CannaGLAP
Grow cannabis in Ontario, CA
Growing cannabis in Ontario: a cooling problem with almost no heating season
Ontario growers can nearly forget one worry. The Ontario International Airport station, 2.9 miles away, records about 2 freezing nights a year. What it records instead is heat: roughly 93 days that reach 90°F, with August the hottest month at about 95°F.
So the useful question is not when the cold arrives. It is how much heat your grow space can shed on an August afternoon, and what happens to the plants when it cannot.
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 an Ontario grow space
The hot stretch is long, and it leans late. July and August bring about 23 and 24 days at 90°F or more, September still brings about 18, and October about 8. A tent in an insulated garage or a live-work loft in the Emporia Arts District has to get rid of lamp heat through all four of those months. Put the lights-on hours at night: August nights average about 65°F, some 30 degrees below the afternoon.
That night air is the cheapest cooling you have. An intake fan running with the lamps after dark pulls it across the canopy. In the worst weeks you may still need an air conditioner, so size the grow to what your cooling can hold, not to the tent.
Humidity runs the other way from the heat. June averaged 63 percent, the dampest of the five warm months measured, then July 55, August 52, September 56 and October 47. That suits flower: late buds ripen in drier air, and bud rot pressure stays modest. A fall batch of seedlings may need a humidity dome while it roots in.
Winter is the easy season. December, the coldest month, averages about 67°F by day and 44°F at night, and the average night stays above 45°F from January 23 to December 8. A garage tent rarely needs more than the lamps' own warmth. January's outdoor humidity of 69 percent counts for little inside a heated house, which dries the air; a dehumidifier then is worth it only in a room you leave unheated and damp.
| Month | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Avg high °F | 68 | 68 | 72 | 76 | 80 | 86 | 94 | 95 | 91 | 83 | 75 | 67 |
| Avg low °F | 45 | 46 | 49 | 51 | 56 | 60 | 65 | 65 | 64 | 57 | 49 | 44 |
| Humidity % | 69 | 63 | 55 | 52 | 56 | 47 |
Highs and lows: NOAA 1991 to 2020 normals, Ontario International Airport station (2.9 miles away). Humidity: Open-Meteo historical model data, average daily humidity in 2024, for the months shown. Planning averages, not a forecast.
Outdoors in San Bernardino County
State law sets the cap at 6 living plants per private residence, however many adults live there. The plants must be in a locked space, not visible from a public place, and local governments get a say: cities and counties can regulate and may ban outdoor grows. Read Ontario's own ordinance before a plant goes in the ground.
If outdoor plants are allowed at your address, sow seeds indoors between March 4 and March 18 and move plants outside from April 15. Frost almost never sets the date here; the risk is a plant that reaches flower in August's 95°F heat. Shade the pots in the afternoon, mulch to keep roots cool, and since six plants covers the whole residence, train a few into a low, wide canopy that stays inside its locked space.
| Frost | Rare: about 1.7 freezing nights a year |
|---|---|
| Nights stay 50°F or warmer | Apr 1 to Nov 12 |
| Start seeds or clones indoors | Mar 4 to Mar 18 |
| Move plants outdoors | Apr 15 or later |
Public city, private grow
Ontario lives a lot of its life in public: the Ontario Art Walk and Loft Tour, Christmas on Euclid, a summer evening at Celebration Park. A loft that opens to visitors, or a garage sharing a wall with the neighbors, is where a grow gets noticed. Your notes on it need not be.
Canna GLAP encrypts the journal entries on the phone itself, ahead of any upload. A server breach would hand the intruder grow records in unreadable form, with no key to open them. The cooling schedule that got your plants through August stays yours.
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 Ontario ask
Does Ontario, California get frost?
Rarely. The airport station averages about 2 freezing nights a year, so frost almost never sets the planting date; summer heat is the main limit.
How do I keep a grow tent cool in an Ontario summer?
Schedule the lamps for the dark hours, when August air cools to about 65°F, and draw that air through the tent. Daytime August averages about 95°F, with about 24 days at 90 or more, before any lamp heat is added.
Can I grow cannabis outside in Ontario?
Only where the city allows it. California caps a private residence at 6 living plants in a locked space not visible from a public place, and cities and counties can regulate and may ban outdoor grows. Where Ontario permits it, plants can go out from April 15.
Growing near Ontario
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