CannaGLAP

Grow cannabis in Foster City, CA

Growing cannabis in Foster City: damp bay air and the hour the lamp goes dark

Foster City's air stays damp all summer. On the bayshore between San Mateo and Belmont, humidity averaged 76 percent in June and July 2024 and 81 in August and September. Summer nights in the mid 50s cool that air further.

Cooler air holds less water, so as the temperature drops, the relative humidity climbs. When the lamp in your tent clicks off and the space cools, what happens to the air around your buds?

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 Foster City tent from lights-on to lights-off

That switch is the moment to watch. When the lamp goes dark, the tent cools, and the same water in cooler air reads as higher humidity. In a room open to that bay air, that can push dense buds toward bud rot (gray mold that starts inside the bud). A hygrometer that records the overnight high, not just a reading at noon, shows whether it is happening in yours.

Timing the lights helps twice. The Redwood City station, 5.9 miles away, averages lows from about 40°F in winter to 57°F in July. Run the lamp through those night hours and its heat keeps the tent above the outside chill; the dark period then falls in the warmer afternoon, when the drop at lights-off is smaller.

Then take the water out. A dehumidifier sized for the room, not the tent, does the steady work on a humidistat (a switch that runs it only above a set humidity). A small fan keeps air moving through the canopy in the dark. An air conditioner dries the air only while it runs, and with about 18 afternoons a year at 90°F or hotter, it would sit idle most nights.

Winter adds cold to the damp. December and January lows run near 40°F, the station counts about 6 freezing nights a year, and January's outdoor humidity averaged 85 percent in 2024. A heated home dries that out; an unheated garage does not, so a garage dehumidifier stays on into winter.

MonthJanFebMarAprMayJunJulAugSepOctNovDec
Avg high °F596265697379818179756559
Avg low °F414345475054575754504440
Humidity %857676818171

Highs and lows: NOAA 1991 to 2020 normals, Redwood City station (5.9 miles away). Humidity: Open-Meteo historical model data, average daily humidity in 2024, for the months shown. Planning averages, not a forecast.

Outdoors on the bayshore

California permits 6 living plants per private residence, however many adults live there. They belong behind a lock, where no one standing in a public place can spot them. Indoor grows can face reasonable local rules too, and outdoor ones can be banned outright, so look up the local rules that cover your address before you plant outside.

If your address permits outdoor plants, sow seeds indoors March 4 to March 18 and plant out from April 15. Frost has come by November 24 only one fall in ten, about 223 days later. The damp is the real limit. In 2024, September stayed as damp as August and October eased to 71 percent, though nights averaging 50°F or warmer end around October 14. Grow airy, mold-resistant varieties, thin the inside of each plant, and check the fattest colas (the top bud clusters) after still, damp mornings.

Last spring frostJan 17 in a typical year, Feb 16 to be safe (later only 1 year in 10)
First fall frostDec 12 typical, as early as Nov 24 (1 year in 10)
Frost-free seasonabout 333 days
Nights stay 50°F or warmerMay 12 to Oct 14
Start seeds or clones indoorsMar 4 to Mar 18
Move plants outdoorsApr 15 or later

Your bayshore grow, on your terms

With shared walls, or a rental where a manager has a key, a grow can feel very close to other people. The settings that finally kept an August harvest clean took work to find, and they belong to you.

Canna GLAP locks each grow record on your phone before upload, and only your passphrase unlocks it. Someone who forced their way into our servers would find only unreadable grow data. Encryption guards the journal, not the room: a neighbor can still notice a humming fan or a flowering smell.

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 Foster City ask

Why do my buds get moldy in Foster City?

The air is damp for most of the season, and it gets damper inside a tent when the lights switch off and the air cools. Lights on at night, a room dehumidifier and moving air through the canopy all work against it.

Does Foster City get frost?

A few nights a year, mostly in December and January. The typical last frost comes around January 17, and one spring in ten brings a frost after February 16.

When should cannabis finish outdoors in Foster City?

In 2024, October was drier than August and September, so a plant that ripens into early October would have missed some of the damp. Nights cool after mid October, so do not push the harvest too far past it.

Growing near Foster City

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