When can seedlings be put under lights or in the sun?

Not sure whether to grow your cannabis seeds in the sun or under lights? This article explains how much light you need, how to prevent your seedlings from falling over, and when to plant them outside.

“When can cannabis seedlings be put under lights or in the sun?” is a common and sensible question that is often asked by novice cannabis enthusiasts after germinating seeds indoors under lights.

For those who are growing indoors, cannabis seedlings may be put under lights as soon as they emerge from the soil or growing medium.

How much light for cannabis seedlings?

When using HID lighting (usually a metal halide lamp), young seedlings should be kept at least 50cm from the bulb.

With compact fluorescent lamps (usually 100w or more), a distance of around 15cm should be maintained between the top of the young seedlings and the bulb.

If using normal fluorescent tubes (18-36w), seedlings can be kept within a few centimetres of the light source.

Cannabis seedlings growing under lights should always be given a gentle breeze from an oscillating fan (a small household fan on the lowest setting is fine), as constant gentle movement will strengthen their stems significantly.

Are your cannabis seedlings falling over?

If cannabis seedlings grow tall and then fall over, this is almost always a result of growing in an environment with static air.

Seedlings intended for outdoor growing should be kept by a sunny window for the first week or two after emerging from the soil.

When they have grown their second or third set of serrated leaves (after the the round cotyledons that initially emerge from the seed) seedlings are usually hardy enough to flourish in direct sunlight.

Seedlings can be kept by a sunny window, until they’re ready to grow outside.

When to plant your cannabis seedlings outside?

If outdoor temperatures are suitable, seedlings may be acclimatised to direct sunlight  by giving them progressively longer daily exposure to outdoor conditions.

Starting with about three hours outside, at the sunniest time of day, seedlings can be given an extra hour of outside exposure each day, so that within about  two weeks they can be left outdoors permanently.

  • Disclaimer:
    Laws and regulations regarding cannabis cultivation differ from country to country. Sensi Seeds therefore strongly advises you to check your local laws and regulations. Do not act in conflict with the law.


3 thoughts on “When can seedlings be put under lights or in the sun?”

  1. I grow autoflowers, sometimes a seedling will pop high, a trick is to take 2 bread ties and burn off the plastic about halfway up one end, insert the metal tie away from seedlind, bend the tie towards the seedling, make a loupe and wrap arond seedling, this will hold seedling in place untill it’s mainstem is able to support itself on it’s own, works everytime, you can allso take a plastic sraw and cut a slit in it like a little cast, but the bread tie method works best, have been running autos for allmost 10 years!

  2. I had 20 plants just fall over I growning them inside .I put outside for a few hours got up the next morning.they were laying over.and I like to know why?

    1. Peter Lesperance

      Lights were too high and you had no wind. The lights have to be right above the seedlings the distance depends on o the wattage of lights you’re using if you put a small fan in the room that blows on them a bit it will make the stalk stronger the other thing that can help is to pinch the top to stop it from growing upwards force it to branch. Then when that branch is as tall as the initial stem pinch it again to force it to make another branch this will let the plant get strong then you can encourage it to grow tall.

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    Sensi Seeds

    The Sensi Seeds Editorial team has been built throughout our more than 30 years of existence. Our writers and editors include botanists, medical and legal experts as well as renown activists the world over including Lester Grinspoon, Micha Knodt, Robert Connell Clarke, Maurice Veldman, Sebastian Maríncolo, James Burton and Seshata.
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