when to give seedlings light

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.

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.

When cannabis seedlings have grown their second or third set of serrated leaves they are usually hardy enough to flourish in direct sunlight. Learn when that is and how to do that now.

When Do I Put My Seedlings Under LIGHT?

Last updated: 10/07/19

Adequate knowledge is needed for starting seeds indoors. The more you know or are able to apply to your gardening, the best your plants turn out at the end of the day.

Like you, when I just started indoor seed starting, I also had a lot of questions to ask, like “when do I put my seedlings under light” or “how far my seeds should be from the LED”.

I was particularly keen on getting satisfactory answers to these questions to prevent my plants from dying.

So, to help you, I’ll be providing all the answers you want and need in making your first experience with indoor seed starting a very easy one (maybe even better than the one I had).

Green thumb Gardener occasionally links to product and/or services offered by vendors to assist you with all your gardening needs. Some of these may be affiliate links, meaning we earn a small commission if items are purchased.

Want to Download a ​Garden Hack Guide​ for FREE

Enter your email below and we will send you a guide to help you SAVE money in your garden.

You’re thinking of starting seeds in your house and don’t want them to grow weak leggy stems. Our solid advice for you to do would be to put them under grow lights once you notice the first few sprouts.

Placing your trays or pots by your window to catch the sun is a good idea. However, it is not a great one.

Your seedlings need plenty of energy but won’t be able to get that if you place them beside your windows.

Artificial lights are the best option for indoor seed starting. You want them to flourish after they start germinating, then you need to place them under LED or fluorescent lights.

What Does It Take To Start My Seed Indoors?

Indoor seed starting can be time-consuming because it requires a great deal of attention and dedication.

However, if you ponder on the benefits of breeding from the ground up, you’ll realize that it’s all worth it in the end.

Doing everything exactly how it should be done ensures that your seeds grow into the plants you want and can always admire.

Truth be told, a considerable amount of your money may go into starting seeds. Most of the cost is upfront for some equipment such as containers, trays, lights, and soil medium.

You might look to spend about $50-$150 dollars depending on the lighting you choose.

But whatever you spend is actually a sacrifice to make things right. It also guarantees perfect results in the long run.

You can cut your costs down to what you can easily afford if you think it’s going to be too much for you. This way, you make things work for yourself as well as for your plants.

Many people don’t know this, but every little detail matters when seed starting. It’s not just about planting and gardening alone.

You want to be sure each of the processes is done correctly, for beautiful and sprightly plants like those you may have seen at a friend’s or at the garden center where you got your seeds from.

Let’s talk about when it is the right time to give your seedlings light…

When To Give Your Seedlings Light

When your seeds have started to sprout is the most ideal time to give them some light.

Light is as important as food is to human growth and development for plants. Let me repeat that, it is vital…

We don’t have to go into the phototropism or photosynthesis jargon right now, but I think it’ll be right to simply say it is food for plants.

Just as we humans need food for our survival and strength, so also do your seedlings need light. The energy derived is what they then convert into all the chemical energy they use for their growth.

Giving your seedlings light should be as early as possible. Even before starting seeds, you need to have gotten a good light to use.

Once they start sprouting from the soil, they will need adequate light for the development of strong and healthy stems. You definitely want them to flourish well, so pay close attention to your trays until you notice the first sprouts.

How Far Should Seedlings Be From Grow Light?

The distance between your seedlings and your grow light is also important.

Let me give you a picture; if you eat more food than you need, you’ll definitely feel very uncomfortable, right?!

Well, the same thing applies here too, when your LED is too close to your seedling, it can make them uncomfortable.

In fact, it can affect their growth or even burn them.

Hanging your HID (High-intensity discharge) or Fluorescent too high can cause them to stretch and wither (You know, too much of everything isn’t really good).

At the same time, hanging them too low can also burn them.

So, to give your seedlings just the essential amount, which they can convert into energy for their survival, you need to make sure the light is not too low or too high.

Brightness or intensity of your light is another factor to also consider. Some lights offer more heat and spectrum than the others.

You have to be careful while hanging them over your trays or pots.

There are different types. The first is the LED grow lights which have a wider range but generate less heat than Fluorescent and HID.

You may want to hang them higher away to prevent them from causing your sprouts to bleach. The average length to keep them away from your tray is 20″(51cm).

HID have a wider range and also generate more heat. Keeping them away at an average of 14″(36cm) would be a great idea.

For Fluorescent, strength and spectrum have a balanced share. This is why they are often recommended to new gardeners. They can be used in growing virtually all plants and are best hung at an average height of 12″ above your tray.

Below are the average heights for LED, HID and Fluorescent grow lights, all according to their wattages:


  • 400w – 13″to 20″
  • 600w – 15″to 25″
  • 1000w – 20″to 30″


  • 210w to 400w: 15″to 30″
  • 450w – 550: 20″to 30″
  • 600w – 850w: 24″ to 26″
  • 900w and above: 26″to 40″+


  • CFL – 15″to 20″
  • T5 – 12″to 15″

The heights of your LED, HID or Fluorescent to your tray also has to do with the growth of your plants. So, the higher they grow, the higher your LED, HID or Fluorescent must go too.

Why Is It Better To Use Grow Light When Growing Indoors?

Like I said earlier, using grow light is the best option if you are thinking of seed starting at home.

You may have large windows that freely allow the rays from the sun into the house, however, the energy they get in that position may not be enough for your seedling.

They will actually find it quite difficult to catch the rays. But artificial rays can work in place of the sun.

It is even the best option if you want your sprouts to develop strong stems and form beautiful canopies.

Naturally, the more energy your seedlings get, the higher the yield.

The most important questions of starting seeds is, "When Do I Put My Seedlings Under LIGHT?" We give you the expert details to help you do it correctly.