CannaGLAP
Grow cannabis in Longmont, CO
Growing cannabis in Longmont: nearly half the nights of the year freeze
The Longmont station, 1.3 miles away and sitting at 4,950 feet, counts about 178 nights a year at 32°F or colder. That is nearly every night from November through March, plus about 16 in April and 15 in October.
Then July turns around and reaches 90°F on about 17 days. A Longmont grow has to survive both. Before the next run, ask which one your space is weaker against.
Adults 21 and over: yes (residents). Plant limit: 6 per resident, 3 or fewer flowering; 12 per residential property unless local law allows more.
Enclosed, locked area not openly visible; separate locked space if anyone under 21 lives there; local rules can be stricter. Source: Colorado home grow laws; 12 plant cap: HB 17-1220. Local rules (landlords, HOAs, city codes) can add more; this is a summary, not legal advice.
A Longmont tent for a cold year with a hot middle
For about seven months, cold sets the rules. Lows average about 15°F in December and 14°F in January, and a garage tent in Prospect New Town or a basement near Main Street can sink well below where roots do their work. Run the lights through the coldest hours, keep pots off bare concrete, and add a heater on a thermostat rather than one that runs flat out.
The air in those months is drier than the outdoor figure suggests. January averages 65 percent humidity outside, but freezing air holds little water, and once a furnace warms it the room runs dry. From fall to spring, give seedlings and fresh clones a humidity dome or a tent humidifier.
July reverses the job. Afternoons average 91°F and August adds about 15 days of 90°F heat, yet July nights still drop to about 55°F. Use that gap of about 35 degrees: run the lights overnight from June through August and pull in the cool night air, so the lamps' heat lands when the outside air can carry it away.
Flowering gets a break. September and October average about 49 percent humidity, which gives bud rot less to work with, though a closed tent late in flower still benefits from a small dehumidifier.
Heater safety: use one with tip-over and overheat shutoff, plug it straight into a wall outlet (not an extension cord or power strip), and keep it a few feet from tent fabric and plants.
| Month | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Avg high °F | 45 | 47 | 57 | 63 | 73 | 84 | 91 | 88 | 80 | 66 | 54 | 45 |
| Avg low °F | 14 | 17 | 26 | 32 | 41 | 51 | 55 | 54 | 46 | 33 | 24 | 15 |
| Humidity % | 65 | 47 | 44 | 52 | 49 | 49 |
Highs and lows: NOAA 1991 to 2020 normals, Longmont 2 Ese station (1.3 miles away). Humidity: Open-Meteo historical model data, average daily humidity in 2024, for the months shown. Planning averages, not a forecast.
The Longmont outdoor window
Colorado lets residents grow at home only behind a lock, in an enclosed space not openly visible, with a separate locked space if anyone under 21 lives there, and local rules can be stricter. The count is 6 plants per resident with no more than 3 flowering, and 12 per residential property unless local law allows more.
If your space qualifies, start seeds April 16 to 30 and plant out May 28 or later, when average nights reach 45°F. An early frost can come by September 23, which leaves about 118 days, and warm nights of 50°F or better average only from June 14 to August 30. That calendar suits autoflowers, and photoperiods only when they are bred to finish by mid September.
| Last spring frost | May 7 in a typical year, May 22 to be safe (later only 1 year in 10) |
|---|---|
| First fall frost | Oct 5 typical, as early as Sep 23 (1 year in 10) |
| Frost-free season | about 151 days |
| Nights stay 50°F or warmer | Jun 14 to Aug 30 |
| 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 |
Keep the Longmont notes to yourself
Longmont growers swap stories at the Longmont Farmers Market, at a First Friday makers market on Fourth, or on a summer night at Rhythm at Roosevelt. The stories are for sharing. The log underneath them, the part a short Longmont season taught you, is worth guarding.
Canna GLAP encrypts your grow entries on your phone, and only then sends them up. Someone raiding our servers would turn up locked grow records and nothing to unlock them with. A roommate or a landlord sees only what you show them.
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 Longmont ask
When can I plant cannabis outside in Longmont?
May 28 or later. Frost usually ends around May 7, and May 22 is the safer date, but average nights only reach 45°F on May 28.
Does Longmont's winter hurt an indoor grow?
It can. The station averages about 178 freezing nights a year and January lows near 14°F, so a garage or basement tent needs a thermostat heater and lights scheduled through the coldest hours.
What should I grow outdoors in Longmont?
Autoflowers, or photoperiods that finish by mid September. From a May 28 transplant to an early frost on September 23 is about 118 days.
Growing near Longmont
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