How to store CBD flower: 55-65% humidity, out of the light

Dried flower degrades under four factors: poorly set humidity, light, heat and air. At least one published source has measured each of them.

The title of the ASTM D8197 standard says what it does: maintain a water activity between 0.55 and 0.65 for dried cannabis flower. Each limit has a reason. Staying under 0.65 guards against micro-organisms such as mould, and staying over 0.55 keeps the flower from breaking when handled.

Water activity (aw) differs from water content. It measures the available water, the share that can pass into the air or feed a mould. Inside a closed jar the air ends up in equilibrium with the flower, and its relative humidity in percent then equals the water activity times 100. An aw of 0.55 to 0.65 therefore corresponds to 55 to 65% relative humidity in the jar.

Humidity packs printed with 58 or 62% follow manufacturers' specifications. Both values fall within the standard's range, though neither is a reference on its own. You can check with a small hygrometer left in the closed jar for a few hours. A reading above 65% means the flower is too moist, and our article on mould on CBD flower lists the signs to look for. Humidity is first set during drying, which our guide to drying and curing covers.

The reference work is from 1976. Writing in the Journal of Pharmacy and Pharmacology, Fairbairn and colleagues tracked pure cannabinoids and cannabis samples over two years. They concluded that light is the greatest single factor in cannabinoid loss, above all in solution. Oxidation by air also caused marked losses, smaller when the plant's glands were left intact. Under their conditions, temperature had little effect up to 20 °C.

Kept in the dark at room temperature, the samples stayed reasonably stable for one to two years in the authors' view. In practice, opacity counts for more than the material of the container. A clear glass jar works if it stays in a closed cupboard. On a lit shelf it is in the worst place possible, even in a cool room.

For a 2020 study in Frontiers in Plant Science, flowers and extracts spent a year in the dark at four temperatures: -80, -30, 4 and 25 °C. Milay and colleagues report that whole flowers kept at 4 °C gave the best conditions, and that 25 °C brought the largest changes in cannabinoids. Colder storage did no better than 4 °C.

Extracts showed the widest gap. In ethanol or DMSO, a laboratory solvent, THCA lost about 22% over a year at 4 °C and 85% at 25 °C. In olive oil the difference was much smaller, 18% versus 24%. The two studies are consistent: in 1976 heat had little effect up to 20 °C, and in 2020 a year at 25 °C was the deciding factor.

Format counts as much as temperature. In a 2021 study in Pharmaceutics, CBD was held at 25 °C and 60% humidity. CBD powder stayed stable for 270 days and, after a year in a closed vial, had lost only about 8%. The same CBD dissolved in sunflower oil lost 42% in a year. What this means for bottles is set out in our article on storing CBD oil.

The 2020 study recorded a loss of over 50% of flower terpenoids within four months, at every temperature. Cannabinoids fell by 26% on average across twelve months. Terpenes give the flower its smell and are more volatile, evaporating at room temperature.

A flower can therefore keep most of its cannabinoids after losing half its scent. A faded smell signals that the most fragile fraction has gone; the product is not empty. In the other direction, a figure on a certificate of analysis is valid on the test date and says nothing about a flower badly stored for a year since. Our article on hemp terpenes explains the chemistry, and the one on trichomes shows where the plant stores these molecules.

Ground samples in the 2020 study contained more degradation products than whole flowers, especially at 25 °C. Fairbairn had given the reason in 1976. The plant's glands, the trichomes, shield their contents like tiny containers, and grinding breaks them open while multiplying the surface exposed to air.

Grind just before use, and never a week ahead. A small jar filled with the amount you store also leaves less air around the buds than a large half-empty one. This last point is deduced from the role of oxidation; no published measurement backs it.

Over a year, the 2021 study searched for the products of CBD breakdown. Many partly degraded samples contained CBN, and none contained Δ⁹-THC. The belief that CBD flower "gains" THC with time confuses storage with reactions triggered in the laboratory under acidic conditions.

CBN is the usual marker of ageing. Fairbairn had already observed THC oxidising into CBN in air and in the dark, so a high CBN figure in the analysis of an older batch is to be expected. The difference between the two molecules is set out in our comparison of CBD and CBN.

The fridge gave the best results for whole flowers in the 2020 study. Its drawback is condensation, because a cold jar opened in a warm room fogs up. Leave the jar closed until it is back at room temperature, then open it.

The freezer brought no gain. For whole flowers, the study found -30 and -80 °C no better than 4 °C, so there is no reason to freeze flower you will use in the coming months.

As for an expiry date, no primary source sets a precise shelf life for flower. Reason in terms of storage conditions, and see our guide to the best-before date of flower, hash or oil. Resins and concentrates have different rules, described in our article on storing rosin and crumble. If you are choosing a flower, the CBD flower page lists the variety and reference level of each product.