For indoor photoperiod cannabis, you trigger flowering by switching your light schedule to 12 hours on and 12 hours off. Most growers make this switch after about four weeks of vegetative growth, though you can extend that period if you want larger plants. The timing depends on your setup, your strain, and whether you’re growing indoors, outdoors, or with autoflowering genetics.
The Light Switch for Indoor Plants
Photoperiod cannabis stays in its vegetative stage as long as it receives 16 or more hours of light per day. To initiate flowering, you abruptly change the schedule to 12 hours of light and 12 hours of uninterrupted darkness. This mimics the shortening days of late summer and tells the plant to start producing buds. The darkness period is the critical part: even brief light interruptions during those 12 dark hours can stall or reverse the transition.
Research from controlled growing environments confirms that 12/12 is the standard, but it’s not the only option that works. Plants flowered under 14 hours of light and 10 hours of dark still entered bloom, though yields and timing varied. For most home growers, sticking with 12/12 is the simplest and most reliable approach.
How Long to Veg Before Flipping
There’s no single correct answer here because it depends on your grow space. Four weeks of vegetative growth is a common baseline, but the real factor is how tall you want your plant at harvest. Cannabis typically doubles in height after the flip to 12/12. Sativa-dominant strains can triple in height or more. So if your tent is five feet tall and your light hangs a foot from the ceiling, you need your plant to be no taller than about two feet when you flip.
You can technically keep a photoperiod plant in veg indefinitely, growing it as large as you want before switching. Some growers veg for eight weeks or longer to maximize yield from fewer plants. Others flip early at three weeks to keep plants small in tight spaces, a technique sometimes called “sea of green.” The tradeoff is straightforward: longer veg means bigger plants and potentially more bud, but it also means more time, more electricity, and more vertical space needed.
Checking for Sex Before You Flip
If you’re growing from regular (non-feminized) seeds, identifying plant sex before flowering saves you from wasting weeks on male plants that won’t produce usable buds. Pre-flowers appear between the fourth and sixth nodes of the plant, usually three to six weeks after germination. Male pre-flowers look like small, smooth, egg-shaped sacs. Female pre-flowers show up as tiny V-shaped structures with white or pink hairs (pistils). Females tend to show slightly later than males, typically between weeks four and six.
If you’re using feminized seeds, sex identification is less of a concern, but it’s still worth watching for the occasional hermaphrodite plant, especially under stress.
Outdoor Flowering Timing
Outdoor plants flower on nature’s schedule. As days shorten after the summer solstice, nights eventually grow long enough to trigger the bloom response. In most of the northern hemisphere, outdoor cannabis begins flowering naturally in late July or August, when uninterrupted dark periods consistently reach about 12 hours. In the southern hemisphere, the equivalent window falls around February or March.
You don’t control the flip outdoors, but you do control when you start. Planting seeds or transplanting clones too early in spring, before the last frost, risks killing young plants. A good guideline is to wait until a few weeks after the spring equinox: April or later in the northern hemisphere, October or later in the southern hemisphere. The longer your growing season, the more vegetative growth you’ll get before nature triggers flowering.
Autoflowers Follow Their Own Clock
Autoflowering strains don’t wait for a light change. Thanks to genetics inherited from Cannabis ruderalis, they transition to flowering based on age alone, typically between weeks three and five after germination. By week five, most autoflowers are showing clear pre-flowering signs regardless of the light schedule they’re under.
This means you can run autoflowers on 18/6 or even 20/4 light schedules from seed to harvest, giving them more photosynthesis time during bloom than photoperiod plants get. The downside is you can’t extend the vegetative phase to grow a bigger plant. What you see at three to four weeks is roughly what you’re working with.
Preparing Your Plant for the Flip
The transition into flower is a high-energy period. Many growers do a round of pruning and training on the same day they switch to 12/12 or in the days just before. “Lollipopping,” which means removing lower branches and foliage that won’t receive much light, is commonly done right at the flip. A second, lighter round of leaf removal around three weeks into flowering helps improve airflow and light penetration to developing bud sites. After that second session, most growers leave the plant alone until harvest.
Any high-stress training, like topping or heavy bending, should be finished during veg. The flowering stretch (that doubling in height) happens in the first two to three weeks after the flip, and the plant is already redirecting energy toward bud production. Stressing it with major structural changes at this point can reduce yields.
Dialing In Your Environment
Flowering plants prefer slightly different conditions than vegging plants. During veg, an ideal vapor pressure deficit (VPD) range is 0.8 to 1.1 kPa. Once flowering begins, the target shifts to 1.0 to 1.5 kPa. In practical terms, this means you want slightly lower humidity during flower than during veg. A common approach is to keep daytime temperatures around 75 to 80°F with relative humidity dropping from 55 to 65% in veg down to 40 to 50% during mid-to-late flower. Lower humidity during bloom reduces the risk of mold forming inside dense buds.
Nutrient Shifts at Flower
Vegetative plants consume a lot of nitrogen to build stems and leaves. Once flowering begins, the plant’s demand shifts toward phosphorus and potassium, which support bud development. Most bloom-specific fertilizers reflect this by reducing the nitrogen ratio and increasing phosphorus and potassium. During the first week or two after the flip, the plant is still stretching and growing vegetatively, so a gradual transition from your veg nutrients to bloom nutrients works better than an abrupt swap. By week three of flower, when stretch slows and bud sites are forming in earnest, you’ll want to be fully on your bloom feeding schedule.
Signs That Flowering Has Started
After switching to 12/12, the first visible changes usually appear within a week. The plant begins stretching noticeably, with internodal spacing increasing as stems elongate. By the second week, female plants start pushing out white pistils at the nodes, which are the first signs of bud formation. This is also when you can most confidently confirm plant sex if you haven’t already. The stretch typically continues through weeks two and three, then tapers off as the plant focuses its energy on filling out bud sites for the remainder of the flowering cycle, which runs eight to twelve weeks total depending on strain.