Harvest, drying & curing – post-harvest science

TL;DR

• Harvest window: 70-90 % milky, 10-30 % amber trichomes.
• Dry at 18-20 °C, 55-60 % RH, 10-14 days – slow protects terpenes.
• Cure in airtight jars at 62 % RH (Boveda packs) for 2-8 weeks.
• Fast drying locks in chlorophyll = grassy smoke.

Curing – what happens in the jar

During curing, enzymes keep breaking down chlorophyll and sugars while residual moisture equalises. Anaerobic fermentation is unwanted – hence daily burping.

Harvesting, Drying and Curing: where quality is gained or destroyed

Between the time of harvest and the finished product lies the process step with the greatest leverage on quality. Ideal is slow drying over 10 to 14 days at 18 to 20 °C and 55 to 60 % relative humidity in the dark. Rapid drying at higher temperatures expels monoterpenes — precisely the volatile compounds that carry the aroma. The result smells flat and hay-like, regardless of how good the genetics were.

During the subsequent curing in a closed jar, the residual moisture balances out between the core of the bud and the surface. Two to four weeks at 58 to 62 % relative humidity breaks down chlorophyll degradation products and rounds off the aroma. Brief daily airing during the first week prevents anaerobic ammonia formation — if the jar smells sharp, the goods were stored too damp.

• Drying 10–14 days, target residual moisture 10–12 %
• Stem snap test: if the side branch snaps, the drying process is complete
• Curing at 58–62 % RH, air daily during the first week
• Store in a cool, dark place — UV light measurably degrades cannabinoids

Why the harvest date belongs on the certificate

Freshness is the most important yet poorest declared quality factor for flowers. A certificate of analysis with a harvest or production date allows an assessment of how much terpene content has already been lost since testing. If the date is missing, even a good laboratory value says little about what is actually in the package.

In Switzerland, the total THC value (THC + THCA × 0.877) alone determines whether a plant is considered industrial hemp or a narcotic. Below 1 %, cultivation is generally permitted; above that, only within the framework of an authorised pilot project under BetmG Art. 8b or with a medical prescription under Art. 8a. Anyone who cultivates themselves bears the burden of proof for the value — the analysis of the harvested batch is decisive, not the variety specification of the seed dealer.

Swiss THC limit values and the influence of the harvest time

In Switzerland, a strict limit of less than 1 percent total THC applies so that cannabis does not fall under the Narcotics Act. This value is calculated from the active THC and the precursor THCA using the factor 0.877. Since the THC content increases as the trichomes mature, the harvest time is a critical factor for Swiss CBD producers and home growers of legal hemp. Anyone who harvests too late risks the flowers exceeding the legal limit, thereby becoming illegal.

Different rules apply to participants in the state-approved Swiss pilot trials under Article 8b of the Narcotics Act, such as Züri Can or Weed Care in Basel, as THC-rich hemp may also be legally cultivated and distributed here. Outside of these scientific projects or without a medical prescription, however, compliance with the one percent limit remains mandatory. If this limit is exceeded, legal consequences and the confiscation of the harvest may result. Therefore, regular laboratory analyses of the harvest batches to determine the exact total THC value are essential in Swiss practice.

• Calculation of the total THC value according to the formula: THC + THCA × 0.877.
• Risk of exceeding the limit value if CBD strains are harvested too late.
• Exceptional regulations only apply to approved pilot trials or medical purposes.
• Personal responsibility and the burden of proof regarding the THC content lie with the grower.

Wet or dry trimming: Choosing the manicuring method

Before drying, a decision must be made whether to remove the sugar leaves from the buds while they are still wet (wet trimming) or only after they have dried (dry trimming). Wet trimming makes the work easier because the leaves are still flexible, stand out, and can be cut cleanly at the base. In addition, the risk of mould during the drying phase is reduced because a large portion of the wet plant matter is removed immediately. However, this causes the buds to dry significantly faster, which can lead to a rapid loss of terpenes if the indoor climate is not sufficiently controlled.

With dry trimming, on the other hand, the entire plant or large branches are hung up along with their leaves. The leaves wrap protectively around the buds and slow down the drying process, which helps preserve volatile monoterpenes and yields a more uniform result. The disadvantage becomes apparent during the subsequent work, as the dried leaves are brittle and curl closer to the bud. This method also requires precise control of the humidity to prevent stagnant moisture within the dense inflorescences.

• Wet trimming reduces the risk of mould by immediately removing moisture from the leaves.
• Dry trimming delays the drying process and protects the sensitive terpenes.
• Increased workload during dry trimming due to sticky, closely adhering leaves.
• The choice of method influences the required drying time and the necessary indoor climate.

Mould prevention and hygiene in the drying room

The drying phase is the most critical phase for losing the entire harvest to moulds such as Botrytis. Since the buds still have a very high water content at the beginning, the humidity in the drying room is naturally high. Insufficient air circulation quickly leads to stagnant moisture in the core of the buds, creating optimal conditions for spores. To prevent this, fans must ensure continuous but indirect air movement without blowing directly onto the buds, as this would lead to uneven, rapid drying.

In addition to air movement, meticulous hygiene in the drying room plays a decisive role in success. All tools, nets, hangers, and walls should be thoroughly cleaned and disinfected before use. Regularly checking the buds for soft, greyish spots or unusual odours is also essential in order to immediately and generously remove infected plant parts. Should a mould infestation nevertheless occur, the affected batch is unusable and must not be consumed under any circumstances for health reasons.

• Indirect air circulation prevents stagnant pockets of moisture on the buds.
• Thorough disinfection of all tools and the room before hanging.
• Daily visual and olfactory inspection of the drying flower batches.
• Immediate disposal of mould-infested flowers to prevent spore transmission.

Technical equipment for controlled drying and curing

In practice, precise maintenance of the optimal climate requires the use of specialised measuring and control devices. A simple thermo-hygrometer is often insufficient to detect fluctuations at different room heights. Calibrated sensors with data logging are recommended to trigger an early alarm if the relative humidity exceeds the critical limit. In damp cellars or during warm summer months, the use of controllable dehumidifiers or air conditioning units is almost indispensable to keep the target climate stable.

An often neglected aspect is the development of odour during the first days of drying, which can be problematic in Switzerland due to consideration for neighbours and legal requirements. A powerful exhaust fan with a fresh activated carbon filter is therefore also mandatory in the drying room. For the subsequent curing, special stainless steel containers or food-safe buckets have established themselves alongside classic mason jars. The use of two-way humidity regulators significantly facilitates humidity control in the jar and protects against drying out.

• Use of thermo-hygrometers for continuous monitoring of temperature and humidity.
• Activated carbon filters to prevent odour nuisance in the living environment.
• Dehumidifiers to safeguard against excessively high humidity in the initial phase.
• Two-way humidity packs to automatically stabilise the target humidity in the curing vessel.

FAQ