What Light Schedule Should I Use for Seedlings, Autoflowers, and Photoperiods? 🌱💡Updated 21 days ago
If you’re used to growing from clones, starting from seed can feel a little different at first — especially when it comes to lighting.
The good news is that once a seedling breaks through the soil, it’s ready for light.
Should Newly Sprouted Seedlings Stay in the Dark?
Once the seedling has come through the soil, give it light.
You don’t need to keep a newly emerged seedling in darkness. In fact, leaving it in the dark too long can cause it to stretch while searching for light.
Before the seed actually emerges, darkness is fine. Once you see that little green sprout, it’s time to turn the grow light on. 🌱
How Strong Should the Light Be?
Seedlings need much gentler light than mature plants.
Too much intensity can stress or bleach young leaves, while too little light can cause the seedling to become tall, thin, and stretchy.
A general target for young seedlings is around:
100–300 PPFD
As the plant gets larger, light intensity can gradually increase.
A rough guide is:
🌱 Seedlings: 100–300 PPFD
🌿 Vegetative growth: 300–600 PPFD
🌸 Flowering: 600–900 PPFD
These are general ranges, not strict rules.
How High Should My LED Be?
There isn’t one correct height for every LED.
A small 100-watt light and a powerful 600-watt fixture can produce very different intensity at the same distance.
For that reason, we recommend starting with the seedling height and intensity recommended by your light manufacturer.
Many modern LEDs also have dimmers, which makes it easier to start gently and increase intensity as the plant grows.
If your seedling is stretching a lot, the light may be too weak or too far away.
If the leaves look bleached, curled, or stressed, the light may be too intense or too close.
Autoflower Light Schedule 🌿
Autoflowers do not need a change to 12/12 lighting to begin flowering.
They flower mainly based on age.
Popular schedules include:
18 hours ON / 6 hours OFF
or
20 hours ON / 4 hours OFF
You can usually keep the same schedule from seedling through harvest.
For most beginners, 18/6 is a simple and reliable place to start.
Photoperiod Light Schedule 🌱➡️🌸
Photoperiod plants work differently.
During vegetative growth, a common schedule is:
18 hours ON / 6 hours OFF
When you’re ready for the plant to flower, switch to:
12 hours ON / 12 hours OFF
That longer uninterrupted dark period signals the plant to begin flowering.
What About Seedlings From Photoperiod Seeds?
You can start photoperiod seedlings on the same 18/6 schedule you would use during vegetative growth.
There’s no need for a special seedling light schedule.
Just keep the intensity lower while they’re young.
Can Autoflowers Grow Under 12/12?
They can, but it usually isn’t ideal.
Because autoflowers don’t need 12 hours of darkness to flower, giving them only 12 hours of light means they have fewer hours each day to photosynthesize and grow.
If you’re growing autos by themselves, 18/6 or 20/4 is usually the better option.
Can Autos and Photoperiods Share the Same Tent?
Yes — during vegetative growth.
Both can be grown under 18/6.
The challenge comes when you switch the photoperiod plants to 12/12 for flowering. The autos will still grow under that schedule, but they may not perform as strongly as they would under longer lighting.
Watch the Plant, Not Just the Numbers 👀
Light recommendations are starting points.
Your plant will tell you a lot:
Too little light:
Tall, thin stretching
Large gaps between nodes
Weak stems
Too much light:
Bleaching or very pale leaves
Leaf edges curling upward
Leaves appearing stressed near the top
Small adjustments are usually better than making one big change.
The Simple Version 🌱
For new seedlings:
Give them light as soon as they emerge from the soil, but keep the intensity gentle.
For autoflowers:
Use 18/6 or 20/4 from seedling through harvest.
For photoperiods:
Use 18/6 during vegetative growth, then switch to 12/12 when you want flowering to begin.
And when it comes to LED height and intensity, always use your specific light manufacturer’s recommendations as the starting point — every fixture is a little different.