CannaGLAP

Grow cannabis in San Rafael, CA

Growing cannabis in San Rafael: the hard season is the wet Marin winter, not the summer

San Rafael summers treat a grow room kindly. Its winters are the ones that test it. January air outside averaged 86 percent humidity in 2024, the highest of the months measured, while December and January nights average about 42°F.

Cold and wet together is what an unheated garage in Marin County faces in winter. So if your tent sits somewhere the furnace never reaches, what happens to its air the minute the lamp clicks off?

California home grow rules (as of October 2026)

Adults 21 and over: yes. Plant limit: 6 living plants per private residence, however many adults live there.

Locked space, not visible from a public place; cities and counties can set reasonable rules for indoor grows (but cannot ban them) and may ban outdoor grows; a landlord may prohibit growing on a rental. Source: Department of Cannabis Control; conditions: Prop 64 text as passed in 2016 (H&S Code 11362.2 and 11362.45). Local rules (landlords, HOAs, city codes) can add more; this is a summary, not legal advice.

A San Rafael grow room, built for winter first

The figures come from the Kentfield station, 1.4 miles away and 145 feet up. In a heated bedroom, January's outdoor humidity hardly matters: warmed air runs dry. In a space that tracks the weather, the lamp warms the tent while it runs, then shuts off and lets the air cool toward 42°F, and as it cools its humidity can climb fast.

Run the lights through the night, the coldest hours, so the lamp's warmth arrives when the room is losing heat, and let the tent sit dark through the milder afternoon. Then give that unheated room a dehumidifier for winter, plus a seedling mat under young plants. Kentfield logs about 5 freezing nights a year, nearly all from December through February, and a garage tent needs a thermostat-run heater for those.

Summer is the easier half. Highs run about 80 to 83°F from June through September, and nights average a cool 52 to 54°F, so an intake fan brings in useful air after dark. The station's 27 or so hot afternoons a year peak in September, at about 7.

Harvest timing comes with a choice. Humidity rose from 59 percent in June 2024 to 67 in September, then fell to 54 in October. A run that ripens in October finishes in that drier air; one that finishes in September ripens through the damper weeks, and that is the price of the early cut.

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.

MonthJanFebMarAprMayJunJulAugSepOctNovDec
Avg high °F576165697480838382766456
Avg low °F424344465052535453504542
Humidity %865960656754

Highs and lows: NOAA 1991 to 2020 normals, Kentfield station (1.4 miles away). Humidity: Open-Meteo historical model data, average daily humidity in 2024, for the months shown. Planning averages, not a forecast.

Planting out in Marin County

The state cap is 6 living plants for each private residence, however many adults live in it, inside a locked space hidden from anyone in a public place. Cities and counties can set reasonable rules for indoor grows (they cannot ban them) and may ban outdoor grows. Read San Rafael's rules first.

If outdoor plants are permitted at your address, get seeds going indoors in the March 4 to 18 window and set them out from April 15. Frost is rarely the limit: one spring in ten brings a frost after February 25. Cool nights are. They average 45°F only from March 25 and 50°F from May 22, so April plants start slowly. The season then runs about 229 days to the early first frost of November 30, long enough for a photoperiod plant to finish in October's drier air.

Last spring frostJan 16 in a typical year, Feb 25 to be safe (later only 1 year in 10)
First fall frostDec 18 typical, as early as Nov 30 (1 year in 10)
Frost-free seasonabout 334 days
Nights stay 50°F or warmerMay 22 to Oct 16
Start seeds or clones indoorsMar 4 to Mar 18
Move plants outdoorsApr 15 or later

Your San Rafael grow, off the record

You may run into other growers at the Sunday Marin Farmers Market, the Marin County Fair or a PorchFest afternoon. Encryption keeps your journal private; it cannot stop a neighbor from catching the smell of a late-flower plant over the fence.

Canna GLAP scrambles the grow journal on your own phone before it is uploaded. Should someone break into our servers, they would carry off grow records that read as noise, with no key to make sense of them. The winter setup that finally kept your garage tent clean belongs to you alone.

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 San Rafael ask

Do I need a dehumidifier for a winter grow in San Rafael?

Only if the tent is in an unheated or damp space, such as a garage. January air outside averaged 86 percent humidity; a heated room dries it down, while an unheated one holds it.

When can I plant cannabis outside in San Rafael?

From April 15, with seeds started indoors March 4 to 18, if your local rules allow outdoor plants. They need a locked spot hidden from public view, with 6 living plants per residence at most.

Is September or October better for a San Rafael harvest?

In 2024, October was the drier month, at 54 percent humidity against 67 in September. A later finish ripens in that drier air; an earlier one trades it for damper weeks.

Growing near San Rafael

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