Grow cannabis in Great Falls, MT

Growing cannabis in Great Falls: a long winter indoors and a 112-day summer out

In Great Falls, the grow that matters most is the one that runs from October to May. The city logs about 143 freezing nights a year, January lows average 18°F, and even October brings about 11 nights at or below 32°F. The warm season is real. It is also short, and it shows up late.

So before you order seeds, settle one thing: are you planning a summer grow with a winter attached, or a winter grow with a summer bonus?

Montana home grow rules (as of October 2026)

Adults 21 and over: yes. Plant limit: 2 mature + 2 seedlings per adult; residence cap twice that (4 + 4).

Locked space; not visible from a public place; own the home or have written permission from the owner. Source: MCA 16-12-106. Local rules (landlords, HOAs, city codes) can add more; this is a summary, not legal advice.

A Great Falls tent, from the coldest months to July

Cold is the first design problem. December and January lows sit near 20°F and 18°F, and a basement tent or an insulated garage a few blocks off Central Avenue loses heat through the floor all night. Set pots on a pallet or a sheet of foam board instead of bare concrete, slide a warming mat under the seedling tray, and let the light period cover the small hours, when the house is coldest and the lamp's warmth does the most good.

Then July turns the tables. Highs average 86°F, about 13 days reach 90°F, and humidity falls to about 39 percent, the driest month measured. A tent stacks its own heat on top, so run the lights at night through July and August, and keep a dome or humidifier on fresh clones and seedlings for their first two weeks.

Flowering gets a break. September and October average about 46 percent humidity together, low enough that bud rot is rarely the threat it is in wetter towns. In Great Falls, the bigger danger to a late harvest is the calendar, not the damp.

Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Avg high °F	37	39	48	58	67	75	86	84	73	59	45	37
Avg low °F	18	19	25	33	42	50	55	53	45	36	26	20
Humidity %	60					49	39	50	48	43

Highs and lows: NOAA 1991 to 2020 normals, Great Falls 16st station (1.5 miles away). Humidity: Open-Meteo historical model data, average daily humidity in 2024, for the months shown. Planning averages, not a forecast.

The short outdoor window in Cascade County

Montana law wants plants locked up and hidden from every public place, and the home has to be yours, or you need written permission from its owner. A locked greenhouse or a screened, fenced yard can qualify. Start seeds under lights from April 16 to April 30 and set them outside on May 28 or after. The last frost usually lands around May 6, yet in roughly one year of ten it still strikes on May 19 or later, and nights do not average 45°F until late May.

From there the clock runs fast. Nights drop below that 45°F average after about September 14, and in an early year frost arrives by September 17, which leaves about 112 outdoor days. Autoflowers and quick photoperiod strains are the realistic choices. North-central Montana is also windy, so stake plants early and put a greenhouse wall or windbreak on the open side of the yard.

Last spring frost	May 6 in a typical year, May 19 to be safe (later only 1 year in 10)
First fall frost	Oct 1 typical, as early as Sep 17 (1 year in 10)
Frost-free season	about 147 days
Nights stay 50°F or warmer	Jun 18 to Aug 28
Start seeds or clones indoors	Apr 16 to Apr 30
Move plants outdoors	May 28 or later
Season note	Frost can arrive before mid October here: long flowering plants may not finish outdoors

A small town, and a log that stays put

Great Falls is a town where people know each other. Spend a Saturday at the Great Falls Farmers Market or a day at the Montana State Fair and someone will ask how your plants did. A friendly answer is one thing. The whole log is another, every Montana winter you worked through written down in one place. Most notes apps store all of that where the company behind them is able to open it.

Your grow records are encrypted on the phone before anything is sent. A thief inside our systems would leave with coded records and nothing to decode them. What happens in your basement by the Missouri River is not ours to read.

What is encrypted: your plants, notes, photos, feed logs and harvests. What is not: account details (such as your email, name and plan), a grow name or town if you type one, your team, and your sensors' names. The full list: what a hacker would get.

Questions growers in Great Falls ask

When should I start cannabis seeds in Great Falls?

Start them under lights between April 16 and April 30, then move plants outside on May 28 or later. That date clears the late frosts, which run past May 19 only about once a decade, and the cold nights of early spring.

Can photoperiod cannabis finish outdoors in Great Falls?

Usually not without help. The outdoor window runs about 112 days, and frost has arrived by September 17 in an early year, so most photoperiod strains are still flowering when it closes. Autoflowers or a greenhouse are the safer plan.

Do I need a dehumidifier for a Great Falls flower room?

Less often than in wetter cities. September and October average about 46 percent humidity, and July drops to about 39, so young plants are more likely to need added moisture than buds are to need less.

Growing near Great Falls

All Montana cities · Every state where adults may grow at home