Cannabis Seedling Light Schedule: How Many Hours?
A cannabis seedling light schedule is the number of hours the grow light stays on each day while photoperiod seedlings settle in. Many growers run 18 hours on and 6 off, and some run 20 or 24 hours. Every extra hour adds to the light a seedling gets, so set the hours and the brightness together.
How Many Hours of Light Do Photoperiod Seedlings Need?
Photoperiod cannabis stays in vegetative growth while days are long, so seedlings are kept on a long day from the start. Many indoor growers use 18 hours on and 6 hours off for seedlings and carry the same schedule into veg. Some use 20 or 24 hours. There is no tested number for seedlings, so the schedule is grower practice. Our post on the cannabis light cycle explains how the schedule changes later in the grow.
Is 18/6, 20/4 or 24 Hour Light Better for Seedlings?
Each has fans. Growers who like 18/6 point to the dark period as a saving on power and heat, and some see it as a rest for the plant, though that has not been tested. Growers who run 20/4 or 24 hours want more total light each day. More hours is not always better, because the extra light only helps if the seedling can use it, and a bright light left on all day can be too much for small plants. Many beginners start on 18/6 because it leaves room to adjust.
How Do Extra Hours Change the Light a Seedling Gets?
The total light a plant gets in a day is called the daily light integral. Virginia Cooperative Extension calculates it from the light level at the plant, called PPFD, and the hours the light is on, with 3,600 converting seconds to hours (Virginia Cooperative Extension). Their worked example uses 200 PPFD for 16 hours. Using that same example light level, the table below shows how the daily total grows with each schedule. These are math examples, not targets. Readers who want targets can see our post on daily light integral.
Daily light total by hours on at an example light level of 200 PPFD
Source: Virginia Cooperative Extension SPES-720 formula (PPFD x hours x 3,600, divided by 1,000,000). 200 PPFD is the guide’s worked example value, not a seedling target. Chart y axis starts at 0 mol per m2 per day.
| Hours on | Light level (PPFD) | Daily light total (mol per m2 per day) |
|---|---|---|
| 16 | 200 | 11.5 |
| 18 | 200 | 13.0 |
| 20 | 200 | 14.4 |
| 24 | 200 | 17.3 |
How Do You Avoid Too Much Light on Seedlings?
Set the hours and the brightness together. If you add hours, you may need to dim the light or raise it so the daily total does not jump. Watch the seedlings. Pale, bleached or curled leaves near the light can be a sign of too much, while long, thin stems leaning toward the light can be a sign of too little. These signs also have other causes, such as heat, pH or watering problems, so check those too. A light meter at seedling height helps you see the real level. Our guides to LED grow lights and grow tent setup cover the hardware side.
Starting photoperiod plants? Browse our feminized seeds. For a fixed-schedule plant, see our autoflower seeds.
Sources
The source below is a general plant lighting guide, not a cannabis study. Its formula is used here for math only.
For the example math, Kind Seed Co used a published light formula, and the rest of this seedling light page is general growing information.
FAQ
Do cannabis seedlings need darkness?
Long days are used to keep photoperiod plants in veg, and many growers report seedlings do fine without a dark period. Some still give 6 hours of dark on 18/6 to save power and limit heat. Running 24 hours adds heat and power load, so check canopy temperature. Neither choice is a tested seedling rule.
How far should a light be from seedlings?
It depends on the light. Each fixture has a different strength, so the maker's distance guide is the best start. Bleaching can mean too much light and stretching too little, but heat, pH and watering cause similar signs. A light meter at seedling height shows what the plants actually receive, which is more useful than a fixed distance.
Can seedlings get too much light?
Yes. A light that is too strong or too close can bleach or curl young leaves, and long hours add to the daily total. Seedlings have small leaves and roots, so they handle less light than older plants. If you raise the hours, check the brightness at the same time, and watch the leaves for signs of stress.
How long do photoperiod seedlings stay on a seedling schedule?
There is no fixed switch day. Many growers keep the same long day from seedling into veg and only change the brightness as plants grow. The schedule changes when the grower wants flowering, which for photoperiod plants means a shorter day. Our light cycle post explains how that change works across the whole grow.
