Cannabis flowers when nights grow long enough to trigger a hormonal shift, typically after the plant receives 12 consecutive hours of uninterrupted darkness each day. For outdoor growers in the Northern Hemisphere, this transition begins naturally in late summer. For indoor growers, it starts the moment you flip the light schedule. Autoflowering varieties skip this requirement entirely and begin flowering based on age alone, usually around three weeks after germination.
What Triggers Flowering
Most cannabis is photoperiod-sensitive, meaning the plant tracks how many hours of darkness it receives each night. During long summer days, it stays in vegetative mode, building stems and leaves. Once darkness extends to about 12 hours per night, the plant produces a light-sensitive hormone called phytochrome that signals the switch to flower production. Indoor growers replicate this by changing their light timer from an 18-hours-on/6-hours-off schedule to a strict 12/12 cycle.
That 12-hour dark period needs to be truly dark. Even tiny light leaks, something as small as a single LED indicator light on a power strip, can produce enough light to disrupt the hormonal signal. The threshold is remarkably low: just 0.01 micromoles of light per square meter per second is enough to interfere. Repeated interruptions can delay flowering, cause the plant to revert to vegetative growth, or trigger hermaphroditism, where the plant develops both male and female organs and pollinates itself into producing seeds instead of potent buds.
Outdoor Flowering by Season
Outdoors, the countdown to flowering starts just after the summer solstice, when daylight hours begin shortening. In the Northern Hemisphere, that’s around June 20 to 22, though most growers won’t notice visible flowering signs until late July or August, once the dark period has gradually lengthened enough to sustain the hormonal trigger. Plants started very early in the season, like March, can actually begin flowering prematurely because spring nights are still long enough to mimic the signal before the days stretch out in summer.
In the Southern Hemisphere, the same process begins after the December solstice, with flowering typically visible by February or March. The exact timing depends on your latitude. Growers closer to the equator see less dramatic seasonal shifts in day length, so their plants may behave differently than those in northern temperate climates.
Autoflowering Varieties Work Differently
Autoflowering strains carry genetics from Cannabis ruderalis, a subspecies that evolved in regions with extreme day lengths. These plants don’t wait for a light-cycle change. Instead, they flower automatically once they reach a certain age, typically around 21 days after germination. The full life cycle from seed to harvest runs roughly 8 to 9 weeks total: about 3 to 4 weeks of vegetative growth followed by 4 to 5 weeks of blooming. Some fast varieties begin flowering as early as 3 weeks from germination.
Because autoflowers don’t depend on darkness to flower, they can be grown under long light schedules (18 or even 20 hours of light per day) from start to finish. This makes them popular for beginners and growers working in spaces where controlling light leaks is difficult.
How to Spot the First Signs of Flowering
The earliest visual indicator is the appearance of pre-flowers at the nodes, where branches meet the main stem. On female plants, these look like tiny teardrop-shaped structures. The most definitive sign is the emergence of thin, white, hair-like strands called stigmas poking out from these pre-flowers. Male plants, by contrast, develop small round pollen sacs at the same locations.
Pre-flowers typically show up within the first week or two after the light change for indoor photoperiod plants, or around week 3 to 4 from germination for autoflowers. At this stage, the hairs are sparse and the structures are small enough that a magnifying glass helps with identification.
Week-by-Week Flowering Progression
Once flowering begins, the process unfolds in distinct phases over 6 to 16 weeks depending on the strain.
Weeks 1 to 3: The plant enters a rapid growth spurt called the “flowering stretch,” often nearly doubling in height. White pistils appear in clusters at the tops of branches, but actual buds haven’t formed yet. The plant is still producing new stems and leaves while building out its bud sites. This stretch is important to anticipate because a plant that’s already tall can outgrow its space quickly.
Weeks 3 to 4: The vertical stretch slows down. Small but visible budlets form where the clusters of pistils appeared. All the pistils are still white and pointing outward.
Weeks 4 to 6: Buds begin fattening noticeably. Most pistils remain white, but the buds are gaining real density and weight each day. Training or shaping the plant is no longer needed at this point.
Weeks 6 to 8: The plant stops producing new leaves and stems entirely. All energy goes into bud development. Pistils begin darkening from white to orange or brown, and buds hit their most rapid growth phase.
Week 8 and beyond: Buds fatten quickly in their final push. The tiny, mushroom-shaped resin glands covering the buds (trichomes) begin changing color, shifting from clear to milky to amber. Most growers harvest when the majority of trichomes are milky with a few still clear and a few turning amber. A jeweler’s loupe or pocket microscope makes this much easier to judge.
Indica vs. Sativa Flowering Duration
Indica-dominant strains typically finish flowering in 6 to 9 weeks, making them the faster option. Sativa-dominant strains take considerably longer, often 10 to 16 weeks. This is one of the biggest practical differences between the two types and a major factor in strain selection, especially for outdoor growers in short-season climates who need plants to finish before the first frost.
Many modern hybrids fall somewhere in between, finishing in 8 to 10 weeks of flowering. Seed banks usually list an expected flowering time for each strain, which gives a useful starting estimate even if the actual finish depends on growing conditions.
Adjusting Nutrients and Environment for Flowering
Cannabis has distinctly different nutritional needs during flowering compared to vegetative growth. During veg, plants rely heavily on nitrogen for building leaves and stems, with an ideal nutrient ratio of roughly 3 parts nitrogen to 1 part phosphorus to 2 parts potassium. When flowering starts, that ratio essentially flips: phosphorus becomes the priority (1:3:2) because it supports flower formation, resin production, and energy transfer within the plant. Too much nitrogen during flowering delays bud development, reduces density, and can hurt the taste and aroma of the final product.
Temperature and humidity also matter more during flowering. Daytime temperatures between 70°F and 80°F work best, with nights slightly cooler at 60°F to 70°F. As buds develop through mid and late flowering, humidity should drop to between 30% and 50%. Dense, moisture-laden buds in high humidity are an invitation for mold, which can ruin a harvest in its final weeks.