Milk thistles belong to the genus Silybum. You can recognise this flowering plant by the solitary pink or purple flower heads that grows on long stalks. There are only two distinct species of milk thistle but they hybridise easily, resulting in a great variety of characteristics among individual plants. Milk thistle is extremely hardy and is often considered to be a weed plant. However, the seeds and flowers are used extensively in herbal medicine.
Plant the seeds indoors. Push the seeds just below the surface at a depth of about three millimetres. Plant biennial varieties of milk thistle in early summer and annual varieties in early spring. Keep the seeds at a temperature of 12 to 15 degrees Celsius for three weeks so they'll germinate.
Select a permanent site in your garden for milk thistle. It can grow in partial shade, but will need at least 7 to 8 hours of direct sunlight each day to go to seed. Milk thistles can generally tolerate a wide range of soil types and the ideal soil is midway between sand and loam.
Plant milk thistle outside. These plants are quite hardy and can also be planted outside directly from seed. Plant biennials outside in early summer and annuals after the last frost.
Maintain milk thistle. These plants can tolerate both wet and dry soil, so you don't generally need to water them. Prune any dead flower heads before the flowers release their seed. This practice is known as "deadheading" and will encourage the milk thistle to produce more flowers. Remove any unwanted milk thistle plants.