💡 The Perfect Light Schedule for Cannabis (Seedling to Harvest)

By Kabloomi
In no way am I an expert — I’m just learning as I grow and sharing what’s been working for me.

🌱 Why Light Schedule Matters

Light is your plant’s life clock. It tells your cannabis when to grow, rest, and eventually flower.
Getting the light cycle right at each stage can be the difference between a healthy, stacked plant or a small, confused one.

When I first started growing, I didn’t realize how much timing mattered — not just the brightness or distance of the light, but how many hours the plant got every single day.

Here’s what I’ve learned so far about light schedules from seed to harvest 👇

🌱 Seedling Stage: 18–24 Hours of Light

• Recommended Schedule: 18/6 or 20/4 (Light/Dark)

• Goal: Build roots and healthy leaves

Cannabis seedlings love light but still need a little dark time to “breathe.”
I personally use 20 hours on / 4 off — it’s a sweet spot that keeps growth steady without stressing the young plants.

If you’re running autos, they’ll do fine under 18–24 hours of light from seed to harvest.
If you’re running photoperiods, you can keep them in 18/6 until you decide to flip.

Tip: Keep lights 18–24 inches above seedlings — too close/far and they’ll stretch or burn.

🌿 Vegetative Stage: 18 Hours of Light, 6 Hours of Dark

• Recommended Schedule: 18/6 or 20/4

• Goal: Encourage strong stems and leaf growth

This is where your plant bulks up — and trust me, they’ll grow fast under a consistent 18-hour schedule.
Some growers run 20/4 to squeeze a little more growth, I’ve found 20/4 works perfect for me and keeps my plants happier.

Stick to the same time every day. I run mine on a timer (lights on 12AM–8PM) to make it automatic.

Watch for:

• Tight internode spacing (good sign!)

• Droopy leaves near lights-off (normal rest behavior)

🌸 Flowering Stage: 12 Hours of Light, 12 Hours of Darkness

• Recommended Schedule: 12/12

• Goal: Trigger flowering and bud development

When you’re ready to flip your photoperiod plants to flower, switch to 12 hours on / 12 hours off.
This simulates fall daylight hours and signals the plant to start producing buds.

Very important:
The 12 hours of darkness must be completely dark — no light leaks, no phone screens, no tent peeks.
Even a little light can cause stress or hermies (plants that grow seeds).

🌾 Late Flower / Ripening: Keep 12/12 Until Harvest

Once you’re deep into flower (weeks 6–8+), don’t change anything.
Keep the same 12/12 light schedule until you’re ready to harvest.
Changing the timing late can slow bud ripening or confuse your plant.

Optional tip: some growers do 48 hours of darkness right before harvest to push a little more resin — totally optional, but I might test that one day and write about it here.

⚙️ Light Schedule Quick Chart

Grow Stage	Light Schedule	Notes
Seedling	18–24 hrs	20/4 is a safe middle ground
Veg	18/6	Most popular indoor cycle
Flower	12/12	Must be 100% dark during off hours
Autoflowers	18–24 hrs entire grow	Ignore the flip — they flower automatically

🔌 KaBloomi Setup Tip

Right now, I’m running a 1000W Aglex LED on a 20/4 schedule for veg with a timer, fans on 24/7, and I’ll switch to 12/12 when my plant fills out.

Using a timer makes everything easier — no missed flips, no stress, and your plants get used to the rhythm just like you would.

🌿 Final Thoughts

The perfect light schedule is the one that fits your grow setup, strain, and daily routine.
Some growers push their lights 24/0 for max growth — others swear by giving their plants rest.

At the end of the day, your plants will tell you what works.
Keep your timing consistent, give them total darkness when it’s time to flower, and they’ll reward you with the kind of growth that makes it all worth it.