Growing Milk Thistle: A Complete Profile
By Growing Home Greens Team · August 20, 2026
Milk thistle is handsome, widely sold as a medicinal herb, and illegal to let grow in parts of the country.
Check your state first
It is a regulated Class A noxious weed in Washington, where eradication is required state-wide. It is listed by the California Invasive Plant Council. King County records that it forms dense patches that push out other species and that it is a prolific seeder.
That is not a caution about tidiness. In some jurisdictions growing it is a legal problem, and this is worth ten seconds of checking before you sow a packet.
It has killed livestock
Milk thistle accumulates nitrates, and grazing animals that eat it can suffer lethal nitrate poisoning. If your garden borders pasture or you keep animals, that is a second reason to leave it alone.
What it looks like if you do grow it
Three to five feet of spiny rosette and flower stalk, with deep green leaves marbled in white along the veins, topped by purple thistle flowers. Hardy zones 5 through 10 in full sun, grown as an annual or biennial.
Deadhead before seed sets, without exception.
A note on sourcing: the horticultural figures here come from general references rather than an extension profile, so treat them as indicative. The weed status is documented by the state authorities cited.
Source: Washington State Noxious Weed Control Board, Milk Thistle.
Companion planting guide for Milk Thistle · Find your planting dates
Typically viable in USDA zones 5-10. Not sure your zone? Look it up by ZIP code.
Method: Direct sow: Direct-sow outside. Milk Thistle doesn't transplant well, or doesn't need the extra time indoors.
See exact start-indoors, direct-sow, and harvest dates for your zone →
How many Milk Thistle plants fit per square foot?
Spacing: about 4 square feet per plant (square-foot-gardening spacing). Mature height: 3-5 ft.
Where to buy milk thistle seeds
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See it growing
Search YouTube for "growing milk thistle" videos → (a live search, not a hand-picked video; results vary and aren't vetted by us)
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Growing Milk Thistle: A striking marbled thistle that is a Class A noxious weed in Washington with eradication required by law.
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