identify in its early stages, however it can be easily seen in colder months because by September or October it has already begun developing next years flush and is one of the few grasses in Southwest Montana that do so. When it develops seed heads, they appear to be hanging off the stock and appear greyish-green in color. By mid July, most of this species is curing or starting to cure. Much of the leaf structure will turn a red or yellow color. Large populations will have a canopy appearance velvety in nature if they are not disturbed. Cheat Grass is rapidly gaining public attention as being one of the most prominent threats to our native ecosystems. It should by no means ignored, but is a difficult species to control.

Hoary Alyssum

Hoary Alyssum has only recently come into the spotlight. This weed is relatively small and creates chutes that are rigid and hairy. In its flowering stage, it creates very small white flowers. It is competitive in soil that is poor in fertility, often times in washed out gravel beds or gravel roads or in gravely portions of hay fields. Although not terribly invasive, it has gained attention due to the fact that it is toxic to horses, even when cured in hay.

Dalmation Toadflax

This weed has become a terrible problem in much of our state. It has become an especially big problem in Yellowstone Park where pesticide application is prohibited. This weed produces yellow snapdragon-type flowers that run at the tip of the stem for anywhere from 3 to 9 inches. The leaf color is blue-green and they have a very waxy coating. The weed can reach heights of up to three feet though most, especially if growing in open conditions, reach one and one half to two feet. The deep root system and waxy coating on leaves makes this weed very resistant to herbicide. It is important to note that there are two look-alikes to this weed. One is yellow toadflax, which is not common. The other is Golden Banner, which occurs often and frequently in Southwest Montana. There are many ways to distinguish the two, but the easiest is that Golden Banner matures very early in the year, producing popcorn looking flowers by the first of June. Dalmation Toadflax matures much later and thrives in more arid environments than Golden Banner. This weed is suspected toxic to foraging animals if consumed in large quantities.

The above listed weed species are those that occur frequently in Southwest Montana, although there are many more weed species have identified by the State of Montana to be invasive. The above list does not include any species that are not currently designated as noxious weeds by this State, however there are many other plant species that should be considered problem species. Below, some of these species are listed. Many of these species, while they have not made the State list, are designated by many counties as being invasive or problem weeds.

Black Henbane

This plant thrives especially well in recently disturbed areas and in sandy or rocky soils such as ditch banks. In its early stages, Henbane sort of resembles a thistle plant. It’s leaves are a dullish green and it has small prickly spikes sort of like thistles, although they aren’t quite as prominent. Typically, by July, the plant has made a series of woody shoots that host a large amount of yellow flowers that turn black at their centers. Each flower creates a seed pod that carries thousands of seeds, and each plant makes anywhere from 10 to 100 seed pods. The seeds are viable for a long time, and this plant spreads very easily. The plant can be anywhere from a couple inches to 5 feet high. If conditions are right the plant can be about the size of a small Christmas Tree. Once the plant dies it leaves behind a very distinct woody skeleton with lines of seed pods. If you find last years skeleton, be sure to look around the area. Henbane is also easy to identify by the foul odor that it emits.

Wild Licorice

Licorice is one of the latest plants to emerge in the growing season. Look for this weed to start showing up from late July to mid August. It has fern like leaves that develop clusters of small cream colored flowers. Flowers turn into clusters of sticky burs that are about the size of a small marble. Licorice easily out-competes many native species of plants and spreads to distant areas because it’s burs stick to passing animals. The plant leaves behind a small woody stem that is reddish brown in color that are typically from six to 30 inches tall. The woody stems hold the bur clusters up high so as to snag on passing animals.

Common Mullein

Mullein is common throughout our area. It is easily identified by it’s very velvety leaves. The plant makes a cluster of leaves that grow closely to the ground before making a single woody shoot, at the top of which sprout a series of yellow flowers on the top 6-16 inches of the plant that sort of resemble an ear of corn. Mullein can grow very tall, sometimes over 10 feet but the average plant will peak at 4 to 5 feet. Mullein is a biennial plant, therefore in it’s first year of growth it never leaves the rosette stage. After a winter dormancy, it will make its woody shoot. Mullein requires lots of sunlight and loose, open soil to grow. It is common on dry creek and river beds, barrow ditches, and cut banks. It’s hairy leaves make it difficult to control.

Wild Iris

While Montana designates Yellow-Flag Iris as a noxious weed in Montana, it’s purple flagged cousin is not on the list because it is Native to Montana. Although it is Native, it is an extremely aggressive weed that will easily take over in marshy or subby ground. The leaves resemble grass but are much thicker and often grow in a ring-like structure on the ground with a radius of one to two feet. In mid-June the plant rapidly grows shoots that bloom into a purple flag like flower that is dark purple on the tips and lightens toward the center, where it turns yellow with black spots. The flower exists for a week or two, and then the plant goes dormant. Iris is difficult to control as it has a high resistance to most herbicides.

Kochia

Kochia is a very problematic weed both in residential and agricultural settings. It is a dull green plant with alternating flowers. It is an annual and spreads entirely by seed. This weed is extremely invasive. Small, almost invisible, pods produce microscopic seeds by the thousands. The plant never really appears to flower. It will take over gardens, yards, barnyards, fallow, and pastures very rapidly. This is the most common plant that turns into “tumbleweeds”. It is thought that this is how the plant spreads seeds… the seeds fall out of the pods as the skeleton of the weed is blown across the ground. Everyone has seen kochia and probably knows about it. It is very persistent and difficult to get rid of.

Burdock

Burdock is common in our region and is the plant responsible for the olive-sized burs that we find in our clothing and on our pets. This weed can survive in a vast array of environments, but seems to like rocky soils and shade. The plant generally has massive heart-shaped leaves that somewhat resemble that of a tobacco leaf. It will grow anywhere from one to 5 feet off the ground. The leaves can be big enough to cover a nightstand table. A small purple colored flower, similar to that of knapweed or Canada thistle, is produced in pods that then turn into brown burs. Although not horribly invasive, this weed is unsightly and the burs it produces can be very annoying.

Showy Milkweed

Showy Milkweed starts to show mid to late June. At first, small red shoots with single or double leaves will poke about 8 inches out of the ground. The plant will then develop into a large plant, usually with multiple stocks coming from a single point in the ground. It has large tobacco-like leaves. Later in the season the plant will develop spherical clusters of pinkish-red flowers that later turn into seed pods that are about the size of a tennis ball but oblong in appearance. When broke open, the veins of the plant will emit a thick milky colored liquid. This plant is extremely difficult to get rid of. It has a very high tolerance to herbicides.

Musk Thistle

Musk thistle is very hard to miss. It is a biennial which produces a rosette of thorny, dark green leaves its first year that grow across the ground but not very high. In its second year it grows into a very wicked looking thistle plant. The entire plant is covered with thorns. It produces large, spherical shaped flowers that are very dark purple in color. The plants can be massive, up to 5 or 6 feet tall. This weed is very invasive and will come on very strong if seeds are present in disturbed areas. The plant sucks up a great deal of nutrients and water from its environment, and makes competition difficult.