Missouri Marijuana Seeds and the 6-6-6 Home Grow Rule

Missouri’s adult-use rules are written into the state constitution by Amendment 3, which voters approved in November 2022 (Article XIV). Adults 21 and older may possess up to 3 ounces, and they may grow at home after registering with the Department of Health and Senior Services for a personal cultivation card.

The plant limit is often called 6-6-6: up to six flowering plants, six nonflowering plants over 14 inches (36 cm) tall and six clones under 14 inches. Plants must be kept in an enclosed, locked facility. Medical patients grow under a separate patient cultivation card.

Growing outside these rules is a crime. NORML’s penalty summary lists unlicensed cultivation as a felony, with penalties that rise with weight. Check the Division of Cannabis Regulation for the current application, annual fee and renewal steps before your first seed goes in water.

Missouri spans USDA zones 5b to 8a on the 2023 hardiness map, with the warm end in the Bootheel and the cold end along the Iowa border. NOAA’s 1991-2020 normals give St. Louis a median last spring frost of April 2 and first fall frost of November 4, about 215 frost-free days. Kansas City runs April 13 to October 24, about 193 days, and Springfield sits between them at about 199.

That is a generous season, but Missouri summers are hot and humid, and July afternoons often reach the 90s (above 32°C). Heat stress can slow bud development, so afternoon shade and deep watering help. Spring brings severe thunderstorms and hail, a good reason to keep seedlings in containers until late April. Early fall is often milder and drier than midsummer, which helps late-flowering plants finish clean.

Missouri’s frost-free window is long enough for most feminized photoperiod seeds, including hybrids that flower for 9 to 10 weeks. Because the law counts flowering plants separately, feminized seeds let you fill all six flowering slots with females and keep the nonflowering slots for your next round.

Heat tolerance matters more here than in northern states. Sativa-leaning genetics with Haze or Thai backgrounds often handle July heat well but may need until late October to finish, which works in St. Louis and is risky near the Iowa line. In northern Missouri, fast flowering seeds are the safer pick.

Outdoor seeds bred for vigor and mold resistance suit most backyard grows, and autoflowers make a good early-summer crop that is finished before the late-August heat.

Missouri requires an enclosed, locked facility either way, so the choice comes down to climate and space. Outdoors, a locked greenhouse or fenced garden makes full use of the 190-plus-day season and can produce big plants. Our guide on when to plant outdoors helps with timing.

Indoors, the 6-6-6 structure suits a perpetual setup: clones and nonflowering plants in a veg tent while six plants flower in another. Plan for strong cooling in summer, since grow lights plus Missouri heat can push a tent past 85°F (29°C). Keep your cultivation card current whichever route you choose.