Drying Cannabis – Hanging, Screens, Climate and End Point

Freshly harvested cannabis flowers are three quarters water. Drying cannabis means bringing them down to about 12–14 % water. Two things pull against each other: if the flowers stay moist too long, mould grows; if they dry too warm, the aroma evaporates. Drying works when it is cool and brisk and the end is checked with a hygrometer.

Hanging or screens

Two methods are common, and both work. They differ in when you trim, that is, when you cut away the small leaves around the flower.

With hanging, whole plants or large branches go upside down on a line, and trimming waits until they are dry. This works because the stem keeps feeding water to the flower for a while: it dries more slowly and more evenly. A survey of more than 2,000 samples from three years of greenhouse growing supports this method. Hung flowers trimmed dry carried clearly less yeast and mould than flowers trimmed wet and dried on racks. The researchers put this down to wounds: trimming wet flowers injures the leaf remnants, and microbes settle there. The drawbacks of hanging: it takes space, and the foliage on the branches pushed the microbe count up during the first three days. So the large fan leaves come off before hanging.

With screens, the flowers are cut off right after harvest, trimmed wet and laid on screens. Without stems and foliage they dry faster, and several screens stacked on top of each other take little room. The drawback: large flowers take much longer than small ones, so drying becomes uneven. That is why you sort them by size and lay them out singly with space between them. The higher microbe load from the greenhouse survey applies to this method. It was measured on machine-trimmed flowers, though; whether careful trimming with scissors carries the same risk was not compared.

In practice: if you dry in a damp room or have had mould before, hanging is the safer choice. If you dry on screens, you make up for the drawback by cutting cleanly, sorting the flowers by size and checking the end with a hygrometer.

The right climate

Commercial operations dry at 18–21 °C, a little cooler than a living room, and at 50–55 % relative humidity. Relative humidity says how much water vapour the air carries compared with the most it can hold. Under these conditions drying takes five to six days. The cool temperature protects the terpenes, the plant’s aroma compounds; the moderate humidity draws the water out before mould has time to grow.

Many growers deliberately dry more slowly, over ten to fourteen days, because they expect more flavour from it. That benefit has not been measured, but the risk has. Ten days of drying at 16 °C and 50 % humidity raised yeast and mould counts a hundredfold, while drying within a few hours kept them at the level of fresh flowers. In a second trial at 15 °C, flowers drying in open air were still not dry after fourteen days, and in one cultivar the mould Alternaria had infected 80 % of the flowers by day twelve. Heat does not help in the other direction: dried at 80 °C, only about half the terpenes remained.

In practice: dry cool, but not wet. A dehumidifier holds the humidity at 50–55 %, a small fan keeps the air in the room moving. If the branches are still soft after a week, lower the humidity.

When the flowers are done

The quickest check is the snap test: a thin stem breaks when bent instead of flexing. It works as a first sign but often signals too early. In one trial, flowers that counted as done by this criterion still held 16–19 % water, against 13 % for normally dried ones.

A hygrometer, a small humidity meter, is the reliable check. Put it in a jar with a handful of flowers, close the lid and read it after one to two hours. The value shows how much water the flowers still give off to the air. Divided by 100 it is called water activity: 65 % equals 0.65. Below 62 % hardly any mould can grow. Between 62 and 70 % some frugal species still survive, and that is exactly where commercial drying usually ends. If the jar shows 65 % or less, the flowers can go into curing; if it shows more, they keep drying.

The bucket method

Some growers slow down the last stage: they cut the almost dry branches into smaller pieces, put them into lidded buckets and let them finish drying on the stem for a few days before the flowers come off. The aim is for the remaining moisture from the inside of the flower and from the stem to spread evenly before the flower is fully dry. Physically, the bucket does what any closed container does: the moisture evens out between flowers, stems and air. Because the stems still give off water, the air in the bucket can end up damper than expected.

In practice: every bucket needs a hygrometer. If it stays below 62 %, hardly any mould can grow. If it climbs above 65 %, leave the lid off or put the branches back into the open air until the value drops.

What is lost during drying

The lightest aroma compounds evaporate first. After one week of air drying they made up only 46.5 % of the fragrant oil of a flower, against 58.7 % when fresh. What has evaporated does not come back: overdried flowers only get water back when rehydrated, not aroma. The active compounds, the cannabinoids such as THC and CBD, change too. The plant makes them as a precursor, the acid form, and part of it already turns into the active form during drying, markedly more with long air drying than with short drying. More on this in the article on decarboxylation.

In practice: better to stop at 62–65 % in the jar than to dry too far. What happens next in the jar is described in the article on curing.