Trichome colour shows roughly how much THC a cannabis flower holds. The best day to cut also depends on aroma, bud weight, mould risk and how much drying space you have. This part explains those trade-offs, what controlled trials found about flushing, and whether to cut the whole plant at once or in stages. It ends with a checklist for harvest day.
A short recap of Part 1: under a microscope at 200× or more, the heads of the stalked trichomes go from clear to cloudy to amber. Cloudy is linked with the highest THCA, the acid form in which the plant stores THC. Amber means THC is breaking down into the weaker cannabinol (CBN). Pistil colour is only a rough first signal.
This is Part 2. Part 1: Harvest Timing: Trichome Maturity, Flushing, and the Science Behind the Rituals
The Aroma Peak Comes Before the Potency Peak
Terpenes are the aroma compounds that trichomes make alongside cannabinoids. They evaporate much more easily than THCA, so the living plant loses them all the time. The loss is faster when:
- the canopy temperature is above 26–28 °C
- UV is strong, from direct sunlight or from grow lights with added UV
- the air is dry. High VPD (vapour pressure deficit, a measure of how strongly the air pulls water from the plant) also pulls terpenes off the resin.
Growers therefore often report that aroma peaks a few days before THCA does. If aroma and flavour are your main goal, you can harvest a little before the mostly-cloudy stage. You give up a small share of THCA in exchange for fresher, more complete aroma.
This advice comes mainly from growers’ experience. It rests on the fact that terpenes evaporate more easily than THCA.
Translator’s note: Outdoor and greenhouse canopies in Thailand and Southeast Asia are often above 26–28 °C on sunny afternoons, and tropical sun carries strong UV. Plants here can therefore lose more aroma before harvest than plants in the cool climates where most harvest advice comes from. In the last weeks of flowering, keep the canopy below 26–28 °C where you can, for example by running fans or air-conditioning in the hottest hours.
What to do: If aroma matters most to you, run a small test. Harvest one plant a few days before it reaches mostly cloudy, and harvest a second plant of the same cultivar at mostly cloudy. Dry and cure both the same way, then compare the smell. You will learn whether an earlier cut is worth it for that cultivar.
Keeping the aroma after the cut
Harvest timing only sets the starting point. Drying and curing decide how much of the aroma survives. The key factors are:
- drying conditions
- water activity, a measure of how much free water in dried flower is available to mould and microbes
- jar curing
- burping, which means opening curing jars briefly to let moisture out
All of these are covered in the Harvest, Drying & Curing overview and in the detailed guide Drying & Curing: The Science of Not Ruining Your Harvest.
Translator’s note: Outdoor air in the Thai rainy season can reach 85–95% relative humidity. Five practical setups for drying under those conditions are compared in Drying Cannabis at 85–95% Ambient Humidity.
What to do: Read those guides and set up your drying space before you cut. Do not start working it out on harvest day.
Flushing Before Harvest
Flushing means watering only with plain, pH-adjusted water for the last one to two weeks before harvest. It is one of the most debated practices in cannabis growing.
Controlled trials have not found a clear, consistent benefit from flushing for cannabinoid or terpene levels. Rx Green Technologies, a company that sells cannabis nutrients and substrates, ran an in-house trial in 2019. It was not peer-reviewed. Flushes of 0, 7, 10 and 14 days made no significant difference to THC, terpenes or yield, or to taste in a blind test. A 2024 peer-reviewed study by Saloner, Sade and Bernstein in Industrial Crops and Products tested five cultivars. Flushing had only limited effects. Cannabinoid levels changed in some cultivars, rising in two and falling in one, and the authors still considered flushing an acceptable practice. Many growers think the harsh “green” taste comes more from chlorophyll, the green pigment in leaves, than from leftover nutrients. Chlorophyll stays in the flower when drying and curing are done poorly.
Many experienced growers still report smoother smoke from flushed plants in their own conditions. Both sides of the evidence are reviewed in Pre-Harvest Flushing: Science or Myth?. That topic also covers the cases where flushing really helps as a way of fixing a problem.
What to do: Test it in your own grow. Take two plants of the same cultivar, flush one and not the other, and dry and cure both the same way. Ask someone else to label the samples so you do not know which is which, then compare them. This removes your own expectations from the result.
Highest Potency Versus the Best Day to Harvest
These two ideas are often mixed up, but they are different.
Peak THCA happens at roughly the mostly-cloudy stage. At that point, the trichome heads are fully opaque and the calyxes (the swollen cups around each flower) are swelling at their fastest rate. Harvest here if you want the highest THCA on a lab test or the most extract per gram of flower.
The best day for dried whole flower depends on four more factors:
- Bud density and calyx swelling. Growers report that many cultivars keep gaining real weight, from extra calyx mass and resin, for a week or two after THCA peaks. Cutting at peak THCA can give up that extra yield.
- Aroma. The light monoterpenes keep evaporating after the THCA peak. The longer the plant stays, the heavier and less fresh its smell becomes.
- Mould. In humid conditions or with dense buds, every day after peak THCA raises the risk of Botrytis (grey mould, also called bud rot). If you are mainly trying to avoid disease, the best day may be earlier than the day of highest potency.
- Work and market. Room space, staff schedules and buyer demand do not always allow you to harvest at the ideal biological moment.
No single answer is right for everyone. The best day depends on which goal you put first.
Translator’s note: For outdoor and greenhouse crops in the Thai rainy season, mould risk usually outweighs the other factors. A crop lost to Botrytis is worth nothing, however much THCA it held.
What to do: Before flowering ends, write your goals down in order, for example: 1. no mould, 2. aroma, 3. weight. On harvest day, when the signs point different ways, follow that list.
Cut the Whole Plant at Once or Harvest in Stages?
Cutting the whole plant at once
The whole plant is cut in one session. The advantages are:
- The work is simpler. There is one harvest, and the room can be cleared and reset right away.
- All the flower is processed together, so it dries and cures as one even batch.
- Total labour during the harvest window is lower.
Harvesting in stages
The top colas (the main bud clusters at the branch tips) are cut first. The lower buds stay on the plant and keep developing under the light that now reaches them.
Why it can work: Light is uneven through the canopy. Over the whole flowering period, the top buds receive far more light than the lower buds. Growers measure this light as PPFD, the amount of usable light reaching a spot each second. In many cultivars, growers report that the lower buds mature about 7–14 days after the top colas. Once the top is cut, direct light reaches the lower buds. They finish noticeably denser and more resinous than they would in a single whole-plant harvest.
The case against it:
- Each cut on the remaining plant is a way in for disease, especially in humid conditions.
- New leafy shoots can grow from the cuts in the remaining weeks. Nodes can also switch back to leaf growth (revegging). These shoots are usually unwanted.
- Several harvests and several drying batches make post-harvest work harder.
In the Thai rainy season, high humidity late in flowering raises Botrytis pressure. Outdoors and in greenhouses, harvesting in stages may not be practical. The disease risk at the cuts and the extra days of exposure can outweigh what the lower buds gain.
Translator’s note: Indoors, where you control humidity, harvesting in stages is more realistic. A power cut can still stop the air-conditioner or dehumidifier for hours, though, and that leaves fresh cuts exposed to damp air.
What to do: Outdoors or in a greenhouse in the rainy season, cut the whole plant at once. Indoors with humidity control, try harvesting in stages on one plant first. Inspect the cut sites for mould every day, then compare its lower buds with those from a plant cut all at once. You will see whether the extra work pays off in your room.
Harvest-Day Checklist
| Sign | Target |
|---|---|
| Pistil colour | Over 70% orange, brown or red (varies by cultivar) |
| Trichomes on top colas | Mostly cloudy; amber share according to your goal |
| Trichomes on lower buds | More clear and cloudy if you plan to harvest in stages |
| New pistils | Stopped or very few, a sign that calyx formation is finished |
| Calyx development | Bracts visibly swollen; no new growth from the growing tips (meristems) |
| Aroma strength | At its peak; smell is strongest in the early morning before the lights come on |
| Botrytis check | Any visible mould means harvest immediately, whatever the trichomes show |
| Drying space | Ready before you cut: dark, cool, humidity controlled, with gentle, indirect airflow |
| Cultivar information | Treat the breeder’s stated flowering time as the minimum, not a fixed date |
What to do: Print this table and tick each row for every plant or batch on harvest day. Keep the sheet with the run’s records so you can compare it with how the dried flower turns out.
After the Cut
Harvest quality is not settled on the day you cut. Drying and curing explain a large part of the difference between identical plants processed under different conditions.
What to do: Start with the Harvest, Drying & Curing overview, which summarises all three stages, and then read the full drying and curing guide. Before harvest day, run your drying room empty for a day and record its temperature and humidity. This shows whether it can hold the conditions you need once it is full of wet plants.
Sources Mentioned in This Article
- Rx Green Technologies, “Cannabis Flushing: What a Controlled Trial Actually Found”, in-house trial run in 2019 (report dated November 2019), not peer-reviewed: Cannabis Flushing Myth: What Our Trial Found | Rx Green
- Saloner A., Sade Y. and Bernstein N. (2024). “To flush or not to flush: Does flushing the growing media affect cannabinoid and terpenoid production in cannabis?” Industrial Crops and Products 220, 119157. Redirecting
- Receptor-binding studies on cannabinol, cited in Part 1 for CBN binding the CB1 receptor roughly ten times more weakly than THC. Figures vary between studies and methods, for example Rhee et al. (1997), Journal of Medicinal Chemistry 40: 3228.
If you have tested flushing or harvesting in stages in your own grow, post your results below this topic. Results from Thai and Southeast Asian conditions are especially useful to other growers.