Noxious weeds are a growing problem around the globe, and purple starthistle is not an exception. It not only invades but also degrades lands in which it grows, leaving them devoid of nutrients to support other vegetation.

It grows as a biennial in most cases. However, in some instances, it is perennial or annual. Where it grows as a biennial plant, it has a life cycle of two years. When it matures, its exterior features sturdy and sharp spines.

This weed also goes by other names, some of which may be familiar to you. They include golden star thistle, red star thistle, and St Barnaby star thistle. It relates to about eleven California non-natives which have also been a problem to farmers in the region.

One of these non-natives is the yellow star thistle, which is more widespread than the purple star thistle. However, that does not make it any less of a threat as it can spread at a rapid rate.

Origin and distribution

This weed is native to the Mediterranean region in southern Europe as well as northern Africa. Its introduction to California took place towards the start of the twentieth century. From then, it spread to other sites as a rangeland and pastureland invader.

At present, it is not uncommon to find it in far-flung areas such as Washington, showing how invasive this plant is. Surprisingly, its sole mode of reproduction is seeds. These seeds, bearing no pappus, can spread quickly through machinery, human beings, cars, hay, and straw.

The seeds also get dispersed by way of wind once they fall off the parent plant. You can thus see how easy it was for the weed to spread back then, and how that ease remains to date.

Reports of Purple Starthistle in the US

How to identify Purple Starthistle

As the name would suggest, this weed boasts of purple flowers, around which you would find stout spines. It grows in a mound, which is one of the telling signs that you could be dealing with a noxious weed.

As the plant grows in its first season, it forms a rosette with lobed leaves and small hairs covering the midribs. In the second growing season, the plant bolts.

Purple starthistle flower

Purple starthistle flower

As the leaves grow, spines encircle the center. When mature, the plant should be about one to four feet high with tons of branches adorning its stem.

From these arise tons of flower heads that are edible when young. In each flower, you will find about 25-40 flowers.

centaurea calcitrapa plant

centaurea calcitrapa plant


The purple starthistle is quite similar to the Iberian star thistle, which is also common in the California region. However, the Iberian thistle’s seeds have a crown of bristles at the top, which is unlike what you would find in the purple one.

This weed is also similar to the yellow star thistle, and you can tell them apart by the color of the flowers. However, in some cases, this could be hard, especially where the flowers take on a pale hue.

In this case, it would be best to rely on the form of the flowers before they bloom. For the yellow star thistle, the plant would feature wing-like edges along the stem.

Purple Star Thistle habitat

Where land is recently or often disturbed, such as in grazing land or roads, it is possible to come across this weed. It generally prefers regions that are at least three thousand feet above sea level.

It also has a preference for fertile soils with a heavy leaning on clay and bottomland soils. Examples of common habitats are waste sites, roadsides, cattle trails, grazed areas, and uncultivated fields.

Growth Pattern

It takes about a year for this weed to go through the prostrate rosette stage. However, this applies where the thistle grows as a biennial. As an annual plant, it would follow a different life cycle. As a biennial, it bolts and flowers in the second growing season.

This weed can grow in dry conditions, and lack of water would not affect its growth. Studies show that the hot months do not deter its growth, and it can keep growing even as other plants fall back.

Flowering starts at around June and lasts till August. By the time of flowering, annual grasses have already seeded. However, it is essential to note that this will depend on the climate of the region.

When the conditions are wet, the flowering will occur towards the end of summer. When the conditions are hot and dry, the flowering will take place at the beginning of summer.

Once the bright and beautiful flowers are out, bees frequent them in search of nectar. Mature seeds lack a pappus and therefore fall under the parent plant where they create a sort of seed bank in the soil. These seeds can remain viable for years on end, and where the conditions are favorable, they sprout.

Seed heads break off easily, and animals and people frequenting the area can disperse them across a field. The same goes for moving water where the plants are in irrigated fields or other wet areas.

What’s more, these heads can stick to rubber tires and other such surfaces, making dispersal by cars and machinery also possible. The heads could also break off and get carried by wind over long distances.

Purple Starthistle threats

Why is the purple starthistle a danger? One would wonder how such a beautiful plant could be of danger to animals, human beings, and land. But there is more to this plant than meets the eye.

Take San Francisco as an example. Here, this purple starthistle has caused so much loss in the pasturelands by invading fields. In so doing, it has created dense infestations that affect the quality of the grazing lands.

Animals and farmers have thus suffered the effects of this. With this invasion, the cattle-carrying capacity of the land also reduces, limiting the cattle one can feed.

Purple starthistle, when left unmanaged, can take over productive land. One year, you have enough land to till and harvest crop from, and the next, the fertile land has reduced in size. How much would that affect your returns?

Forage production also reduces when this thistle affects a field. As it takes more space and remains unmanaged, you get less forage, and animals and farmers end up on the losing end.

As if that’s not bad enough, this weed crowds out native plants by pushing them out of the habitat. It creates conditions unsuitable for their growth, and soon enough, desirable plant numbers begin to drop.

Where this purple star thistle makes its way into parklands, it greatly reduces the land available for recreational use.

With all these happening, it would help if animals could feed on this thistle, but that is not the case. For one, it has spines covering its seed heads and flowers, making it hard to eat. And even if the animals were to get to it, they would not appreciate its poor taste.

It is thus crucial that people take measures to not only prevent its growth but also control infestations before they become a bother.

Management of Purple Starthistle

Managing purple starthistle is the only way to preserve un-infested lands while reclaiming those with dense infestations. It is important to note that management is quite intensive as it involves a lot of monitoring and management.

Seeds in the seed banks can remain dormant for many years. However, given ideal conditions, they can sprout and create many problems. The monitoring of sites is, therefore, necessary to deal with current infestations and prevent future occurrences.

You could eradicate all purple starthistles in one area, unaware of a seed bank in the soil. Then two years later, thistles would start emerging from the soil, and you would wonder where they were coming from. Seeds can sprout years after they fall off the parent plant, more so when heavy rainfall is in play.

Here is how it works. When the seed falls off the parent plant, it is often during hot weather. As such, there could be a crack, and the seed could sink deep into it.

When heavy rain falls, this seed can float on the water and come to the surface or the upper soil layers where it can sprout. That is the first complication in dealing with this thistle.

The second complication involves the fate of the lark sparrow. This beautiful bird is quite selective as to where it will nest. And as chance would have it, the bird prefers purple star thistle infestations. Not an easy decision to make there.

The third and often-faced issue would be suitable methods in dealing with this thistle. While organic farmers, as well as most people, would prefer to stay away from chemicals, this is not entirely possible.

Studies show that the best results come from integrating different management methods. Using monitoring and physical removal would, therefore, have few effects if not coupled with herbicide applications.

With this out of the way, we can now get into considerations you should make before using any control method and what would work.


First off, you need to visit the infested site, as this will help you assess the extent of the infestation. Here is what you should think about:

Affected people

Every weed falls under a category, and depending on how devastating its effects are; a different agency will handle it. Thus, look up who handles purple starthistle in your area and their guidelines as to how you should deal with the situation.

History of the site

Look at the current state of the site and consider its past too. This information will cover the annual rainfall received, the condition of the soil, animals, and plant species in the area. You can also gauge if the vegetation on site can compete with the purple star thistle.

As you do this, think back to a time when you had weeds in the area. How did you deal with them? What was the source? Did they recur?

Is the purple starthistle a recent problem? Have you dealt with it in the past? If you have, how did you control it? How did it get to your field? Can you prevent this from happening in the future?

Present Use

How did you use the land? How do you intend to use the land? All of this will affect the control methods you choose. Suppose you plan on having a given crop in the land, you need to look at its interactions with your chemical of choice.

These questions may seem many, but having clear answers to them greatly aids in finding a long-lasting solution to this weed. You can now move on to suitable management strategies you could use in controlling the weed as follows:

What you should keep in mind

As much as dealing with the thistle should take priority, you need to take time in planning as to how you will go about it. Also, think of how your strategy could affect the environment. Here is a guide as to the same:

The Law

Different laws govern how you can deal with purple starthistle. You should thus look at whether you would be in line with the said laws before doing anything.


Control methods not only call for time, but they also require that you invest labor and money in the process. Consider your ability to carry out your chosen control method without exhausting your resources.

You can also think of collaborating with people dealing with such an issue to reduce on resources spent. Another idea would be to work with an agency dealing with the eradication of weeds.


In this case, you would start by looking at the various control methods you can use. From this list, you can narrow down your choices based on their suitability for your field.

The Environment

Will your control method of choice adversely affect the environment? If so, what are the alternatives to this? Note that using an unsuitable control method could also encourage the growth of thistle or other weeds.

Control Level

There are many control levels when dealing with this weed. You thus need to choose the one you would like, and one you can sustain without depleting your resources.

The first is containment, where you identify an infested area and keep the weed from spreading to other places. This method works best for people who are short on money and time.

It also works for large infestations where dealing with the weeds at once is not possible. As you contain the weeds, you need to ensure that seed production also stops, as this further encourages new growths.

The second method is a reduction, where you reduce the number of purple starthistles in an area. It works for both new growths and large infestations. People pursuing this method should be ready to invest time and money in the processes.

The third is the Bradley method, which is a combination of reduction and combination. It provides better results and is an in-between for people who want to reduce populations and lack adequate resources.

The last method is the most time-consuming and will cost a lot as compared to the others. However, it is the most effective, and using it will ensure there are no more weeds in the area. Where you have relatively new weeds in an area, this technique would be ideal.

Monitoring Infestations

Having records as to purple star infestations in the past, present, and future helps in eradicating them. Start by locating them and mapping out the area, specifying where they are mainly found. Ensure that you update the map whenever you come across a new infestation.

You could work with an agency to enlighten the public on the presence of this weed and how they should go about reporting its sightings. In this way, new outbreaks get dealt with fast before they spread to new areas.

Once you have a clear picture of what you are up against, you can now come up with a plan. Decide where you will start, given the resources available to you. That means that you can use a method and continue follow-ups without depleting your resources.

It is best to focus on one area and keep at it as opposed to working on numerous sites and failing to monitor the situation. The best place to start would be the infestation that affects your economic wellbeing or the state of the environment.

The control method in place should be in line with the life stages of the weed if you wish to use fewer resources. It also helps in the effectiveness of the processes.

As you work on the weeds, ensure that you update the records, showing what you have done to keep track of all the data. This information should include everything from the date of application to prevailing weather conditions through to the success or lack thereof of the process.

Remember that you should only take on what you can handle at a given time, as it is only through this that you can achieve success.


We have now covered the groundwork. Let us look at some of the ways you can keep this weed from spreading. While herbicides are effective in controlling infestations, it would be simpler and cheaper to prevent the spread of purple starthistles. Here is how you can do this:


You should be on the lookout for any new infestations in your field and deal with them before they spread. Work with your neighbors in controlling new infestations as you engage them on outbreaks. In this way, they can alert you when there are new infestations in your area.


You should stay away from seeds and forage not classified as weed-free as they could have thistle seeds. Where you are unsure of the quality of the feed, avoid purchasing it.

Also, when moving soil from one area to another, be sure to check if it has purple starthistle seeds. The presence of these would lead to sprouting of the weed in new areas.

Keep people, animals, machinery, and vehicles from infested areas as they could move the seeds to un-infested zones. If they must access the area, conduct checks after they leave to ensure they do not have seeds.

Cleaning the undercarriage of vehicles and machines helps in containing the infestation. Also, create a boundary between the infested and non-infested areas. In this way, you can tell when the weeds spread to other areas.

How to control Purple Starthistle?

It helps to note that dealing with purple star thistle is not a one-time thing. Thus, be ready to dedicate a few years to this process, integrating various methods all the while.

Biological control

At present, there are no known biological means of dealing with this weed. However, some insects have shown some results. One is the Larinus minutus, which works in dealing with knapweed varieties. The other is the Bangasterus orientalis, which deals with yellow star thistle.

Studies show that where these species are unable to find their primary hosts, they can feed on purple star thistle. However, this is not enough. Pathogens have also not borne fruit, with an example being the Puccinia jaceae. This fungus attacks the leaves of the purple starthistle but has little effect on the weed’s growth.

Grazing does not work as livestock do not like the taste of this weed. And that brings us to the end of biological means.

Physical control

You have better luck relying on physical controls than you would with biological means. The reason behind this is that these controls are already in place and have proven to be effective.

Point to note

Mowing is not a recommended means of removal as it is barely sufficient. However, you can schedule it to late summer when the weeds are unable to re-sprout at fast rates.

Hand pulling

This method calls for precision, and it mainly works for areas where the purple starthistle infestations are small. For significant populations, removal by hand would consume a lot of time, labor, and money.

Note that this method has some disadvantages. For one, it could disturb an area and lead to new growths. Also, it could bring seed banks to the soil surface, encouraging germination.

To be effective, you need to map out an area and work on the weeds one section at a time. Pull out the weeds and set them aside where you can later appropriately dispose of them.

Pulling out is effective when you use a long-handled hoe as it can penetrate hard soil and get to the base of the plant. Ensure that you cut out the crown root such that no purple hue remains on the plant. If you can get at least three inches below the plant’s base, you would be successful.

Timing is essential when pulling out purple starthistles. For one, it should ideally not take place after seed production takes place as the movement would spread the seeds. If you have to pull the weeds out at this stage, dispose of the plants in plastic bags immediately.

Two, it is better to deal with older plants as opposed to dealing with plants in their first growing season. The latter plants are hard to find, and it helps to wait for them to bolt.

physical invasive plant removal

Physical invasive plant removal is labor-intensive

Chemical control

Using chemicals allows you to eradicate purple starthistles and replace them with native and desirable plants. Be sure to use a suitable chemical that will not alter the growing conditions. Using a harsh herbicide will get in the way of growing other plants, which is not the aim.

Timing is also vital when using herbicides, as you should wait for when the thistles are vulnerable. When the plants bolt and are yet to seed, their food reserves are low, and they are more susceptible to attacks. Thus, plan the application before the flowers open, and bees start visiting. Note that the timing of the application will depend on the climate.

During application, use the herbicides on the tops of the thistles such that you fully cover them. However, ensure that the mix does not drip onto the surrounding plants or the soil.

In this way, you will create an environment that can support the existing vegetation once the purple starthistles die. If you kill the plants near the weeds, thistles can grow back and take over the field again.

You need to keep track of the treated plants, and you can do this by mixing a non-toxic dye into the herbicide. In this way, you will not miss any purple starthistle, and you can come back and collect the remains.

Where the thistle exists together with grasses, you should be careful in the selection process. During the time of application, the grasses will have seeded. Using chemicals on them would thus get in the way of their spread.

You would have better results with fewer negative impacts on the environment if you applied the weed selectively. You can use a backpack sprayer or a wick applicator for this.

Avoid using herbicides near water sources. In these areas, you can rely on hand pulling as the moist soil will make this task easy.

Remember to follow the guidelines of the manufacturer to ensure you stay safe. That means you should go through the label, dress protectively, and consult an expert where you are in doubt. It also helps to get a chemical with low volatility as it will not spread through the area much.


Dealing with purple starthistle will take years, but as long as you keep at it, you will enjoy success.


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