Most indoor cannabis growers switch to flowering when their plants have developed at least 8 to 10 nodes and stand roughly 16 inches tall. That’s the point of sexual maturity, when the plant’s leaf pattern shifts from paired sets to a spiral arrangement, signaling it’s biologically ready to produce flowers. Outdoors, nature handles the timing: plants begin flowering naturally after midsummer as nights grow longer than 12 hours.
What Actually Triggers Flowering
Cannabis is a short-day plant, meaning it flowers in response to long, uninterrupted nights rather than short days. The trigger is darkness, not light. When a plant receives at least 12 consecutive hours of darkness each night, photoreceptors called phytochromes shift between active and inactive forms, which kicks off the hormonal cascade that produces flowers. Even a brief flash of light during the dark period can disrupt this process, stressing the plant enough to cause hermaphroditism, where a female plant develops male pollen sacs.
Indoors, you control this by switching your light timer from a vegetative schedule (typically 18 hours on, 6 hours off) to a 12/12 schedule: 12 hours of light followed by 12 hours of complete, uninterrupted darkness. Whatever time you choose for lights-off, stick with it every single day. Consistency matters more than the specific hour.
How To Tell Your Plant Is Ready
Node count is the most reliable indicator. Allow your seedlings to grow naturally until they’ve produced 8 to 10 nodes (the points where branches meet the main stem). Around the tenth node, cannabis reaches sexual maturity and begins producing pre-flowers, tiny structures tucked behind a small leaf sheath at each node junction. Female pre-flowers look like a small teardrop with two white hairs emerging from it. Male pre-flowers resemble a tiny cluster of grapes.
If you flip to 12/12 before the plant reaches this maturity point, it simply won’t flower. The spiral leaf pattern that replaces the earlier paired-leaf arrangement is your visual confirmation that the plant is developmentally ready. For most varieties, this happens around 16 inches of height, though genetics and growing conditions can shift that number.
Height and the Flowering Stretch
One of the biggest mistakes newer growers make is flipping too late and running out of vertical space. Cannabis doesn’t stop growing when it starts flowering. During the first two to three weeks of bloom, plants go through a “stretch” phase where they can grow dramatically taller. Indica-leaning strains typically increase their height by 50% to 100%, while sativa-leaning strains can double or even triple in size.
This means a plant that’s 24 inches tall at the flip could finish at 48 to 72 inches for a sativa. If your grow space has a ceiling, work backward from your maximum usable height, account for the distance your light needs to hang above the canopy, and flip early enough that the stretch won’t create problems. Many indoor growers flip when plants are between 12 and 20 inches tall, depending on strain genetics and room height.
Outdoor Flowering Timing
If you’re growing outdoors, the sun handles the light schedule for you. Cannabis stays vegetative through late spring and summer when days are long, then begins budding naturally as daylight hours drop below roughly 12 hours after the summer solstice. In the northern hemisphere, most outdoor plants start showing flowers between late July and mid-August, depending on latitude and strain.
Seeds can go outside in April or later (October or later in the southern hemisphere). Clones are more sensitive and tend to flower prematurely if put outside too early in the season, so waiting until late spring or early summer is safer for clones. If your local climate has a short growing season, choosing faster-finishing strains matters, because most photoperiod plants need 8 to 10 additional weeks of flowering after they start budding before they’re ready to harvest.
Autoflowering Strains Are Different
Autoflowering varieties bypass the light-schedule requirement entirely. They carry genetics from Cannabis ruderalis, a species that evolved to flower based on age rather than day length. Most autoflowers enter the flowering stage around weeks 3 to 5 after germination, regardless of how many hours of light they receive. You don’t need to change your light schedule at all. Many growers keep autoflowers on 18 or 20 hours of light per day from seed to harvest to maximize photosynthesis throughout the entire lifecycle.
Pruning Before the Flip
If you plan to do any major pruning, defoliation, or lollipopping (removing lower growth that won’t receive adequate light), do it about a week before you switch to 12/12. This gives the plant time to recover from the stress before it redirects energy toward flower production. A second, lighter round of cleanup is common around day 21 of flowering, once the stretch phase is winding down and you can clearly see which bud sites will stay in shadow. Avoid heavy pruning after that point, because the plant’s energy is fully committed to building flowers and recovery becomes slow.
Adjusting Your Environment for Bloom
Flowering plants have different needs than vegetating ones. The shift isn’t just about light hours.
Your nutrient mix should change from a nitrogen-heavy formula to one with more phosphorus and potassium. The ideal ratio during flowering is roughly 1:3:2 (nitrogen to phosphorus to potassium). Phosphorus supports flower formation, while potassium aids in the transport of sugars that build dense buds. Cut back nitrogen gradually, since too much during bloom produces leafy, airy flowers.
Light intensity should increase during flowering. The sweet spot for most flowering cannabis is between 600 and 1,000 µmol/m²/s of photosynthetically active light, which is significantly higher than what seedlings or young vegetative plants need. If your light has a dimmer, ramp it up over the first week or two of bloom rather than jumping straight to full power.
Humidity and temperature also deserve attention. A vapor pressure deficit (the relationship between temperature and humidity that drives transpiration) of 1.0 to 1.5 kPa is the commonly cited target for flowering cannabis. In practical terms, this means keeping relative humidity lower than you did during veg, typically in the 40% to 55% range depending on temperature. Lower humidity during late flowering also reduces the risk of mold forming inside dense buds.
Signs Flowering Has Started Successfully
Within the first week of 12/12, you’ll notice the stretch beginning as internodal spacing increases. By days 7 to 14, white pistils (hair-like structures) should appear at the nodes and branch tips of female plants. If you see small round clusters developing instead of pistils, those are male pollen sacs, and the plant needs to be removed before it pollinates your females.
By week three, the stretch slows and true bud formation begins. Small clusters of pistils stack together into what will eventually become the colas you harvest. From this point, the plant is fully committed to reproduction, and your job shifts to maintaining stable conditions, feeding bloom nutrients, and watching for any signs of stress or pest pressure through the remaining 6 to 10 weeks of flowering.