Cannabis stored through a Toronto winter dries out faster than cannabis stored in July, and the reason is physics. Cold outdoor air holds very little water vapour. When your furnace warms it, the amount of water in it does not change but the air's capacity to hold water rises sharply, so relative humidity falls. Air at minus 10 degrees Celsius, heated indoors to 21 degrees, lands near 10 percent relative humidity before any humidifier touches it. Flower stored in that air loses moisture to the room, gets brittle, and crumbles when you handle it.
The short version
- Your home in winter is drier than flower wants. Health Canada recommends indoor relative humidity between 30 and 50 percent. Dried cannabis is generally stored between 55 and 62 percent.
- Forced-air heat compounds it, because moving air accelerates evaporation from anything porous.
- Brittleness is a moisture problem, not an age problem, and it is the first thing you will notice.
- A sealed glass jar plus a two-way humidity pack handles almost all of it.
- Cold itself is not the enemy. Temperature swings are, because a cold container brought into a warm room collects condensation.
Why winter air is so dry indoors
Relative humidity is a ratio, not a quantity. It measures how much water vapour air holds against the maximum it could hold at that temperature. Warm air can hold far more water than cold air, so the same absolute amount of moisture reads as a high percentage when cold and a low percentage when warm. A Toronto January day at minus 10 degrees, even at full saturation outdoors, arrives inside at roughly 10 percent relative humidity once heated to 21 degrees. A humidifier can push a room back toward 35 or 40 percent, which is the right target for a house. It is still well below the 55 to 62 percent range where dried flower is stable.
Forced-air systems add a second effect. Moving air carries evaporated moisture away and replaces it with drier air, so evaporation keeps running rather than slowing as the local air saturates. A container on an open shelf near a heating register sits in a small wind tunnel.
What happens to flower as it dries
Dried cannabis still contains water, generally 10 to 15 percent of its mass after curing, and that water exchanges with the surrounding air until the two reach equilibrium. Put flower in a room at 25 percent relative humidity and it will give up moisture until it matches. Three things follow.
Structure breaks down
Below roughly 55 percent relative humidity, plant material stiffens. Stems snap instead of bending, buds fragment under light pressure, and grinding produces more fine powder and less consistent particle size.
Trichomes become fragile
The resin glands on the surface of the flower sit on thin stalks, and brittle stalks break. That resin then ends up on your fingers and in the bottom of the container rather than on the flower, which is why very cold flower should be handled as little as possible until it has warmed.
Terpenes evaporate
Terpenes are volatile organic compounds. Volatile is a chemical description: these molecules have a measurable vapour pressure at room temperature, so they move from the plant material into the surrounding air continuously. Heat raises that vapour pressure and airflow removes the vapour so more can evaporate. The observable result is that aroma fades. More on these compounds in Terpenes, explained.
Light, heat and oxygen do not take the winter off
THC oxidises to CBN over time, at a rate set by light, temperature and oxygen exposure. A four-year study published in Forensic Science International in 2019 found that samples kept in light at 22 degrees lost close to all of their THC, samples kept dark at the same temperature degraded more slowly, and samples held at minus 20 degrees in the dark changed least.
Winter does not change that chemistry, but winter habits sometimes do. A clear container on a windowsill gets direct light and a cold surface. Since March 2025 Canadian flower may be sold in transparent containers and containers with cut-out windows, which is useful in a store and less useful in your kitchen. We covered that in Why your cannabis package has a window now.
Containers
| Container | Winter notes |
|---|---|
| Original child-resistant container | Fine for short periods, less reliable once the seal has been opened repeatedly |
| Glass jar with a gasketed lid | Impermeable and airtight. Size it close to the volume of flower so there is little air in it |
| Soft plastic bag | Permeable to water vapour and prone to static, which strips trichomes |
| Refrigerator | Repeated warming and cooling causes condensation inside the container every time you open it |
| Freezer | Slowest degradation, but only for material you will not handle until it has warmed, because frozen trichomes shatter |
Humidity packs
A two-way humidity pack holds a saturated salt solution in a permeable membrane. It releases water vapour when the surrounding air is drier than its set point and absorbs it when the air is wetter. Packs are sold at set points commonly of 58 and 62 percent.
Size the pack to the container, because a small pack in a large jar will exhaust itself. Replace it when it turns rigid, which means it has given up its water. And remember it regulates the air inside a sealed container, so it does nothing in a container that leaks. In a dry house, a correctly sized pack in a sealed glass jar is the single change that makes the most difference.
Where to put it
The target is stable temperature, no direct light and no airflow. An interior closet on an interior wall usually works well in a heated flat, and so does a cupboard away from the stove or a bedroom drawer.
Avoid the windowsill, the top of the fridge, any shelf above a radiator or baseboard, anywhere within a metre of a heating register, and the bathroom, where humidity swings from near saturation during a shower to room level an hour later. Avoid an unheated balcony or garage, where condensation is guaranteed.
Condensation, and the rule that follows
When warm room air meets a cold surface it cools, its capacity to hold water drops, and the excess condenses. Bring a cold container in from a delivery, open it immediately, and moisture will condense on the walls and on the product. Let it reach room temperature first. An hour is usually enough for a small jar.
Other formats
Vape cartridges. Cannabis extract thickens as it cools, so a cartridge brought in from the cold will draw poorly until it warms, and forcing a cold draw is a common way to flood or clog one. Store cartridges upright so the oil sits against the wick. More on the hardware in 510 cartridges and vapes, explained.
Edibles. Shelf-stable and the least affected by winter. Keep them sealed, away from heat and within the best-before date.
Hash, rosin and concentrates. More temperature-sensitive than flower. Cool and dark is the requirement, and a humidity pack is not appropriate because there is no plant material to regulate.
The date still matters most
Good storage slows degradation. It does not reverse it. The packaging date on a Canadian label is the most useful number for judging where a product sits in its life, and how to read it is in Packaged on is not harvested on. Our year-round storage notes are in How to store cannabis.
Our staff can walk you through anything printed on a package. They cannot give medical advice, and we do not make claims about what a product will do for you. You can see what is on our shelf in our full range, and our hours and directions are on the visit page.