control musk thistle

The musk thistle also goes by the name nodding thistle. Scientifically, people regard it as the Carduus Nutans L. It can either be a biennial plant or a winter annual. However, in some cases, you could find that it bears the characteristics of a summer annual. In this article we will talk about how to control musk thistle.

Where it occurs as a winter annual or a biennial species, it will give rise to a rosette in the first year. In the year that follows, it will produce flowers and seeds.

Where it grows as a summer annual, the story is quite different as it grows fast. It completes its life stages from sprouting through to seed production in one growing season. The seedlings emerge during the growing season when the conditions are ideal. However, you should note that the plant will be in the rosette stage for most of its life stages. Its length by the end of the season will depend on how favorable the conditions are.

As is the case with the rosette, this species’ height will depend on the prevalent conditions. It can be as short as a few inches, or it can thrive in the ranges of six feet. As a mature plant, the musk thistle will have an extensive taproot that reaches deep into the soil. Near the surface, the root will appear hollow.

Fun Fact

Did you know that mature musk thistle plants die after seed production? Well, now, you do!

How to Identify Musk Thistle

Knowing how to identify this species goes a long way in its eradication. We will start with the hairless leaves, which are easy to distinguish from other plants. These have a dark green hue with a light shade along the midrib. On the side are coarse lobes, which may have a silver-gray tone along the margin. They arrange themselves alternately on the stem and feel smooth to the touch on both sides.

Each of the lobes has at least three points with a spine at the end, which can be white or yellow. The leaves attach to the stem, which is equally hairless. In so doing, they create somewhat of a winged appearance which works great for the outdoors.

In the optimal conditions, this species will flower, often starting in May. When the blooming starts, it lasts at least seven weeks, giving rise to breathtaking flower heads. They have a puffy appearance and will range in hue from violet through to purple. However, it is not uncommon to come across a white flower head. Their puffy appearance distinguishes this thistle from other thistles that have a shaving brush look.

From the flowers comes the seeds, which are ready for dispersal in as little as seven days from the day of blooming. The seeds have a straw-like appearance and will be at least a quarter-inch in length. One side appears straight while the other curves. At the tip, you will find a protrusion while the base is round.

A single plant can give rise to over one hundred heads. Of course, this will depend on how favorable the growing conditions are. One long stem will host one or two heads, which will have individual heads. The lower branches can support as many as between two and nine heads. You can distinguish the main head by its appearance. It will be at least one and a half inches long and will be lone, with a slight bend in its appearance.

It is interesting to note that pre-budding, flowering, and seed maturity take place at the same time on a single plant. No time wasted here!

Musk thistle plant

Musk thistle plant

Origin and Distribution

The musk thistle is not native to the United States. Reports of its sighting in the US started towards the end of the 19th century and the welcoming of the twentieth. States such as New Jersey, Pennsylvania, and New York were some of the affected regions.

In 1932, there was a sighting of this plant in Kansas, where its distribution to date remains dense. You will mostly find it in the northern central and eastern parts.

This species can survive in a wide range of habitats. From roadsides to pastures to meadows, it can take whatever gets thrown its way and survive. Generally, it does not cause much of a problem when it occurs in tilled croplands. However, it can germinate when land is fallow or after the planting of wheat in the fall.

This plant spreads at fast rates and takes over the ecological system. As such, authorities have been on high alert as to its growth. Kansas would be an excellent example in this case. The law in the region prevents anyone from planting this thistle and calls for the eradication of the existing plants.

Musk Thistle US

Reports of Musk Thistle in the US

Threats: Why is the musk thistle a noxious weed?

This thistle competes with native plants for nutrients and water. In so doing, it ends up altering the existing conditions, making it hard for the native species to survive. Forage production thus reduces, and both humans and animals suffer the effects of this. Plus, this weed contaminates hay and decreases its quality.

This taking over of land also affects the economy. Where a farmer loses desirable plants to this weed, the productivity of the land reduces. And so does the income-earning capacity of the farmer in question.

An excellent example of this reduced capacity would be Colorado, where this weed has infested over forty thousand acres. Think of how badly the economy hurts from this infestation.

How to control Musk Thistle

Managing invasive species starts with preventing their further spread. Given that this species reproduces through seed production, you should ensure that mature plants do not get to the seeding stage. Here is how you can deal with these plants effectively:


It is easier to prevent the spread of musk thistle than to deal with established plants. Say, for example, you come across some plants on your farm. You should eradicate them long before they have had a chance to produce seeds.

Where you have mature plants established on your farm, you need to take action immediately to avoid spreading of the thistle.

Mechanical Control

If you see some plants around the farm, you can remove them by hand, ensuring that you get them by the roots. You can then discard the debris in a way that will ensure they do not sprout again.

Digging below the crown of a plant gets in the way of healthy development and has proven to be an effective means of control. Dig up the plant and remove it from the field and discard it in the best way possible. Where the dug-up plants are mature and in their budding or blooming stages, burn them. Failure to do so will encourage the production of seeds. And this could lead to the spread of the thistle to nearby areas.

Mowing is another excellent means of keeping the plant from spreading. Aim to do this when the plants are in full bloom. Note that this is not a one-off control method as subsequent cuttings will be necessary. You will find that once you cut the mature plants, new stems can grow at the base. And failure to cut them in time can lead to the growth of more thistles as the root system will still be healthy.

Chemical Control

Chemicals are useful in the control of musk thistle plants. However, you should exercise caution when using chemicals as they could affect desirable plants in and around your farm. Applications can take place during two seasons: spring and fall.

Tordon 22K, Milestone, Transline, Perspective and 2,4-D are commonly used chemicals to control Musk thistle.

Spring Applications

Herbicides used on the thistle plant in spring are effective. You should time the application to when the thistles are in active growth, during the rosette stage. At this time, the soil has thawed, and the flower stalk is yet to start developing, which is often towards the start of May.

Note that the application’s effectiveness significantly reduces when the air temperatures fall, or rainfall starts. The same goes for when other activities delay the application, such as farm work. When the flower stalks start developing, the herbicides will not have much effect on the plants.

Note that the use of herbicides will not prevent seed production in blooming plants. But instead, it will reduce the number of seeds per plant. Thus, it is best to start the application before flowering begins.

Fall Applications

Fall is another great time to apply herbicides to the musk thistle plants. Aim for the beginning of October when the soil starts freezing. At this time, the musk thistles are in the rosette stage. However, the soil moisture content and cold temperatures may get in the way of effectiveness.

Fall is the best time for herbicide applications, as it shows better results as compared to applications in spring. For one, there is a reduced chance of drifting of the spray to desirable plants. Also, you have more time available to apply the chemicals than you would in spring.

What is the best time for application?

You will achieve better results if the musk thistle plants are in ideal conditions. That is, the soil moisture content is high, and the temperature is at least 90 degrees Fahrenheit. Aim to apply the chemical at least twelve hours before rain falls as this will give the plants more time to absorb the herbicides.

Things to consider when using chemical methods

The use of herbicides, though useful, is not always the best option. These chemicals may affect desirable plants on your farm. Some can remain in the soil for weeks leading to months, changing the growing conditions of other plants. Also, a lack of caution during application may prove dangerous. You should, therefore, use the chemicals as per the stated guidelines and use those suited for controlling musk thistle weeds.

Biological Control

This technique involves the introduction of natural enemies to infested areas. Not only does this reduce the number of plants in an area, but it also ensures that the plants do not spread to neighboring regions. In this control method, you have two options: the flower head weevil and the rosette weevil.

Both these species are native to Europe and have undergone extensive research to ensure that they would not harm desirable native plants. However, it is essential to note that the flower head weevil has been known to attack other thistle varieties. Using it on land with other thistles you intend to keep would therefore not be an ideal choice.

These weevils find their way to the musk thistle populations in two ways: introduction to infested areas and spread. Suppose you introduce them to your field, this would fall under a release mechanism. As they feed on the musk thistle weeds, they will move from one area to the other in search of other such plants. And this would constitute spreading.

Using natural enemies to control the spread of musk thistle is time-consuming. In most cases, you will find that they will work on the infested areas for up to ten years. During this time, they can wipe out at least half of the population.

Weevils are an excellent choice for areas where other control measures would not be viable. Also, they have proven to be effective in long-term control. Let’s take a closer look at these weevils:

The Flower Head Weevil

These weevils emerge as adults in the spring. When mature, they have slender bodies with some golden spots on their wings. They measure about a quarter of an inch and have a distinctive short and broad snout.

They actively seek musk thistle rosettes and feed on the foliage of the same. However, feeding on the leaves does not affect the plants much as little damage accrues from this. As the adults feed, the females mate and start laying eggs as the thistle begins to bloom. The eggs thus lie on the flowers covered with chewed plant material, which gives them a scale-like appearance.

One female can lay as many as 100 eggs in one lifetime. The more the weevils, the more the eggs there will be on the flower bracts.

The eggs remain on the bracts for at least six days before they hatch. Now, here comes the fun part. The larvae make their way through the tunnel into the flower where they can feed on the young seeds. As a result, the stem holding the flower can turn brown, indicating damage to the plant parts. Alternatively, the flower head’s hue could change to brown.

The feeding on the young seeds takes place for about three to four weeks. From here, the pupation stage takes place, and this lasts for about one to two weeks. All this takes place in the flower, and when the adults form, they seek new sites under developing rosettes, wooded areas, or ground litter. Here, they remain dormant as they await the next spring.

These weevils give rise to one generation per year.

Rhinocyllus conicus, Flower Head Weevil

Rhinocyllus conicus, Flower Head Weevil

The Rosette Weevil

Compared to the flower head weevil; this species is much smaller. Unlike the flower head weevil with a broad snout, this weevil has a narrow one. A similarity to the flower head weevil would be that they both give rise to one generation each year.

Adults remain dormant in the summer and will emerge towards the start of October. They, too, feed on musk thistle foliage and will work on the leaf tissue from the underside.

The females lay eggs in fall and winter, which they lay on the underside of the leaves. Alternatively, the females can lay the eggs on the rosette crown. As the larvae emerge, they create tunnels into the plant crown, where they damage the plant parts. Once they complete the larval stage, they head to the soil where pupation begins.

These weevils, given time, can show more results than the flower head weevils. While the flower head weevils work to stop seed production, they do not weaken the plant. As such, they are best for use when preventing the spread of the musk thistle. But the rosette weevils attack the plant, which could weaken it such that it can produce fewer flower heads. Also, it can kill the plant and put an end to seed production from the given plant.

Using these two weevils together would work great as they do not compete with each other. Also, you can integrate other methods to deal with the musk thistle populations. All you need to do is to ensure that mechanical and chemical means of management complement the use of weevils.

Say, for example, you have the flower head weevil in your farm; you can spray the musk thistle with chemicals between March and April. Also, you could mow the plants towards the end of July when the weevils’ life cycle is over.

Where you have both species, you can limit the use of herbicides to the fall. Using the chemicals in spring will kill the rosette weevil larvae, which feed on the crowns during this season.

While using weevils alone could give you the results you need, adding another control measure would work to speed up the process.


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