CannaGLAP
Grow cannabis in Walnut, CA
Growing cannabis in Walnut: let the night air do the cooling
A Walnut summer day comes in two halves. August afternoons average about 92°F, and by the small hours the air has fallen near 63°F. July and September open a gap nearly as wide.
A grow tent makes most of its heat while the lamp is on. So when is your lamp on: in the hot half of the day, or the cool one?
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.
Cooling a Walnut grow with the clock
The figures come from the Pomona/Fairplex station, 7.1 miles away at 999 feet. A home in Walnut may run warmer or cooler, so trust a thermometer in your own space. The station counts about 71 afternoons a year at 90°F or hotter, with July, August and September holding about 51 of them.
From June through September, run the lights from evening to morning. July through September nights average in the low 60s, so an intake fan pulling in that air does real work while the lamp burns. Warming that air under the lamp also lowers its humidity, and the dark period falls in the afternoon, when the lamp is off and adds nothing to the day's heat.
Night air falls short in one place: a garage that bakes through an August afternoon can stay warm well after dark. Shade or insulate its sunny wall, or move the tent to an inside room for August. An air conditioner helps on hot spells, but it dries only while it runs. In 2024, outdoor humidity held between 61 and 68 percent from June through October, so a small dehumidifier on a humidistat (a switch that holds a set humidity) covers the lights-off hours.
Winter turns the schedule around. December lows average about 42°F, and the average night drops below 45°F from about November 26 to February 21. Keep the lamp on through the coldest hours and put a mat under seedlings. January's 75 percent outdoor reading fades in a heated house; an unheated garage holds onto it.
| Month | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Avg high °F | 68 | 67 | 72 | 76 | 78 | 84 | 90 | 92 | 89 | 81 | 74 | 67 |
| Avg low °F | 43 | 45 | 47 | 50 | 54 | 57 | 62 | 63 | 60 | 54 | 47 | 42 |
| Humidity % | 75 | 68 | 65 | 62 | 64 | 61 |
Highs and lows: NOAA 1991 to 2020 normals, Pomona/Fairplex station (7.1 miles away). Humidity: Open-Meteo historical model data, average daily humidity in 2024, for the months shown. Planning averages, not a forecast.
Outdoors in the eastern San Gabriel Valley
California's limit is 6 living plants per private residence, however many adults live there, grown in a space with a lock that is not visible from any public place. Local governments may set reasonable rules for indoor grows, but not ban them, and may ban outdoor grows outright. Check the rules for your address first.
Where outdoor plants are allowed, start seeds indoors March 4 to 18 and plant out from April 15, or sow autoflowers, which bloom on age rather than day length, about a week before that. Frost rarely matters here. Heat does: a photoperiod plant, which starts flowering as nights lengthen, usually begins its buds in August, the hottest month. Afternoon shade, deep watering and wide spacing help it through, and cooler, slightly drier October air suits the finish.
| Frost | Rare: about 2.7 freezing nights a year |
|---|---|
| Nights stay 50°F or warmer | Apr 18 to Nov 2 |
| Start seeds or clones indoors | Mar 4 to Mar 18 |
| Move plants outdoors | Apr 15 or later |
A Walnut grow log, sealed at the source
With Diamond Bar, Rowland Heights and West Covina nearby, a weekly farmers market can put you beside gardeners from several towns. Your summer cooling schedule took seasons to tune, and you decide who learns it.
Canna GLAP locks each grow record on your phone before it uploads, with a key only your passphrase opens. Someone who got into our servers would find grow entries scrambled past reading. That covers the journal, not a fan humming at 2 a.m. or the smell of a ripening plant.
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 Walnut ask
Should I run my grow lights at night in Walnut?
In summer, yes. Night air in the low 60s cools the tent while the lamp is on, and the dark period lands in the hot afternoon.
Does Walnut get frost?
Now and then. The Pomona/Fairplex station averages about 3 freezing nights a year, mostly in December and January.
How humid is Walnut during flowering?
Moderate. Outdoor air averaged 62 percent in August 2024, 64 in September and 61 in October, so a small dehumidifier is usually enough for a flower tent.
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