Cannabis Guide: Vegetative Stage to Flowering Stage

By Rick Smith / March 30, 2022

Leafy vegetative cannabis plant beside an early-flowering plant with a timer between, in a grow tent

Table of Contents

For photoperiod cannabis, the switch from veg to flower is the biggest decision you make in a grow. Until you flip, the plant builds leaves, stems and roots. After you flip, it puts its energy into flowers. Flip too early and you get a small plant. Flip too late and the plant can outgrow your space. Below, you will find what triggers flowering, how to decide when to switch, how to make the change, and what to expect in the weeks after. It draws on University of Georgia Extension and NC State Extension hemp research. Check your state’s home grow law before you start, including plant limits.

What triggers flowering

Photoperiod cannabis flowers in response to long nights. University of Georgia Extension explains that “most hemp cultivars will flower when exposed to day lengths of fewer than approximately 13 to 14 hr.” Indoors, the common grower practice is 12 hours of light and 12 hours of uninterrupted darkness to start flowering.

Autoflowering plants are different. UGA describes “autoflowering (day neutral) cultivars that will initiate flowering a certain number of days after seeding, regardless of the day length.” Autos do not need a switch. This guide is all about photoperiod plants.

How long to veg

There is no single right veg time. It depends on your space, plant count and strain. Common grower rules of thumb:

Situation	Typical approach
Small tent, several plants	Shorter veg (about 3 to 5 weeks from seedling)
One or two plants, training with a screen	Longer veg until the canopy is mostly filled
Tall, stretchy sativa leaning strains	Flip earlier, since they tend to stretch the most
Compact indica leaning strains	Can veg a little longer
Clones	Often flipped sooner, since they start more mature

These are common practice, not Extension figures.

Plan for the stretch

Most plants grow noticeably taller in the first two to three weeks of flower. Many growers expect a plant to grow to roughly one and a half to two times its height at the flip. Measure your headroom: leave room for the pot, the plant after stretch and a safe distance to the light.

How to switch from veg to flower: step by step

Check plant maturity. Seed plants need to be mature enough to flower. Many growers look for alternating branch nodes and several weeks of strong growth.
Measure your space. Make sure the plant can grow to one and a half to two times its height and still fit under the light.
Finish major training. Top, bend and set up any screen before the flip, so plants recover in veg.
Light proof the space. Check for leaks around zippers, vents and doors with the lights off.
Set the timer to 12/12. Pick a dark period that suits your schedule and the room’s temperature, then do not change it.
Write down the flip date. NC State Extension notes that breeders increasingly recommend harvesting “based off of weeks after floral initiation,” so this date matters later.
Switch nutrients gradually. Many growers move toward a lower nitrogen feed as flowering gets going. That is grower practice, not an Extension figure, so follow your nutrient label.
Keep tucking or training during the stretch. Stop major training once buds start to form.
Watch for first pistils. White hairs at the nodes and tops usually appear within one to two weeks.

What changes after the flip

Stretch: fast vertical growth for two to three weeks.
Pistils: white hairs appear where buds will form.
Feeding: demand shifts away from nitrogen toward other nutrients.
Humidity: as buds form, lower humidity and steady airflow matter more.

NC State notes that “often when we notice the crop flowering it has actually been in the reproductive phase for a couple of weeks,” which is a field observation, but it is a good reminder to look closely at the nodes after the flip.

Temperature and humidity after the flip

Flowering plants generally do best a little cooler and drier than in veg. As buds form, keep air moving through the canopy and bring humidity down gradually over the first few weeks. Large swings between day and night temperatures can stress plants and cause condensation on buds later. Many growers aim for daytime temperatures in the 70s °F in flower and humidity that drops toward 40 to 50% by late bloom, which is common grower practice rather than an Extension figure.

Switching outdoors

Outdoors, photoperiod plants flip on their own as nights lengthen in late summer. You cannot control the date, but you can choose strains whose flowering time fits your season, and you can avoid planting so late that plants have little time to veg. In regions with early frost or fall rain, faster finishing strains or autoflowers can help.

Signs a plant is ready to flip

Beyond size, look for a healthy plant with no active pest or nutrient problems. Flipping a stressed plant tends to carry that stress into flower, when it is harder to fix. Clear up yellowing, pests or root issues first. If you trained the plant, give it about a week to recover from topping or heavy bending before you change the light schedule.

Can you go back to veg?

Yes. Putting a flowering plant back under long days (often 18 hours of light) can trigger revegging. It is slow and plants may grow odd, single bladed leaves for a while. It is usually done to save a special plant, not as a routine step. See our guide to revegging.

Troubleshooting

My plants have not started flowering after two weeks of 12/12. Check for light leaks with the lights off. Even small leaks or a timer error can delay flowering.

Plants stretched into the light. Raise the light, bend tops sideways, or use a screen. Next time, flip earlier.

Some plants show pollen sacs after the flip. Remove males or any pollen sacs before they open. Slip a plastic bag loosely over the plant or branch first so pollen does not spread. Light leaks and stress can cause female plants to form sacs.

Lower leaves yellowing in early flower. Some yellowing is normal as feeding needs change. If it moves fast, check pH and your feed.

Buds are forming slowly. Check light intensity and temperature. See our guide on light intensity in flowering.

Related guides

Once you are flowering, see the final weeks of flowering. If you want a shorter path, see feminized vs autoflowering seeds.

FAQ

When should I switch my cannabis from veg to flower?

Switch when the plant is mature and big enough to fill your space after it grows to one and a half to two times its height during the stretch. In a small tent with several plants, that is often 3 to 5 weeks of veg.

What light schedule triggers flowering?

Indoors, the common grower practice is 12 hours of light and 12 hours of uninterrupted darkness. University of Georgia Extension notes most hemp flowers with day lengths under about 13 to 14 hours.

How long after switching to 12/12 do plants show flowers?

Most plants show white pistils within one to two weeks. They also stretch noticeably for the first two to three weeks.

Do autoflowers need to be switched to 12/12?

No. Autoflowering plants flower based on age, not day length, so they can stay on the same light schedule from seed to harvest.

Does a longer veg mean a bigger yield?

Not necessarily. A longer veg gives a bigger plant with more branches, but a plant that outgrows your space or light will not use that extra size well. Genetics, light and care matter as much as veg time.