Milk thistle: planting, effects & uses of the medicinal plant


I studied horticultural sciences at university and in my free time you can find me in my own patch of land, growing anything with roots. I am particularly passionate about self-sufficiency and seasonal food.

Favourite fruit: quince, cornelian cherry and blueberries
Favourite vegetables: peas, tomatoes and garlic

The seeds of milk thistle are used in herbal medicine for liver diseases. We introduce the tall-growing, bee-friendly medicinal plant.

A purple milk thistle flower
A milk thistle flower is made up of many elongated tubular flowers [Photo: Heike Rau/]

Milk thistle (Silybum marianum) is particularly well known for its use in liver problems. Here you will learn everything about the planting, harvesting and effects.

Milk thistle: flowering and characteristics

The milk thistle is also popularly known as blessed milkthistle, variegated thistle, Marian thistle or Mary thistle. The tall-growing plant gets its name from the cream-coloured veins on the leaves, which, according to legend, were created by Mary’s dripping breast milk while nursing the infant Jesus. Milk thistle belongs to the daisy family (Asteraceae) and originates from Southern Europe, North Africa and Asia Minor. In Central Europe it now also occurs wild, but in most countries it has been cultivated for centuries as a medicinal plant.

Silybum marianum plant without flowers
Initially, milk thistle plants form a basal rosette of leaves [Photo: Martina Unbehauen/]

Milk thistle grows into a stately plant of 50 to 150 cm in height. It is annual to biennial, erect, and branches only in the upper part of the stem. Similar to other thistle species, it develops a deep, thick taproot. Above ground, a basal rosette of leaves appears, from which the flowering shoot later emerges. Its leaves are up to 40 cm long, elongated, lobed and thorny-toothed at the leaf margin. The leaf surface features impressive green-white marbling.

The flowering period begins between July and August. One to several spherical thistle flowers form, typically consisting only of elongated tubular flowers resembling coloured threads. The milk thistle flower appears in purple to violet and is visited with preference by wild bees. The flowers have large amounts of pollen and nectar, so beekeepers also value the medicinal plant.

After pollination, the milk thistle fruits are formed: Hard-shelled, shiny black achene seeds with brown spots and an elongated, silky appendage – the pappus. This is mainly responsible for the spread of thistle seeds by the wind.

Field of milk thistle flowers
Pollinators enjoy the purple milk thistle flowers [Photo: Oleksandr Masnyi/]

Danger of confusion with milk thistle: The plant is hard to confuse with other plants because of its distinctive leaf pattern. Similarities are mainly in growth and flowering with cardoon (Cynara cardunculus). However, this one forms even more spreading, non-spiny plants. Milk thistle is most likely to be confused with wild, purple-flowered thistle species such as creeping thistle (Cirsium) or Scotch thistle (Onopordum acanthium), but neither has the typical leaf pattern of milk thistle.

Cotton thistle flowers
Cotton thistle looks very similar tomilk thistle, but its leaves have no white markings [Photo: STUDIO_BEE/]

Planting milk thistles: location and procedure

The optimal location for milk thistle plants is full sun, dry and warm on deep, nutrient-rich soil. Its deep taproot allows it to grow easily on clay or stony subsoils, and the plant can even colonise compacted soils. In no case should waterlogging occur. In principle, cultivation in a container is also possible. Milk thistle then needs at least 10 to 15 litres of soil volume and a pot as deep as possible with good water drainage. A nutrient-rich potting soil such as our Plantura Organic Flower Compost is ideal for this purpose, as it is already pre-fertilised and thus optimally supplies flowering plants in the first period after planting. In addition, it manages entirely without peat and therefore also scores highly in terms of climate friendliness.

Organic Flower Compost, 40L
Organic Flower Compost, 40L
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  • Perfect for all flowering plants in garden beds & pots
  • For beautiful blossoms & healthy plant growth
  • Peat-free & organic soil: CO2-saving composition

The seeds are sown in the spring from March to April in the cold frame or on the windowsill in small pots with growing medium. From the beginning of May, sowing can also be done directly into the open ground. Plants sown from the end of July bloom only the following year. The black-brown seeds are sown only about 0.5 cm deep, as they are light germinators. They are then watered carefully but thoroughly. The optimum germination temperature after sowing milk thistle is 18 to 23 °C. Germination usually occurs within a few days to a maximum of 3 weeks. From May, the seedlings can be placed in the bed or in a pot. The distance between planting is from 50 to 100 cm. Plants often grow larger and more spreading on good garden soils than on compacted, heavier soils.

A small milk thistle seedling
Milk thistle seeds germinate quickly under the right conditions [Photo: revital/]

The right care

When growing milk thistle there is not much to consider, because the plant is extremely hardy and drought tolerant. Only freshly planted or germinated plantlets should be watered from time to time in hot, dry weather. More important is often the nutrient supply of the vigorous medicinal plant. Milk thistle has a relatively high demand for nitrogen and potassium, so fertilisation is useful for planting and flowering. Our Plantura Liquid Flower Food is a liquid fertiliser that is easy to apply through the irrigation water. Especially in pot cultures, the nutrient supply and numerous other flowering plants is therefore very easy to achieve.

Liquid Flower Food, 800ml
Liquid Flower Food, 800ml
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  • Perfect for all flowers & balcony plants
  • Liquid fertiliser for a lush blossom throughout the season
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Milk thistle is conditionally hardy to about -7 °C and should be protected outdoors with brushwood or dry leaves. However, often a plant that has already formed seeds, dies in the winter. Specimens that are not sown until August overwinter as a basal rosette of leaves, making them much hardier than plants that have already flowered. They then bloom much earlier the following year and can be harvested as early as the end of June.

Milk thistle ready for harvest
Milk thistle seeds need to be harvested before they are blown away by the wind [Photo: babetka/]

Harvesting and drying milk thistle

The spiny seed heads, i.e. the former flowerheads, are cut off from the plants as a whole before the fruits are fully ripe from August to October. When they are fully ripe, the seeds are easily carried away in small gusts of wind – so harvesting is done before the parachutes unfold, and ideally in the early morning hours. The whole heads are then post-dried at room temperature for a few days, tapped out, and the milk thistle seeds are freed from their pappus, the parachute-like appendage. They should be stored in a cool, dark and dry place and will keep well for about 3 years.

Thistle flowers and thistle seeds
After harvest, milk thistle plants are dried and the seeds are freed from the pappus [Photo: Soyka/]

Uses and effects

Milk thistle fruit is used in herbal medicine for liver diseases such as cirrhosis, toxic liver damage caused by alcohol and digestive problems. It is often included in liver and gall bladder stimulating blends such as liver tonic, but is also available as pastilles, capsules, and powder made from ground fruit. Often whole seeds are available, which are crushed in a mortar and then brewed with hot water as milk thistle tea. The daily dose should not exceed 12 to 15 g of milk thistle fruit.

The main active ingredient of milk thistle on liver and digestion is called silymarin. The effect of silymarin is even used in acute poisoning with the tuberous leaf fungus and applied as a life-saving infusion. Side effects of milk thistle are not expected when used properly. Milk thistle oil can also be pressed from the fat-rich seeds, which has particularly high antioxidant substances and therefore has a cell-protecting effect.

Milk thistle tea and flowers
Milk thistle tea has long been known to benefit the liver [Photo: Chatham172/]

Are milk thistles poisonous?

Milk thistle is not poisonous, but is even used in cooking. Indeed, young leaves, flower buds and the taproot are considered a delicate vegetable. Especially in Mediterranean countries, milk thistle is used in this way. There is also no danger for animals. The positive effect of the seeds in the form of oil and extracts is even used in veterinary medicine for dogs, cats and especially horses.

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