CannaGLAP
Grow basics · 9 minute read · Updated October 15, 2026
Autoflower vs photoperiod: which should you grow?
Pick the wrong seed type for your space and you spend the whole grow fighting it: autos that finish tiny, or photoperiods that hit the light. What matters more to you this grow, speed or control?
The one real difference: what tells the plant to flower
A photoperiod plant reads the length of the night. While nights are short, it keeps growing leaves and stems. When the dark period gets long enough, it switches to flowers. Indoors, almost every grower triggers that switch with 12 hours of light and 12 of darkness.
An autoflower ignores the night. It flowers based on its own age, whether it gets 12 hours of light or 20. Scientists call this photoperiod insensitive, or day neutral.
The trait is genetic. A 2022 study mapped it to a single location in the cannabis genome, named Autoflower1, and its data fit a simple recessive pattern: a plant needs two copies to autoflower. The trait is often said to come from Cannabis ruderalis, hardy plants from high latitudes, though the same paper notes that recent genomic work does not support ruderalis as a separate group. For growing, the name matters less than the behavior: you cannot talk an autoflower out of flowering.
Side by side
| Autoflower | Photoperiod | |
|---|---|---|
| Flowering trigger | Age, a few weeks after sprouting | Long nights; 12/12 indoors |
| Light schedule | Same all grow, commonly 18/6 or 20/4 | 18/6 in veg, 12/12 in flower |
| Seed to harvest | About 65 to 90 days for classic autos | Veg time you choose, plus 56 to 80 days of flower |
| Size control | Mostly set by genetics; pot size and light help a little | You decide by how long you veg |
| Yield per plant | Usually smaller | Usually larger, with time and space |
| Training | Gentle LST; topping is risky | Topping, LST, ScrOG all work |
| Cloning | Not practical | Yes; keep a mother plant |
| Recovery from mistakes | Little time; stress costs yield | Can add veg time to recover |
| Dark period | Not required | Must be total in flower |
Autoflower timing is from breeder guidance, and a "super auto" can take closer to 120 days. The photoperiod flowering range comes from a 2023 study of ten drug type cultivars, which listed 56 to 80 days from the switch to harvest ready flowers.
Time: how many harvests a year
Speed is the main reason people grow autos. Here is the math for one tent, allowing a week between runs for harvest and cleaning.
| Type | Seed to harvest | Plus a week turnaround | Runs per year |
|---|---|---|---|
| Autoflower | 65 to 90 days | 72 to 97 days | about 3.8 to 5 |
| Photoperiod, short veg | about 14 weeks | about 105 days | about 3.5 |
| Photoperiod, long veg | about 23 weeks | about 168 days | about 2.2 |
More runs does not automatically mean more flower, because each auto plant is usually smaller. But if you want something to harvest sooner, or you are learning and want more practice grows per year, autos give you more chances. Photoperiod growers who want steady harvests often run a second tent instead; see two tent setup.
Light schedules for each type
Photoperiods. 18 hours on and 6 off in veg. Then 12 on and 12 off to flower, and the 12 hours of dark must be truly dark. A light leak or a curious peek at night can disrupt flowering; see light leaks and light schedules.
Autoflowers. Most growers keep autos on 18/6 or 20/4 from seed to harvest. Some run 24 hours of light. A longer day gives the plant more total light each day, and a University of Guelph study found photoperiod cannabis yield rose steadily with light intensity in flower. But we could not find a controlled study comparing 18, 20 and 24 hours for autos, so treat any one schedule as preference, not proof. A short dark period also gives the room a cooler break and cuts the power bill.
Can you grow both in one tent? Yes, with a catch. On 12/12, both types flower, but the autos get less light than they could use. On 18/6, the autos flower and the photoperiods stay in veg. Growers usually run mixed tents on 12/12 once the photoperiods are ready to flip, or keep the types apart.
Which one fits you?
Answer these honestly before you buy seeds.
- How tall is your space? In a short tent or cabinet, an auto's built in size limit is a feature. A photoperiod can outgrow a small space during the stretch after the flip.
- How much do you want to decide? Photoperiods let you choose when to flip, how big to grow and how hard to train. That is control, and also more ways to get it wrong.
- Do you want to keep a strain? Only photoperiods let you keep a mother plant and take clones. See how to clone cannabis.
- How steady is your schedule? An auto cannot wait for you. If you will be away for a week during its first month, a photoperiod gives you room to recover.
- Is your dark period reliable? If you cannot keep the grow space fully dark for 12 hours, autos avoid the problem entirely.
Quality depends heavily on the breeder and on your growing. There is little independent data comparing modern autos and photoperiods for potency, so be wary of anyone claiming a clear winner.
Growing autos well
- Plant in the final pot, or move it early. Any transplant stress eats into a short life. One autoflower breeder advises transplanting 7 to 12 days after sowing and warns against letting seedlings become rootbound. Pick the final size up front with the pot size guide.
- Train gently and early. LST from the 3rd or 4th node. Top only very early, if at all. Training to fill a tent covers how.
- Do not stall it. Overwatering a seedling, a nutrient burn or a cold week all cost size the plant cannot get back. Feed lightly at first and raise it slowly.
- Start flower conditions on time. When pistils appear, move toward flower humidity: 40 to 55% RH and a VPD of 1.2 to 1.6.
Mistakes that come from mixing up the two
- Waiting for an auto to "finish veg". There is nothing to wait for. If you treat an auto like a photoperiod and hold off on flower conditions, the plant flowers anyway, in veg humidity, which raises the risk of mold in the buds.
- Flipping an auto to 12/12 to "make it flower". It will flower on its own schedule regardless. Cutting its light to 12 hours just gives it less light to build buds with.
- Topping a stalled auto. A plant that is already slow does not need another reason to stop. Fix the cause first: water, temperature, pH or feed.
- Flipping a photoperiod too late. The stretch after the flip catches many first growers. If the plants already fill the tent at the flip, they can hit the light by week three of flower.
- Breaking a photoperiod's night. Opening the tent with the room lights on during the dark period, or a timer set wrong after a power cut, can disrupt flowering or stress the plant. Check the timer after any outage.
- Buying the wrong seed by accident. Seed listings say "auto", "autoflowering" or "fem" in small print. Check before you plan the grow around it.
Growing photoperiods well
- Veg to fit the space. Flip when plants fill a little more than half their flowering space. Most will roughly double in height over the next two to three weeks.
- Use the time to train. Topping plus LST in veg builds an even canopy that uses your light well.
- Keep the night dark. Check the tent for leaks with the lights on inside and the room dark.
- Feminized or regular? Feminized seed should give only females. Regular seed gives roughly half males, which you remove when they show preflowers.
Whichever you choose, write down the sowing date, the day flowers first show and the harvest date. After one grow you will know your strain's real timing, not the seed pack's. Canna GLAP tracks each plant's phase and day count, so the autos and photoperiods in a mixed tent each keep their own clock. The full stage by stage plan is in cannabis grow week by week.
Questions growers ask
Are autoflowers easier to grow than photoperiods?
They are simpler in one way: no light schedule change and no dark period to protect. But they forgive less, because a stressed auto cannot be given extra veg time to recover.
How long do autoflowers take from seed to harvest?
Classic autoflowers usually take about 65 to 90 days from sowing, according to breeder guidance. Some larger types take closer to 120 days.
Can you clone an autoflower?
Not in a useful way. A cutting from an autoflower is still an autoflower and cannot be held in veg, so you cannot keep a mother plant going. Growers who want clones use photoperiod plants.
What light schedule is best for autoflowers?
Most growers use 18/6 or 20/4 for the whole grow, and some run 24 hours. We found no controlled study showing one schedule is clearly best for autos.
Do autoflowers yield less than photoperiods?
Per plant, usually yes, because their size is fixed by genetics and they cannot be vegged longer. Per year, faster cycles can close part of the gap.
Read next
- Cannabis grow week by week: the full indoor timeline
- How to germinate cannabis seeds
- How to train cannabis plants to fill a grow tent
- Two tent setup: how to run a veg tent and a flower tent
Sources
- Frontiers in Plant Science (2022): Identification and mapping of major-effect flowering time loci Autoflower1 and Early1 in Cannabis sativa L.
- Plants (MDPI, 2023): Is Twelve Hours Really the Optimum Photoperiod for Promoting Flowering in Indoor-Grown Cultivars of Cannabis sativa?
- Frontiers in Plant Science (2021): Cannabis Yield, Potency, and Leaf Photosynthesis Respond Differently to Increasing Light Levels in an Indoor Environment
- Atlas Seed: Frequently Asked Questions (autoflower timing and transplanting)
Education only, for adults 21+ where home cultivation is legal. Updated October 15, 2026.