Introduction

Milk Thistle or Silybum marianum is an interesting plant. It has gorgeous variegated leaves and will grow in just about any disturbed environment as long as it has sufficient sunlight. It is also a widely used medicinal herb and many people today are still planting it in their garden for this reason. Once planted however, they quickly realize that now they have to remove the Milk Thistle from their yard before it takes over.

About 20 years ago I once planted it in my garden in West Sechelt, British Columbia, Canada for its medicinal properties. I was thrilled by how productive my crop was the first year. However, the second year I was horrified at the number of seedlings popping up where I had not planted them. I had not heard yet of its invasiveness at that time, clearly, or I would never have planted it. So I spent the following two years trying to remove every Milk Thistle seedling I could find. I moved away after that and have often wondered if I managed to stop it from becoming invasive there, or did I start an infestation that got out of control? It literally haunts me a little still to this day.

Until something becomes known as invasive, how are we supposed to know? My suggestion is to do your research before you plant anything new. This is particularly true of medicinal herbs which are often opportunistic species that may invade given the chance. With all of the information available today at our fingertips, it is not necessary to take chances anymore. If it is invasive and you want some raw herb, purchase it online from a reputable source instead.

Description of Silybum marianum

Leaves & Stems

Silybum marianum is part of the Asteraceae (Sunflower) family in the Asterales order of flowering dicot plants. Its stems are either glabrous or slightly tomentose (rough-hairy), lightly grooved, not winged, and grow from 0.3 – 2 m in height. It grows from a basal rosette of leaves up to 1.6 m wide in diameter. Stems are usually branched but not repeatedly and are usually armed with spines. The oldest stems are often hollow.

Its leaves appear strongly variegated in shiny green with milk-white veins. The basal leaves are oblong to lanceolate, coarsely lobed and armed with marginal spines. They are 15 – 65 cm long and are attached to their stems on winged petioles. As the leaves approach the flowers they become smaller and more clasping with an auriculate (ear-shaped) base.

Flowers & Fruits

Silybum marianum flowers anywhere from February to November, depending on the location. The phyllaries’ (floral bracts) appendages are ovate, spreading, and 1 – 4 cm long including their long spiny yellowish tip. The flower heads are from 4 – 12 cm long and 3 – 6 cm wide; they contain disk florets only. The disk florets are 26 – 35 mm long in magenta or purple. The tubes are 13 – 25 mm long, throats are campanulate (bell shaped) and 2 – 3 mm long, and they have 5 – 9 mm long lobes.

Note: disk florets and phyllaries are unique features of Asteraceae family flowers. If you want to learn more about Asteraceae flowers and their unique physical characteristics I suggest you check out the diagrams here https://cronodon.com/BioTech/asteraceae.html . You can also read the Wiki page on Asteraceae, it is very informative. However, the above link has much better diagrams showing the different physical features of the composite flowers.

The fruit is a brown cypsela (a typical angular seed of the sunflower family) with black spots. Although, it is often incorrectly referred to as an achene. Both are simple, dry, indehiscent fruits. However, achenes come from a superior ovary whereas a cypsela comes from an inferior ovary. The distinction is subtle. The cypsela is accompanied by a pappus of hairs or scales that remain attached to the apex that aid in wind dispersal. The pappus is a fluffy white attachment 1.5 – 2 cm long, that is surrounded basally by a yellow ring.

Toxicity

Milk Thistle in some instances can be toxic to ruminant livestock due to nitrogen accumulation. It is not, however, toxic to human beings and is a common and highly popular medicinal herb.

Similar Species Frequently Confused With

There are several native and nonnative thistles in North America that Milk Thistle is frequently confused with. In almost all cases they can easily be distinguished by the unique milky variegation on the leaves of the milk thistle. Sometimes, however, this variegation is more subtle which can lead to occasional misidentification. Similar looking species can be distinguished as follows:

• Cynara cardunculus the Globe Artichoke – popular with gardeners, it has tall stems that are not winged and large, spiny flower heads like Milk Thistle. However, it can be differentiated by its grey-green leaves that are not spiny.
• Cirsium vulgare the Spear Thistle – this has winged stems and its leaves are greenish and not variegated. It has moderate-sized flowers with small spines. 
• Cirsium arvense the Creeping Thistle – has wingless stems but its greenish colored leaves are not variegated and it has smaller and much more slender flower heads.
• Carduus nutans the Musk Thistle – has winged stems but its greenish leaves are not variegated. Its flower heads have small spines and they droop with age.
• Onopordum spp – multiple species in North America that look somewhat similar but all have winged stems and their leaves are either bluish-grey or whitish rather than variegated. They also have smaller flower heads and smaller spines.

Native Distribution of Silybum marianum

Silybum marianum is native to throughout the Mediterranean countries as well as all of Asia from India north to Siberia.

Habitat Types Where Milk Thistle is Found

Milk Thistle grows in open fields, roadsides, agricultural areas, waste areas, any disturbed habitat. It can be found from sea level to 1300 m above sea level.

Silybum marianum requires habitats with full sun and prefers areas that are regularly disturbed. It tolerates drought, full sun, and nitrified soils. While it prefers nutrient-rich soil it can tolerate a wide range of soil types, quality, and moisture levels. It does not seem to prefer rocky soils.

Human Uses of Milk Thistle Silybum marianum

Milk Thistle as Food

Milk thistle has a long history of use in Europe as a food. Leaves with their spines removed were used in salads or cooked and eaten like cabbage. Stalks, roots and flowers were often cooked on their own or added to stews as a vegetable. Seeds were sometimes used as a coffee substitute. While humans use it for a food source it is considered somewhat toxic for livestock to feed on.

Milk Thistle as Medicine

Silybum marianum has been used as a herbal medicine for over 2000 years. It was widely used as a milk stimulant, for liver, kidney and spleen problems, jaundice, gallstones, menstrual problems, and neurological problems.

Currently, Milk Thistle is widely used as a herbal remedy for hepatitis (especially Hepatitis C), cirrhosis, jaundice, diabetes, indigestion, and as part of a gentle cleanse. It is also occasionally used to help treat poisonings from toxic mushrooms, industrial solvents, excess acetaminophen or other over-the-counter or prescription drugs.

Studies have been being done that show it can help as a neuroprotective to aid in treating age-related decline in brain function. Studies are also being done to see if it can help aid and boost the effectiveness of cancer treatments.

The active ingredients in Milk Thistle cypselae are a group of phytochemicals called silymarin. Occasionally it can cause allergic reactions, usually in those with allergies to other plants in the sunflower family. It also may lower blood sugar levels in people with type 2 diabetes so they should be cautious when using it.

Ethical Wildcrafting of Silybum marianum and Making of Medicines

When it comes to truly invasive species with no or little wildlife values there is no need to “ethically” wildcraft. Milk Thistle is a wonderful medicinal herb. If you see it growing in your area and want to harvest pick to your heart’s content. Choose an area free of major sources of pollutants though if you intend to ingest it medicinally.

The achenes are ripe when there is a lot of fluffy white pappus visible. They readily split open at this stage upon picking so keep your bag or basket under the flower head. This will prevent any seeds from spilling on the ground as you harvest. Pick all of the mature seed heads that you find whether you need them all or not. Allow them to continue drying in the bag or basket for a few more weeks until they easily split apart. Process any seeds you want to use. Any excess seeds should be ground before you burn or otherwise dispose of them. Do not throw them directly in your compost, garbage, or anywhere else before grinding them.

Making Milk Thistle Tinctures

To use Silybum marianum as medicine you must remove the fluffy pappus from the achene (seed). Dry the seed thoroughly on a rack or in a paper bag. Then grind it and make it into a tincture using alcohol. You can use vodka or white rum (or other alcohol, they just impart more flavor to the tincture). If you can get your hands on 95% ethyl alcohol this will make the best tincture. Everclear is a common brand but it is not legal for sale in some states and provinces.

To make a tincture put the ground seeds in a jar and cover them with your alcohol of choice. Be sure to add enough alcohol to thoroughly cover the seeds and a little extra to account for any evaporation. When done your tincture should be bright yellow from the resins that contain the active ingredient silymarin. Note that the active ingredient silymarin is almost completely insoluble in water so teas and other water-based extracts will not be effective.

Distribution of Silybum marianum in North America

The species was first brought to North America by the early colonists as a medicinal herb and as a food source. It was intentionally planted and spread with the colonists.

In Canada, Silybum marianum has been recorded in British Columbia, Alberta, Ontario, Quebec, New Brunswick, and Nova Scotia. In Saskatchewan it is reported as ephemeral. Interestingly, it is not reported as present in Manitoba yet the Manitoba government’s department of agriculture has a page on their site telling you how to grow it. They even seem to try to persuade farmers to grow it by spouting the economics and marketing of it. Species known to be invasive in any part of any country should ideally not be encouraged to be grown in other parts of that same country.

In the USA, Milk Thistle is found in Washington, Oregon, California, Nevada, Arizona, Colorado, New Mexico, Oklahoma, Texas, Arkansas, Louisiana, Mississippi, Alabama, Georgia, Tennessee, Virginia, Washington DC, West Virginia, Maryland, Michigan, Indiana, Ohio, Pennsylvania, New York, Connecticut, Massachusetts, and New Hampshire. Since there is still so much habitat suitable for Silybum marianum it is expected that it will be much more widespread in the USA in the future unless adequate control of Milk Thistle is enforced.

In Mexico Silybum marianum has been reported in Baja California, Sonora, Nuevo Leon, Hidalgo, Veracruz, Mexico State, Mexico City, and Puebla. It is likely more widespread than currently reported and will continue to spread into more states throughout Mexico.

Milk Thistle has been introduced on every continent except for Antarctica. It is considered an invasive weed in North America, Australia, New Zealand, and parts of South America.

How Blessed Milk Thistle Spreads

Milk Thistle is primarily spread through long-distance by deliberate human introductions by planting them in their garden. Seeds are still widely sold on the internet and often in garden stores as well due to their popularity as a medicinal herb.

One Silybum marianum plant can produce over 6000 seeds that remain viable in the soil seed bank for 9 years. Furthermore, they can germinate through an extraordinary range of temperatures between 0 – 30 C. Seeds can be transported short-distances by wind, clothes, shoes, equipment, pets, etc.

Habitats at Risk of Invasion in North America

Any open, disturbed habitat is at risk of invasion in North America as long as it has adequate sunlight. This makes agricultural areas, roadsides, new development, riparian areas, clearings, urban fallow, and waste areas particularly at risk. This means that anywhere humans develop the area is at risk of invasion. And since most disturbed habitats have abundant sunlight that means that light requirement is not an issue. Since it is not only drought tolerant but will also germinate at temperatures around 0 C it is expected that Silybum marianum will continue to spread throughout North America. This is a particular concern in the central areas of Canada, the USA, and Mexico where it currently is not found or at least not yet in any abundance.

Impacts of Invasion by Silybum marianum

Silybum marianum frequently invades pastures and agricultural lands where it can rapidly take over and exclude forage grasses and herbs, native plants, and agricultural plants. Due to its height and diameter, a single plant can displace a significant amount of forage plants for livestock.

Milk Thistle is known to accumulate potassium nitrate which can make it toxic to ruminant livestock because the bacteria in their stomachs convert it to nitrate ions. However, it is not considered toxic to humans.

Due to their preference for disturbed habitats they do not pose a very serious threat to undisturbed native environments. However, they will rapidly spread in fallow fields and have occasionally invaded nearby wild grasslands and meadows with proximity to agricultural areas.

Potential Benefits of Invasion

The flowers do produce nectar that is used by both native and agricultural bees. However, the plants that it displaces likely would have also provided the same benefit for the bees.

Methods to Remove Milk Thistle

As always prevention is the preferred method of control. It, like most invasive species, is still widely sold online and in most local garden stores. Do not buy or transport any Milk Thistle. Do not plant it in your yard like I mistakenly did all those years ago.

If you see them being sold online or in your local garden stores please inform them of their invasive status and ask them to do their part and cease selling them. Ask them to instead sell more native species as ecologically friendly garden alternatives to invasive species. And if you are wanting to grow it solely for the medicine, then buy the medicine or the raw seeds online from a reputable source. Just do not plant those seeds in your yard.

Physical Control of Milk Thistle

Once already established, however, physical control is always the most effective means. Physical control is labor-intensive and time-consuming but it usually causes the least amount of environmental damage.

Physical methods for the removal of Milk Thistle involve the cutting down of the plants. Because it is an annual or biennial species removal of the rootstock is not absolutely necessary. The best time to remove Milk Thistle is in the mid-late spring before the flowers have opened or matured. Immature seeds will continue to mature on cut plants, particularly those that remain attached to the stem. So it is important they be cut down before the flowers mature so they cannot go to seed.

Wear gloves to protect your hands from the spines. If the plant already has a stem use hand cutters, machetes, loppers, etc to cut down the above-ground growth. Be sure to then use a shovel to cut it further just below the surface of the soil so that it is not able to re-grow that year. Alternatively, you can then use a weed puller to remove the root. If it is early in the year and you are only dealing with the basal rosette of leaves then weed pullers can be used to remove the root. A shovel can also be used on a rosette to cut the growth off below the soil surface.

Frequent cultivation in agricultural areas is a good method to control Milk Thistle infestations. Cultivation must occur before the plants go to seed. This should be followed with planting of suitable annual forage plants for your livestock. Keep cultivating yearly if necessary until the infestation is gone. Then yearly monitoring can be done to remove any seedlings that may still try to sprout.

Physical Removal Methods of Milk Thistle That Are Not Recommend

Mechanical mowers are never recommended. They help spread the seeds around and they will not actually kill the plant since they do not remove all the green photosynthetic leaves above ground. The remaining plant will simply re-grow and could still set seed that same year.

Controlled fires are not an adequate method to remove Milk Thistle. Milk Thistle seeds are tolerant of low-temperature fires. Furthermore, the release of nutrients the fire creates as well as the disturbance it creates makes a suitable environment for the seedlings to sprout. Fires may actually encourage their spread.

Disposal of Milk Thistle Removed

If you have plants that have any seeds, whether mature or not, they must be solarized. To solarize put the plants under a thick black tarp, or into thick black garbage bags and leave them in the full sun for a good 8 weeks to ensure that all seeds are no longer viable. Many sources recommend shorter time periods for solarization. However, temperature differences throughout the bag, as well as variable sun conditions during the process, can lead to differential survival of some seeds during solarization. Better safe than sorry, so please solarize as long as possible.

Replanting may be necessary, and ongoing monitoring is absolutely essential. See the Integrated Management section below for more information.

Chemical Control of Milk Thistle

Chemical applications are almost never an ideal method of control for any invasive species. That is because chemical alteration of the environment often makes the environment more suitable for invasive species than native species. Furthermore, it is often difficult to keep the chemical control method contained so that it does not directly affect any native species that are there during the application process itself. As a result, plots where chemical control is used usually show a decrease in species richness. On the other hand, in plots where only physical control is used species riches significantly increases.

Chemical control is generally not recommended. However, since Silybum marianum is more often a weed of roadsides and agricultural areas chemical control is sometimes used. Since there are no chemical control methods that effectively target only Milk Thistle then handheld spray applications are recommended rather than non-selective machine spraying. Herbicide availability and legality vary from area to area so if you have a large infestation and want to use this method then please research herbicides used in your area. If at all possible try physical control methods first as Milk Thistle removal by physical means is relatively easy for an invasive species.

Biological Control of Milk Thistle

Biological control involves the use of a predator, herbivore, disease, or some other agent to control an invasive species once it is established in the environment. The problem with biological control is that the agent used must be entirely specific to only the target organism before releasing it into the environment. This is often difficult to determine since the agent of control is also not native to the environment and could behave differently when released there. Take the example of the mongoose and the rat. The mongoose was released in Hawaii in the late 1800s to help control the rat. To this day there are still rats in Hawaii but the mongoose has helped to decimate many native bird populations.

Biological control methods are extremely risky and should only be carried out by professionals after years of rigorous study. The use of biological control methods can never be used alone. They must be part of an integrated pest management approach. This is because the control agent would need to effectively destroy over 99% of seeds to actually control the Milk Thistle on their own. However, using biological control in conjunction with physical control and ongoing monitoring can be very effective when dealing with large infestations over a wider geographical area. These are the biological control methods that have been used in North America in an attempt to help remove Milk Thistle.

Biological Control Organisms Used in North America

• Rhinocyllus conicus is a seed head weevil is reported to do significant damage and is a viable option to control Milk Thistle. It has been released in the USA with some success. However, it also attacks the native thistles so this method is not recommended.
• Alternaria species, a plant pathogen, is currently being studied for possible use in the USA, but to my knowledge has not yet been released.
• Domestic Goats will feed on Silybum marianum without any ill effects. The goats are non-selective, however, and will eat almost anything in their path. Goats should be kept in a fence to prevent them from grazing outside of the infestation area. They should be allowed to graze from spring till fall to allow them to eat any new regrowth. They will need to be returned the following spring to graze continuously the next year to deal with any seedlings that emerge.
• In agricultural areas maintaining vigorous, dense pasture land is the best method to control Milk Thistle. The types of agricultural plants used in the pastures will of course depend on your location so research what is best in your area.

Integrated Pest Management & Ongoing Monitoring

Integrated management is always the best approach. In its simplest and least impactful form this involves physical removal methods, possibly biological control methods, replanting, and ongoing monitoring. No one method alone will ever control an invasive species, or they would not have become invasive in the first place.

Replanting is Often Crucial

In many cases, particularly in agricultural fields where there has been a lot of Milk Thistle removal the area may need to be replanted. Since they are agricultural the type of planting will depend on the agricultural use as well as the location. Where the infestations occur on roadsides there are usually other weeds that will quickly grow up to take their place. In the occasional case where the area is a natural one, or one being converted to a natural area, then replanting should be done with native species. In that case, a replanting program should already be planned and ready to implement immediately upon the removal of Milk Thistle. Use seeds from the local area if possible to replant native areas. If that is not possible then purchase native plants and native seeds from a reputable nursery specializing in native plants in your area.

Ongoing Monitoring is Essential

In all cases of invasive Milk Thistle removal, ongoing monitoring is absolutely essential. Yearly monitoring programs should be put in place to ensure that any surviving individuals are removed so that the population is not able to recover. This is required whether the area is replanted or not. Milk Thistle can be aggressive and may out-compete planted vegetation if yearly monitoring is not put in place to remove young plants before they have a chance to become established.

Simply revisit the site every spring or so over the following years and remove any new rosettes or full plants that emerge before they are given a chance to go to seed. Seeds remain viable for 9 years in the soil so ongoing visits would be required for at least that long.

References and Resources

CABI Fact Sheet on Silybum marianum https://www.cabi.org/isc/datasheet/50304

Canadensys Plant Search https://data.canadensys.net/vascan/search

Dictionary of Botanical Terms – Lyrae’s Nature Blog Dictionary of Botanical Terms

Eflora Plants of North America http://www.efloras.org/browse.aspx?flora_id=1

iNaturalist Plant Search https://www.inaturalist.org/home

Manitoba Department of Agriculture on Milk Thistle https://www.gov.mb.ca/agriculture/crops/crop-management/milk-thistle.html

Milk Thistle Flower Photo from Wikipedia By fir0002flagstaffotos [at] gmail.com Canon 20D + Sigma 150mm f/2.8 – Own work, GFDL 1.2, https://commons.wikimedia.org/w/index.php?curid=1526654

USDA Plants Database https://plants.sc.egov.usda.gov/home

Wikipedia on Silybum marianum https://en.wikipedia.org/wiki/Silybum_marianum

Willis, Lyrae (2022).  Plant Families of North America. Not yet published.

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