Cannabis plants flower when they receive the right combination of light signals, nutrients, and environmental conditions. For photoperiod strains, which make up the majority of cultivated cannabis, the key trigger is switching to 12 hours of uninterrupted darkness per day. But getting the plant to flower is only the first step. Growing dense, resinous buds requires managing nutrition, humidity, temperature, and light intensity across several weeks, then drying and curing properly after harvest.
How the Cannabis Life Cycle Works
Cannabis moves through four distinct stages from seed to harvest. Germination takes 2 to 10 days, during which a seed cracks open and sends out its first root. The seedling stage lasts another 1 to 2 weeks as the first true leaves appear. Then comes the vegetative stage, lasting anywhere from 2 to 8 weeks depending on how large you want the plant before it begins producing buds. Finally, the flowering stage runs 7 to 16 weeks depending on genetics.
Indica-dominant strains tend to flower faster, finishing in roughly 7 to 10 weeks. Sativa-dominant strains take longer, often 10 to 16 weeks. Hybrid strains fall somewhere in between. Knowing your strain’s expected flowering time helps you plan your grow schedule and anticipate harvest.
Triggering the Flowering Stage
Cannabis is a short-day plant, meaning it begins to flower when nights grow long enough. During vegetative growth, growers keep lights on for 16 to 18 hours a day to prevent flowering. When you’re ready, you switch to a 12-hour light, 12-hour dark cycle. This abrupt change signals the plant that “autumn” has arrived and it’s time to reproduce.
The 12/12 light cycle is the standard, but research shows many cannabis genotypes actually begin flowering with slightly more than 12 hours of light. In one study of 15 medicinal varieties, 14 had a critical threshold of 14 hours or more, meaning they’d start flowering even with relatively long days. Still, 12 hours of darkness produces the most rapid flowering response across the widest range of genetics, which is why it remains the go-to approach.
The dark period must be truly uninterrupted. Even brief light exposure during the dark hours can confuse the plant’s internal clock and delay or prevent flowering. If you’re growing indoors, check for light leaks from equipment indicators, door seams, or ventilation openings.
Autoflowering strains are the exception. These varieties flower based on age rather than light cycles, typically beginning around 3 to 4 weeks after germination regardless of how many hours of light they receive.
Shifting Nutrients for Flower Production
Cannabis plants need different nutrient ratios during vegetative growth than during flowering. In the vegetative stage, the ideal ratio of nitrogen to phosphorus to potassium is roughly 3:1:2. Nitrogen drives leaf and stem growth, which is the plant’s priority before it starts producing buds.
Once flowering begins, that ratio flips to approximately 1:3:2. Phosphorus becomes the dominant nutrient because it fuels flower development and energy transfer within the plant. Potassium supports the transport of sugars and water. Nitrogen drops because the plant is no longer focused on leafy growth, and too much nitrogen during flowering can actually slow bud development and affect flavor.
Most cannabis-specific nutrient lines sell separate “grow” and “bloom” formulas that reflect these ratios. The transition doesn’t need to be instant. Many growers shift gradually over the first week or two of flowering.
Light Intensity During Flowering
Switching to a 12/12 schedule tells the plant when to flower, but light intensity determines how dense and productive those flowers become. During flowering, cannabis performs best with a light intensity of 600 to 900 µmol/m²/s (a measurement called PPFD that describes how many photons of usable light hit each square meter per second). Some growers push up to 1,000 µmol/m²/s, though this requires higher CO2 levels and careful temperature management to avoid light stress.
If you’re using LED grow lights, most manufacturers list the PPFD output at various hanging heights. Position your lights so the canopy receives even coverage in that 600 to 900 range. Too little light produces airy, loose buds. Too much can bleach the tops of flowers and reduce quality.
Temperature and Humidity Control
Environmental conditions during flowering have a direct impact on bud density, resin production, and mold risk. A useful metric is vapor pressure deficit (VPD), which describes the relationship between temperature and humidity and how aggressively the plant transpires water through its leaves.
During early flowering, aim for a VPD around 1.0 kPa, then gradually increase to 1.2 to 1.4 kPa during peak bud development. In practical terms, this means keeping temperatures around 75 to 80°F (24 to 27°C) with relative humidity dropping from about 55% in early flower to 40 to 45% in late flower. The higher VPD in late flowering encourages resin production while reducing the risk of mold forming inside dense buds.
What’s Happening Inside the Flower
Understanding basic flower anatomy helps you assess how your buds are developing. The main structure of a cannabis bud is built from bracts, small pear-shaped pods that are often mistakenly called calyxes. Bracts house the highest concentration of resin glands on the entire plant, which is why they account for most of the weight, cannabinoids, and terpenes in harvested buds.
White hairs emerging from the bracts are pistils (specifically, the stigma portion). Their job is to catch pollen for seed production. In an unfertilized female plant, these pistils gradually darken from white to orange or brown as the flower matures. The small, mushroom-shaped resin glands covering the bracts and surrounding leaves are trichomes. These produce the cannabinoids and aromatic compounds that define a strain’s effects and flavor.
Knowing When to Harvest
Timing your harvest correctly makes a significant difference in potency and effect. The most reliable method is examining trichomes under magnification. A jeweler’s loupe (30x to 60x) or a USB microscope works well.
Early in flowering, trichomes appear clear and glassy. As they mature, they turn milky white, which indicates peak cannabinoid content. Eventually, some trichomes shift from milky to amber, signaling that THC is beginning to degrade into CBN, a cannabinoid associated with more sedative effects. The sweet spot for most growers is when 70 to 90% of trichomes are milky with some turning amber. Harvesting earlier in that window produces more energetic effects; waiting until more amber appears produces a heavier, more relaxing result.
Other visual cues include pistils darkening and curling inward (at least 50 to 70% should be brown) and a general swelling of the bracts in the final week or two.
Drying After Harvest
Proper drying preserves the terpenes and cannabinoids you spent weeks developing. The target environment is 65 to 70°F (18 to 21°C) with 50 to 55% relative humidity. Keep conditions as stable as possible: temperature shouldn’t swing more than about 2°F and humidity shouldn’t vary more than 3% throughout the process.
Most growers hang whole branches or individual stems upside down in a dark room with gentle air circulation. A small fan moving air in the room is fine, but don’t point it directly at the buds, as that causes uneven drying. Depending on your setup and whether you trimmed the leaves before or after hanging, drying takes anywhere from 5 days to about 2 weeks. Buds are ready for the next step when small stems snap cleanly rather than bending.
Curing for Flavor and Smoothness
Curing is the step many growers skip, and it’s the one that makes the biggest difference in final quality. After drying, trim the buds (if you haven’t already) and place them in airtight glass jars, filling each jar about three-quarters full to leave room for air exchange.
The target humidity inside the jars is 58 to 62% relative humidity. Humidity control packs designed for this purpose help maintain that range consistently. For the first week, open each jar once or twice a day for a few minutes to release moisture and exchange stale air, a process called “burping.” After the first week, you can reduce burping to once every few days.
A minimum cure of two weeks improves smoothness noticeably, but four to eight weeks produces the best results. During curing, residual moisture redistributes evenly through the flower, chlorophyll breaks down (reducing the “green” harshness), and terpene profiles develop more complexity. Well-cured cannabis smokes smoother, tastes better, and stores longer without losing potency.