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.

Comments

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

  1. OK so question it is winter where I live it get down in negative temperatures so I’m obviously growing indoors in a tent. That being said I also still have the tent in my basement in the cold room where the heat doesn’t really get to so I have an electric heater outside the tent and it maintains a constant temperature around 80 to 85 inside the tent ….. but my question. Is the first day and half it was way to hot about 103 inside the tent one of my seedlings grow sideways and I’m assuming was trying to go the opposite way of the heat . But once I got the condition back to about 80 it started to grow upright again so it kinda looks like a capital L laying on its side … is it still going to be OK now that it is growing upright and also is it OK now to add my 650w led to the tent now? Or should I use something smaller for now and how long should I leave them under said light

    1. Mark - Sensi Seeds

      Good afternoon Magoo,

      Thanks for the question,
      Unfortunately, legal restrictions mean we can’t answer grow-related questions or give grow advice on this blog. However, cannabis is a very robust plant, so I’m sure it will recover. In general, plants prefers steady temperatures in the day, with fluctuations of 2-4 °C during the evening.
      You might also enjoy our article on the Top 6 Cannabis Plant Training Techniques.

      Thanks again, and have an excellent day.

      Mark

    2. I’m growing outdoors and I’m wondering if I am too late to start. I have seedlings right now and I’m just curious if that is enough time for the plant to get big enough before flower

  2. 1 have 2 tomatoe plants I’m growing but the weather has been unsettled so due to lack of sun can I put my tomatoes seedlings under a light to help 🙂

  3. Hey I have germinated two seeds. Do I put in a greenhouse that gets 9 hours of sunlight a day or put out in direct sunlight for same amount of time? Only popped through soil 3 days ago.

    1. Mark - Sensi Seeds

      Good afternoon Jon,

      Unfortunately, legal restrictions mean we can’t answer grow-related questions or give grow advice on this blog. However, other readers of this blog will often answer questions like yours.

      In general, 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 seedlings are usually hardy enough to flourish in direct sunlight.

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

      Please check out the following articles on our blog which I think you may find of use, and I hope you continue to enjoy the blog!

      The life cycle of cannabis: From seed to harvest

      Growing cannabis in greenhouses

      Thanks again, and I hope you continue to enjoy the blog!

      With best wishes,

      Mark

  4. My sprouts are about 2 1/2 – 3 inches tall. They have produced the first and second smooth leaves, as well as the first serrated leaves. They also have the second, tiny serrated leaves coming out. But the initial leaves have turned slightly yellow. I’m concerned that I am, perhaps, overwatering? Any suggestions will be welcomed. (Thanks for the suggestion for moving air. I will try that to strengthen the stems because they will go outside once the threat of frost has passed.)

    1. Mark - Sensi Seeds

      Good morning Susie,

      Unfortunately, legal restrictions mean we can’t answer grow-related questions or give grow advice on this blog. However, other readers of this blog will often answer questions like yours.

      Please check out the following articles on our blog which I think you may find of use, and I hope you continue to enjoy the blog!

      What are the best conditions for seedlings

      How to water a cannabis plant

      With best wishes,

      Mark

    2. andrea manchia

      Ciao Susie il consiglio che ti do io x esperienza trentennale, pianta direttamente nel terreno, è più difficile ma dà risultati del 30% in più specie nella quantità. Allora metti il seme in acqua almeno 15 ore,dopo pianta il seme (anche a 23 centimetri sotto terra) innaffia bene e dopo ogni giorno inumidisci fino a quando non spunta la piantina, una volta germogliata io personalmente non la innaffio x almeno una settimana (dipende dalla temperatura esterna) ha lei i nutrienti x sopravivere almeno 10 giorni. E poi vedi se cresce più forte. Io faccio dei mostri di 6 metri

  5. Bonjour,
    Concernant les tubes fluorescents normaux ( 18-36 W ), que me conseiller vous ?
    Tubes normaux de couleur blanche, ou tubes horticoles de couleur rosée ( comme ceux qu’ont peut trouver sur les aquariums ).
    Merci pour vos réponses.

  6. Michael Dore

    First time grower here. I didnt germinate I just put them in soil. Do I need to put light on them or just grow till till I can see them a little and then put light??

  7. If they lean or tip over, I use the soil around them to “mound” around the stem. Makes them wicked thick in maturity.

  8. I am growing in a pretty old basement there just seedlings it’s a cherry glue I’m going to just build a room down there should I put a humidifier in there or would it just cause mold added to the condition of the basement.

    1. Balearic Gardener

      If the basement is damp that’s why you would have mould. Yes totally get a dehumidifier! Run it so you get the basement at the right humidity first, It will dry out the room and give you a stable base. Then aim to keep the grow room around 55% I think and around 24 degrees and have several small fans that can be moved about easily. It’s also really useful to keep the humidity down if you’re using it to dry too. That’s when you have to really worry about mould! But on your bud not the room! Good luck 🤞

      1. Mark - Sensi Seeds

        Hey Balearic,

        Thank you so much for sharing your experience.

        I hope you continue to enjoy the blog.

        With best wishes,

        Mark

  9. 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!

  10. 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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    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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