Autoflower vs Photoperiod Seeds: The Complete Comparison
An autoflowering cannabis seed grows into a plant that begins flowering after a set number of weeks of age. A photoperiod cannabis seed grows into a plant that begins flowering when its daily dark period gets long enough. Age against light. Every other difference between the two comes out of that one.
This guide puts them side by side in a single table, states the ranges as ranges, and explains where each difference comes from. It describes how the two plant types behave. It is not an instruction to germinate anything: cannabis seeds are sold on this site as collectible souvenirs, and germinating them is illegal in the UK.
What is the difference between autoflower and photoperiod seeds?
An autoflowering plant flowers on a timer it carries in its genetics, typically starting three to five weeks after the seed sprouts, and it does this under any light schedule. A photoperiod plant flowers only when the light changes. Left under long days it will keep growing indefinitely; when the dark period reaches roughly twelve hours, indoors by choice or outdoors by the season, it switches to flowering.
That difference sets everything else. Because an autoflower will not wait, its whole life is short and its final size is fixed early. Because a photoperiod plant does wait, its growth stage can be extended for as long as there is room, which is why the same genetics can finish at half a metre or at three metres depending on how long it was held.
| Factor | Autoflowering | Photoperiod |
|---|---|---|
| Flowering trigger | Age. Flowering starts on its own, commonly three to five weeks after the seed sprouts | Light. Flowering starts when the daily dark period reaches roughly twelve hours |
| Seed to harvest | Commonly eight to twelve weeks in total | Commonly sixteen to twenty four weeks, and longer if the growth stage is extended |
| Typical height | Commonly 30 to 120 cm | Commonly 100 to 300 cm, and taller outdoors |
| Light needs | One schedule from start to finish, usually 18 hours on and 6 off, or 20 and 4 | Two schedules: long days for growth, then a 12 hour day to start flowering |
| Difficulty | Fewer decisions, but no second chances. The plant does not wait for anything to be put right | More decisions, more room to recover. The grower chooses when flowering starts |
How long do autoflowers take from seed to harvest?
Most autoflowering lines finish in eight to twelve weeks from the seed sprouting. Roughly the first three to five of those weeks are growth, and the rest is flowering. A photoperiod line covering the same ground commonly takes sixteen to twenty four weeks, because the growth stage is held open until the grower shortens the day.
The figure worth watching is which one a seed bank is quoting. Flowering time and seed-to-harvest time are different numbers and are often printed as though they were the same, which is where most of the contradictory flowering times online come from. TAG publishes the flowering stage on its own, and explains the distinction in How to Read a Cannabis Seed Spec Sheet.
How tall do autoflowering plants get?
Autoflowering plants commonly finish between 30 and 120 cm. They set their final size early, because the growth stage ends on schedule whether or not the plant has filled its space. Photoperiod plants commonly run from 100 cm to 300 cm indoors and can go well past that outdoors, since the growth stage lasts as long as it is allowed to.
Height is the most reliable practical difference between the two types, and it is the reason autos are associated with small spaces.
Do autoflowers yield less than photoperiod plants?
Per plant, usually yes. A smaller plant with a shorter life makes less flower than a large one that was held in growth for weeks. Comparing a single auto to a single photoperiod plant is not a fair comparison though, because the auto finishes in roughly half the time. Over a full year the gap narrows, since more short runs fit into the same period.
The honest position is that autoflowers trade peak size for speed and predictability. Anyone quoting a single number for either type without stating the conditions behind it is guessing.
Are autoflowers considered easier than photoperiods?
Autoflowers are usually described as more forgiving to plan and less forgiving to fix. There is one light schedule instead of two and no decision about when to switch, so there is less to get wrong. The flip side is that the plant runs to its own timetable, so anything that sets it back early is still there at the end. A photoperiod plant can be held in growth until whatever went wrong is sorted out.
Neither type is harder in a general sense. They fail in different ways.
Which seed types does TAG sell?
TAG splits its catalogue into four types rather than simply into autos and photoperiods, because “autoflower” on its own hides the thing that separates one auto from another: how much of the plant comes from ruderalis and how much from the photoperiod parent.
- Photoperiod Seeds: flower on light, the reference genetics everything else is bred from.
- Photoperiod-Derived Autos: the autoflowering trait bred into a photoperiod line and then bred back towards that parent.
- Ruderalis × Photoperiod Autos: the standard autoflower cross, with a recent ruderalis parent.
- Ruderalis Autos: the lines closest to the wild day-neutral plant.
What Is a Photoperiod-Derived Auto? explains how those four relate to each other, or browse the whole catalogue in the seed shop.
Frequently asked questions
Can an autoflower be turned back into a photoperiod plant?
No. Day-neutral flowering is genetic, not a state the plant can be moved in and out of. Changing the light schedule does not hold an autoflowering plant in the growth stage the way it holds a photoperiod plant.
Do autoflowers need a dark period at all?
No. Day-neutral flowering does not depend on darkness, which is why long schedules such as 20 hours on and 4 off are common with autos. Whether continuous light is better or worse than a short dark period is still argued over by growers and is not something anyone has settled with published data.
Are autoflowering seeds always feminised?
Not always, though most sold today are. Autoflowering and feminised are two separate traits: one decides what triggers flowering, the other decides the likelihood of the plant being female. A seed can be either, both or neither, and a seed bank should say which.
Which type has more THC?
There is no reliable answer at the type level, only at the strain level. Early autoflowers carried far more ruderalis than most sold today and were widely regarded as weaker than the photoperiod strains they came from, and that reputation has outlasted the genetics that earned it. The only figure worth anything is the one measured on the specific line, which is what the spec table on each strain page is for.