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.
Mature height: 3-5 ft
Method: Direct sow: Direct-sow outside. It doesn't transplant well, or doesn't need the extra time indoors.
When to plant milk thistle, by zone
Every zone milk thistle grows in. Pick your zone in the header and it will be highlighted here. Each zone links to its full page.
| Zone | Start indoors | Sow or plant out | Harvest |
|---|---|---|---|
| 5a & 5b | Not usually | Apr 26–May 3 | Aug 24–Aug 31 |
| 6a & 6b | Not usually | Apr 11–Apr 18 | Aug 9–Aug 16 |
| 7a & 7b | Not usually | Mar 27–Apr 3 | Jul 25–Aug 1 |
| 8a & 8b | Not usually | Mar 11–Mar 18 | Jul 9–Jul 16 |
| 9a & 9b | Not usually | Feb 1–Feb 8 | Jun 1–Jun 8 |
| 10a & 10b | Not usually | Jan 16–Jan 23 | May 16–May 23 |
Harvest dates are about 120 days after the sow or plant-out date.
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
True Leaf Market (each supplier's 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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