Do Autoflowers Need a Light Change to Flower? Here's the Answer

No: with autoflowers, you don't have to change the light cycle to trigger flowering: they bloom on their own once they reach a certain age, so most growers run one schedule (usually 18/6 or 20/4) from seed to harvest. Unlike photoperiod plants, which need a set stretch of darkness to start flowering, autos run on their own internal clock.

After a short vegetative stretch following the seedling stage, these plants switch to flower based on their age, not on how many hours of darkness they get, so there's no “flip” to make like there is with photoperiod strains.

Autoflower light schedules vary, but the common options are 24 hours of light and 0 of darkness (24/0), 20 hours on and 4 off (20/4), or 18 hours on and 6 off (18/6). The best one for you comes down to your grow setup and your own preferences.

Keep in mind that light cycles mimic the seasons in nature: more light reads as summer, more darkness reads as winter. Knowing how to tweak your light schedule can make a real difference to the yield and health of your autoflowers.

How many hours of light does an autoflower need?

The answer depends on each grower's preferences and the needs of each strain. That said, most growers agree that 18 to 24 hours of light a day is the sweet spot for healthy autoflower growth.

As a grower, you'll want to dial in your autoflower light schedule based on the strain you're running and the conditions in your grow space. Growers use a handful of common schedules, from nonstop 24-hour light (24/0) down to shorter cycles like 18 hours on and 6 off (18/6).

Autoflowers need at least 12 hours of light a day to thrive, but within the recommended range you can experiment with different schedules (19/5, 16/8, 22/2 and so on) to find what works best for your setup.

Some experienced growers try unconventional schedules like 6 hours on and 18 off (6/18), but if you're new to growing, stick with the standard schedules above to give your harvest the best shot. Light schedule plays a huge role in the growth and development of your plants, so picking the right one is key to maximizing their health and yield.

Light spectrum: what kind of light do your plants need?

Light spectrum is a big deal when growing cannabis, because it can significantly shape how your plants develop and how much they yield. Depending on the growth stage, a plant needs a specific light spectrum to get the most out of its growth and production.

Cannabis has two distinct life stages, vegetative and flowering, and each one responds best to different wavelengths of light. In the vegetative stage, plants want blue light, roughly 400 to 500 nanometers (nm). In the flowering stage, they want red light, with wavelengths between about 600 and 700 nm.

Indoor light cycles: can you still get a harvest without full spectrum?

You can pull off a successful harvest without giving your plants the full light spectrum, but results are usually better when they get the right spectrum at each stage. That's why it's worth using lights that put out a full spectrum, or at least the spectrum each specific stage needs.

This applies to every lighting technology used for growing cannabis, from traditional bulbs to LEDs. Quality LEDs will give you full-spectrum output, while mixed-spectrum bulbs can also be a solid option for covering your plants' light needs across their life cycle.

Which light schedule works best for autoflowers?

Knowing when to run light on your weed plants is key to getting the most growth and yield out of autoflowering strains. Unlike photoperiod strains, which need a change in the light cycle to start flowering, autos flower based on age.

Even so, giving them the right amount of light is still essential for maximizing yields. Here are the most effective light schedules for autoflowers and how each one can shape their development.

The 18/6 light schedule

Cannabis plants, autoflowers included, photosynthesize while the lights are on, and autos don't depend on photoperiod to flower. That's why lots of growers run an autoflower light schedule of 18 hours on and 6 hours off. It strikes a solid balance between vigorous growth and energy efficiency.

Autos don't need a long dark period to flower, so 18/6 is usually plenty to keep them growing healthy.

The 24/0 light schedule

Some growers keep the lights on nonstop for the plants' entire life cycle, running 24 hours of light and 0 hours of darkness. Their argument is that it maximizes vegetative growth by cutting out any rest period. Others say a short dark period helps plants recover and keeps their metabolism running efficiently.

Both approaches have produced good results, and choosing between them usually comes down to personal preference and growing conditions.

The 12/12 light schedule

It's less common with autos, but some growers run 12/12 during flowering, meaning 12 hours on and 12 hours off for that phase. It can help keep heat down in warm grow rooms, or make sense when you're running photoperiod strains alongside autos in the same space, but it generally means smaller buds. Still, it can be a workable option on a tight budget, since it cuts your electricity bill.

Bottom line: the right light schedule for autoflowering strains depends on several factors, including your preferences, your grow environment and your yield goals. Experimenting with different schedules will help you figure out which one fits your setup best.