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what kind of light do autoflowering cannabis seeds need

Autoflowering cannabis strains take approximately 8 to 9 weeks from germination to harvest, which is significantly shorter than photoperiod weed varieties which need an average of 9 to 12 weeks. Autoflowering cannabis strains are probably the easiest strains to grow as it requires much less attention and gardening effort compared to photoperiod.

Tip: check all our high quality autoflowering cannabis seeds

Why you should grow an autoflower

Some growers give their plants 24 hours of light with no darkness at all. Some growers argue that the Ruderalis hemp genetics derive from northern areas of the world where the sun hardly sets in summers, and so native plants are exposed to nearly 24 hours of sunlight in the natural world. However, we do not recommend this as it tends to overwhelm the plants and puts them in unnatural stress. Some growers report nutritional problems (such as calcium, magnesium, zinc, and iron deficiency) and other problems that can arise from constant lighting.

As mentioned earlier, growers of autoflowering cannabis do not need to manually set the lighting duration to 18 hours in the growing phase then to reduce it to 12 hours a day for flowering.

This form of cannabis blooms automatically due to its genetic makeup. Since autoflowering cannabis starts flowering only 20 to 28 days after germination and remains in the flowering phase for the rest of its life cycle, it requires different types of lighting and intensities at different times compared to photoperiod cannabis.

Autoflowering cannabis strains flower based on age, rather than a change to their light cycle. Nonetheless, providing your autos with the right amount of light remains crucial to ensuring the best possible harvest. Read on to learn all there is to know about lighting for autoflowering cannabis strains.

Cannabis is a C3 plant, meaning it can absorb CO₂ for photosynthesis even during light hours. And since autoflowering plants have short vegetative phases and often grow shorter than photoperiod strains, you’ll typically want to give your autos at least 18 hours of light. This allows for robust growth, without overspending on energy. Your autos love light just as much as traditional photoperiod strains, they just don’t rely on it to begin bloom.


Some growers still decide to keep their autos under a 12/12 light cycle during flowering. And that’s fine, as autos grown in these conditions can still produce a fair harvest. Just remember that the buds you harvest under a 12hr light cycle will be noticeably smaller than what you’d get under a 18–24hr cycle. Some reasons you might consider keeping your autos under 12/12 include:

Some growers will even give their autos a full 24 hours of light, arguing that this helps maximise vegetative growth. Growers who stick to 18-hour light cycles, on the other hand, argue that this gives their plants a short “recovery” period that is essential for healthy growth.

There’s no real consensus on whether autos grow better under 18 or 24 hours of light, and we’ve seen growers achieve great results with both methods. To make up your mind, we recommend playing around with both variations and sticking to what works best for you.

In fact, many growers use Metal Halide or other more “blue” colored lights for the entire life of the plant and still produce good results. Some growers even prefer these lights for budding.

Auto-Flowering Vegetative & Flowering Stage

Yellow Light for Flowering – yellow/orange/red light (like from HPS bulbs or “soft white” colored CFLs/fluorescents) tend to produce the best growth and densest buds in the flowering stage. Their color mimics the reddish light from the autumn sun. It is completely okay to give your plant yellow light for its whole life. In fact, that’s what I prefer to do as yellowish light encourages stretchy growth, and I want an auto-flowering plant that grows fast!

Light Color Spectrum

18/6 Pros

However, marijuana plants are weeds, and will usually grow at least okay under either spectrum of light during either stage.

There are growers who believe auto-flowering plants need a dark period and won’t be as healthy if they get 24 hours of light a day.

All cannabis strains, whether they’re photoperiod or auto-flowering, have two main stages of life: the vegetative (first) stage, and the flowering (second) stage. Think of them as the “children” and “adult” stages of marijuana growth. During the vegetative stage, a cannabis plant grows only leaves and stems. In the flowering stage, the plant stops growing leaves and stems and starts growing buds/flowers.