Flowering is the stage that decides final quality and yield. The plant stops putting energy into new stems and leaves and puts it into buds instead. That is when the buds produce cannabinoids and terpenes (the aroma compounds). Depending on genetics, flowering lasts 7–12 weeks. After harvest, drying and curing decide how much of that quality you keep.
This part covers how flowering starts, flowering week by week, reading trichomes to time the harvest, and the harvest, drying and curing steps. It ends with a table of common mistakes from flowering to storage. Light schedules are written as hours of light / hours of dark, so 12/12 means 12 hours on and 12 hours off.
This is Part 2. Part 1: The Cannabis Life Cycle: From Seed to Harvest — A Complete Guide
How flowering starts
Photoperiod plants start flowering when the nights get long enough. The leaves sense light with a pigment called phytochrome, which comes in two forms. Light turns the Pr form into Pfr, the form that absorbs far-red light. In the dark, Pfr slowly changes back into Pr.
According to the original guide, flowering starts once each unbroken dark period is long enough. Indoors, growers use 12 hours of darkness. By then, enough Pfr has turned back into Pr. The leaves then release florigen, a flowering signal that travels to the shoot tips and starts flower growth.
Any light during the dark period (a light leak) can reset this process. The plant gets stressed and may turn hermaphrodite, meaning it grows male flowers as well as female ones (see sex determination).
Autoflowers carry a gene from Cannabis ruderalis that starts flowering by plant age. They do not depend on day length. See the autoflower guide.
Translator’s note: Thai nights stay close to 12 hours all year. That is why photoperiod plants grown outdoors here can start flowering early, sometimes before they have grown much. See why photoperiod plants flower at six weeks in southern Thailand and flowering photoperiod at Thai latitudes. Indoors, power cuts cause a different problem. A mechanical timer stops during an outage and runs late afterwards, so the lights can come on in the middle of the dark period. After every outage, reset the timer to the real time.
Flowering week by week
| Phase | Timing | What happens | What to do |
|---|---|---|---|
| Stretch | Weeks 1–3 | The plant grows 50–100% taller, the gaps between nodes get longer, pre-flowers appear | Raise phosphorus and potassium, lower nitrogen step by step, raise the lights |
| Bud development | Weeks 3–6 | Buds form and swell; trichomes start to appear | NPK about 1-3-2, add calcium and magnesium, RH 40–50% |
| Ripening | Weeks 6–8+ | Buds get dense, 50–70% of pistils turn brown or orange, trichomes turn cloudy | Lower the EC (feed strength), flush in the last 1–2 weeks, night temperature 18–24 °C |
| Harvest | Weeks 8–12 | 70–90% of trichomes cloudy and 10–30% amber; 70%+ of pistils brown | Optional 48 hours of darkness before harvest; cut early in the morning |
Pistils are the hair-like parts of the female flower that catch pollen. RH is relative humidity, the amount of moisture in the air. For how much extra phosphorus actually helps, see bloom booster fertilizer. For a week-by-week description of bud growth, see cannabis flowering stages.
Translator’s note: In the Thai rainy season, average outside relative humidity in Bangkok is about 75 to 80%, and it often rises to about 90% at night (Thai Meteorological Department, 1991–2020 averages). You cannot hold 40–50% RH with fans alone. You need a closed room with an air conditioner and a dehumidifier sized for it; see dehumidifier and air-conditioner sizing. A night temperature of 18–24 °C also needs air conditioning in most Thai rooms. Outdoor and greenhouse growers cannot control either one. Plan those runs so ripening does not fall in the wettest months (see outdoor cannabis in Southeast Asia). Indoors, record RH every morning and every evening for one week of flowering. That record tells you whether your equipment can hold the target.
Trichomes: reading them to time the harvest
Trichomes are tiny resin glands on the buds. Under magnification, they look like little mushrooms. They hold most of the plant’s cannabinoids and terpenes. Their colour shows how ripe the bud is. See the guide to harvest timing and trichome maturity.
| Trichome state | Colour | Cannabinoid level | What it means for harvest |
|---|---|---|---|
| Clear | Glassy, transparent | Low; THCA (the acid form of THC in the living plant) has not built up yet | Too early |
| Cloudy | Opaque, milky white | Highest; THCA at its peak | Main harvest window |
| Amber | Yellow-brown | THC is breaking down into CBN by oxidation | Later harvest; more breakdown products |
The working rule: harvest when 70–90% of trichomes are cloudy and 10–30% are amber. Check them on the calyxes of the main bud. A calyx is the small swollen pod at the base of each flower. Do not judge from the sugar leaves, the small resin-covered leaves inside the bud. Trichomes on sugar leaves often look riper than those on the bud, and you would harvest too early. For what CBN and THCA are, see the cannabinoid family.
From week 6, check the same main bud every 2–3 days with a jeweller’s loupe or a clip-on phone macro lens. Write down the share of cloudy and amber trichomes each time. You will see the trend and can plan the cutting day several days ahead.
Translator’s note: If you grow for a licensed buyer, agree on the harvest target and the lab test with them before week 6. Their product specification, not the grower’s preference, should decide the cutting day. See how to read a certificate of analysis.
Harvest
Cutting on the right day is only the first step. Drying and curing decide how much of the bud’s quality you keep, and poor drying loses volatile terpenes.
- Flush: for the last 7–14 days, water with plain water at pH 6.0–6.5 and no fertiliser. The topic on pre-harvest flushing weighs the evidence for and against.
- Darkness before harvest (optional): keep the plant in complete darkness for 24–48 hours before you cut.
- Cut early in the morning: many growers harvest before the lights come on.
- Wet or dry trim: a wet trim (straight after cutting) is easier and gives neater buds, but the buds dry faster. A dry trim (after drying) keeps terpenes better but needs more hanging space. See wet vs dry trimming.
Before harvest day, measure the space in your drying room. It tells you whether you have room for a dry trim or should trim wet.
Drying
| Factor | Best value | Too low | Too high |
|---|---|---|---|
| Temperature | 15–20 °C (60–68 °F) | Below 10 °C: drying is too slow, mould risk | Above 25 °C: terpenes evaporate, flavour is lost |
| Humidity (RH) | 55–65% | Below 45%: dry outside, wet inside | Above 70%: mould grows fast |
| Duration | 10–14 days | Under 5 days: chlorophyll has not broken down | Over 21 days: higher mould risk |
| Light | Complete darkness | — | Light breaks THC down into CBN |
| Airflow | Gentle and indirect | No airflow: damp pockets form and mould starts | Strong direct air: drying is too fast |
The snap test: bend a small branch. When it snaps cleanly instead of bending, the buds are dry enough to cure. See drying and curing.
Translator’s note: A Thai room without air conditioning is usually far above 20 °C, and in the rainy season the air can reach 85–95% RH. You cannot hit 15–20 °C and 55–65% RH without an air-conditioned room and a dehumidifier. Five practical Thai setups are costed in drying cannabis at 85–95% ambient humidity. A power cut stops both machines, and humidity in the room climbs quickly. After every outage, check the hygrometer and the buds before you leave the room again. Damp buds can grow mould that leaves toxins behind; see mycotoxins on cannabis flower.
Put a thermometer-hygrometer at bud height in the drying room. Record it twice a day for the whole drying period. That record shows whether your room actually holds the target, or whether you need more dehumidifier capacity.
Curing
Curing is slow, controlled drying in closed jars. It lets the remaining moisture spread evenly through each bud.
- Put the buds in airtight glass jars, filled to 70–75%.
- Burping: for the first 7–10 days, open the lids for 5–10 minutes, 1–2 times a day. This lets out moisture and fresh air in.
- After 2 weeks, open the jars only once every 2–3 days.
- Cure for at least 2–4 weeks. The best quality comes after 6–8 weeks. Premium strains can cure for up to 6 months.
- Keep the jars at 16–21 °C, with 55–62% RH inside, in complete darkness. A Boveda 62% humidity pack in each jar helps hold that level.
Translator’s note: In Thailand, 16–21 °C usually means keeping the jars in a closed cabinet inside an air-conditioned room. Boveda packs may have to be imported, so check local stock before you rely on them. Whether or not you use a pack, put a small digital hygrometer in one test jar. If it reads above 62%, burp more often. Fresh buds also hold more moisture in humid weather, so this matters most in the rainy season. For long-term storage, see storage: humidity, light, oxygen and shelf life.
Mistakes from flowering to storage, and how to fix them
| Stage | Mistake | What happens | Fix |
|---|---|---|---|
| Flowering | Light leaks in the dark period, even from LED standby lights on equipment | Hermaphrodite plants, seeds in buds, or the plant goes back to vegetative growth | Seal the room completely; stand inside at night with all lights off and look for leaks |
| Flowering | Too much nitrogen | Dark green leaves, loose buds, grassy smell after drying | Lower nitrogen from the stretch onward |
| Flowering | RH above 60% from week 4 | Very high risk of botrytis (grey mould) destroying buds, especially dense ones | Dehumidifier, fans, remove some leaves; keep RH at 40–50% |
| Flowering | Harvesting too early, while trichomes are clear | Lower cannabinoid content than the plant could have reached | Wait for 70–90% cloudy and 10–30% amber |
| Post-harvest | Drying too fast (above 25 °C, under 5 days) | Grassy smell, terpene loss | 15–20 °C, RH 55–65%, 10–14 days |
| Post-harvest | Skipping the cure | Harsh, flat-tasting product | Cure in sealed jars for at least 2–4 weeks |
| Post-harvest | Jars filled over 80% | Moisture builds up and mould grows inside the jar | Fill to 70–75%; use a Boveda 62% pack |
Translator’s note: If botrytis appears despite these steps, cut out the infected bud and remove it from the room in a closed bag. Spray only products registered with the Department of Agriculture (กรมวิชาการเกษตร), and use them exactly as the label says. See what you may legally spray under Thai GACP.
Thai rules that apply to this guide
This guide is for education and technical information only. It is not legal advice. If you grow outside Thailand, check the law where you live.
- Cannabis flower (ช่อดอกกัญชา) is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568. It was issued under the Thai Traditional Medicine Protection Act B.E. 2542 and has been in force since 26 June 2025.
- Flower may be sold only with a PT 33 (ภ.ท.33) prescription, for at most 30 days. Advertising and online sale of flower are prohibited.
- The Ministerial Regulation on Controlled Herbs (No. 2) B.E. 2569 has been in force since 30 April 2026. It tightens the licence criteria for cannabis flower.
- The Ministerial Regulation on Cannabis or Hemp Extracts B.E. 2569 has been in force since 26 April 2026. Licences for extracts above 0.2% THC go only to Thai juristic persons that are not “foreign” under the Foreign Business Act B.E. 2542, to state agencies and to the Thai Red Cross Society.
- Unlicensed research, export, sale or processing of a controlled herb carries up to 1 year in prison and/or a fine of up to 20,000 baht (Section 78). A licence can be suspended for up to 90 days per order (Section 52) or revoked (Section 54). After a revocation, you cannot get a new licence under the Act for two years.
- Cannabis products must stay within the THC limits set by current law. The topic on controlled herb vs Category 5 narcotic explains those limits and how samples are tested.
- The draft Cannabis Control Act is not law yet. The Cabinet approved the draft on 22 September 2026 and sent it to the House.
Before you harvest for sale, confirm that your licence covers what you plan to do in Licensing and Compliance in Thailand. Read the original legal texts in Thai Cannabis Law and Compliance.
References
- Chandra, S., Lata, H., & ElSohly, M.A. (Eds.) (2017). Cannabis sativa L. – Botany and Biotechnology. Springer. doi:10.1007/978-3-319-54564-6
- Rodriguez-Morrison, V., Llewellyn, D., & Zheng, Y. (2021). Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Frontiers in Plant Science, 12, 646020.
- Bernstein, N., Gorelick, J., Zerahia, R., & Koch, S. (2019). Impact of N, P, K, and Humic Acid Supplementation on the Chemical Profile of Medical Cannabis (Cannabis sativa L). Frontiers in Plant Science, 10, 736.
- Jin, D., Jin, S., & Chen, J. (2019). Cannabis Indoor Growing Conditions, Management Practices, and Post-Harvest Treatment: A Review. American Journal of Plant Sciences, 10(6), 925–946.
- Backer, R., et al. (2019). Closing the yield gap for cannabis: A meta-analysis of factors determining cannabis yield. Frontiers in Plant Science, 10, 495.
- Caplan, D., Dixon, M., & Zheng, Y. (2019). Increasing Inflorescence Dry Weight and Cannabinoid Content in Medical Cannabis Using Controlled Drought Stress. HortScience, 54(5), 964–969.
- Saloner, A. & Bernstein, N. (2021). Nitrogen supply affects cannabinoid and terpenoid profile in medical cannabis (Cannabis sativa L.). Industrial Crops and Products, 167, 113516.
- Danziger, N. & Bernstein, N. (2021). Light matters: Effect of light spectra on cannabinoid profile and plant development of medical cannabis (Cannabis sativa L.). Industrial Crops and Products, 164, 113351.
- Stack, G.M., Toth, J.A., Carlson, C.H., et al. (2021). Season-long characterization of high-cannabinoid hemp (Cannabis sativa L.). GCB Bioenergy, 13(4), 546–561. doi:10.1111/gcbb.12793
- Eichhorn Bilodeau, S., Wu, B.-S., Rufyikiri, A.-S., MacPherson, S., & Lefsrud, M. (2019). An Update on Plant Photobiology and Implications for Cannabis Production. Frontiers in Plant Science, 10, 296.
Post your drying-room readings, trichome photos or harvest questions below this topic. Please include your temperature, humidity and the season, because those numbers decide most of the answers.