Scotch cotton thistle, known botanically as Onopordum acanthium, is a biennial flowering herb species that belongs to the aster family. Scotch thistle produces cerise pink to purple flowers in the spring and summer. Scotch thistle foliage has copious spines all along the stems. It will grow in poor soils with just natural seasonal rainfall and quickly become large and invasive in fertile and irrigated soil.
Don a pair of heavy-duty garden gloves when working with or around your Scotch thistle plant to prevent pricks, scrapes and cut injuries. Leather gardening gloves such as those used by rosarians are ideal.
Water your thistle well in the summer and fall to speed up germination of the seeds ahead of fall rains. Supplement natural rainfall throughout the year to keep the soil lightly moist at all times.
Cut back any flower heads, damaged branches or diseased parts of the plant in the fall to prepare it to overwinter. In climates where the top growth does not survive the winter, cut down the entire plant to 1 or 2 inches above the crown. Cut up the clippings into manageable pieces and discard into the trash. Bypass the compost bin which can harbor the Scotch thistle seeds and spread them widely.
Mulch around the base of the plant or over the sheared crown of the plant with at least 2 inches of organic mulch. Used shredded bark, compost, leaf mold or cocoa bean hulls for best effect and apply over the soil after watering in the fall.