Short answer
THC tracks trichome number, trichome age, and harvest timing. The capitate-stalked glandular trichomes on female flower bracts hold most of the THC in the plant. Growers raise THC by picking high-THCA genotypes, keeping resin heads intact, and cutting in the window when heads are cloudy. Light, temperature, and post-harvest handling move the final number up or down. No technique adds THC that the genotype cannot make.
trichome maturity color and harvest time
Three trichome types
Cannabis grows three glandular trichome types. They differ in size and in cannabinoid output.
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- Bulbous: 10 to 15 micrometers. Low resin volume.
- Capitate sessile: 25 to 30 micrometers. Resin head sits on a short base.
- Capitate stalked: 50 to 500 micrometers. Largest head, highest cannabinoid and terpene content.
Trichomes sit on bracts, calyxes, and sugar leaves. Bract trichomes carry the highest THC load. Sugar leaves mature first and test lower.
Development timeline
Timing shifts with cultivar. Photoperiod types run 8 to 11 weeks of flower. A general sequence:
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- Weeks 1 to 3 of veg: glandular trichomes form on new growth at low density.
- Flower week 1 to 3: stalked trichomes appear on bracts and calyxes.
- Flower week 3 to 6: density climbs and resin fills the heads.
- Flower week 6 to 9: THCA content peaks. Heads shift from clear to cloudy.
- Late flower: heads turn amber as THC oxidizes to CBN.
The THC pathway
CBGA is the shared precursor. THCA synthase converts CBGA into THCA. CBDA synthase converts the same molecule into CBDA. Plants with high THCA synthase activity and low CBDA synthase activity make more THCA. THCA is not psychoactive. Heat removes the acid group in a step called decarboxylation and yields THC. The mass conversion factor is 0.877. A lab report showing 25% THCA equals about 21.9% THC after decarboxylation. Total THC = THC + (THCA x 0.877).
What raises THC
- Genetics: the largest single factor. Two clones grown side by side can differ by 10 points of THC from different seed lines.
- Light: intensity and spectrum. Flower rooms run 800 to 1200 PPFD. UV-B trials show higher THC in some cultivars and no change or lower yield in others. Treat UV-B as a possible small gain, not a promise.
- Temperature: enzyme activity falls at high heat. Most trials find temperature effects smaller than genotype and light effects.
- Minerals: sulfur and phosphorus feed the pathway. Excess nitrogen pushes leaf growth and can cut cannabinoid per gram of flower.
- Water: late drought stress can raise concentration per gram while cutting total yield.
- Handling: trichome heads are fragile. Every brush, squeeze, and shake costs resin.
Harvest window
Check heads with a 30x to 60x loupe on bracts, not sugar leaves. Target most heads cloudy, few clear, few amber. Clear heads hold mostly CBGA and low THC. Amber heads mark THC loss to CBN, a cannabinoid with low psychoactivity. Peak THC sits in the cloudy window.
Post-harvest losses
Dry at 60 to 70 F and 55 to 65% relative humidity for 7 to 14 days. Cure 2 to 4 weeks in sealed jars at 58 to 62% RH. Store away from light, heat, and air. THC degrades to CBN over months. Ground flower loses THC faster than whole flower because more surface sees oxygen. Labels measure the jar at test day, not the jar at week 12.
Open questions
UV-B dose, blue to red ratios, and the exact point of peak THCA per cultivar are not settled. Cultivar-specific harvest data is thin. Test batches at two harvest dates if the number matters.