How to Grow a Silybum Marianum

Annual Milk Thistle

• 1

  Plan to start milk thistle seeds indoors in March to grow milk thistle plants as annuals. Make a potting mix using 50 percent peat moss and 50 percent perlite or vermiculite, or use a commercial, peat-based potting mix that contains perlite and sand. Pour water over the potting mix, and stir it until it is completely moist. Place a 2- to 3-inch layer of the moistened potting mix into seed flats, trays or other small containers that have bottom drain holes.

• 2

  Plant milk thistle seeds in the containers' moist potting mix at a depth equal to one to two times each seed’s width. Leave about 2 inches of space between seeds. Put the containers in a clear, plastic, zipper bag and seal the bag, or set a piece of clear plastic or plexiglass on top of the containers to hold in moisture.

• 3

  Move the containers to a sunny room, but do not leave them exposed to direct sunlight. Maintain a potting mix temperature of 70 to 75 degrees Fahrenheit. Moisten the potting mix if the top of the soil begins to dry.

• 4

  Select an outdoor planting location where the milk thistle will be exposed to at least four hours of direct sunlight per day. Milk thistle grows in soil that has a neutral, acidic or alkaline pH level. Mix a 2- to 4-inch layer of aged cow manure, compost, peat moss or leaf mold into the planting site's soil.

• 5

  Plant the milk thistle seedlings in the planting site right after the last expected hard frost in spring. Wear gardening gloves for protection from the plants' prickly foliage. Space the plants 1 to 1 ½ feet apart.

• 6

  Water the milk thistle's soil once per week when rain does not fall. Check the plants once or twice per week for caterpillars, snails and slugs. Remove them from the plants as they appear, and put them in a bucket of soapy water or crush them. Lay wooden boards on the ground near the plants to collect snails and slugs. Check underneath the boards for slugs and snails during afternoons, and remove the slugs and snails. Sink shallow cans into the soil so that the top of each can is at soil level, and fill each can with beer to trap slugs and snails.

• 7

  Remove the milk thistles' flowers as they begin to fade.

Biennial Milk Thistle

• 8

  Select an outdoor planting location where milk thistle plants grown as biennials will be exposed to at least four hours of direct sunlight per day. Milk thistle grows in soil that has a neutral, acidic or alkaline pH level.

• 9

  Incorporate aged cow manure, compost, peat moss or leaf mold into the soil in fall or early spring.

• 10

  Plant milk thistle seeds directly in the planting site's soil between May and August. Water the soil daily, keeping it uniformly moist.

• 11

  Thin the seedlings after they grow to a few inches tall, removing some so those that remain are to 1 to 1 ½ feet apart. Keep the soil slightly moist for the plants' first summer. Do not overwater the milk thistle plants. They do not tolerate overly wet or muddy, slow-draining soil.

• 12

  Remove most of the flowers as they fade after the milk thistle blooms the following year. Enclose the few flowers remaining on the plants in paper bags when they begin to dry. Doing so allows you to collect seeds for planting the next year. Store the seeds in a can or plastic bag in a refrigerator until spring.