grow light distance seedlings

How to Give Seedlings Enough Light to Be Healthy

Why your seedlings aren’t getting enough light

Light is the number one underestimated factor of growing, even at the seedling stage. If seedlings don’t have enough light, they grow more slowly and can become leggy. Overall, growers who do not give seedlings enough light have longer crop timelines and higher rates of seedling mortality.

Why do growers mistake the level of seedling light?

  • They don’t have enough lighting.
  • They place lighting too far from seedlings (bad coverage).
  • They don’t leave lights on long enough.

Usually, these problems are easy to fix: add lights or adjust your seedling light schedules! Here are some general seedling light guidelines to speed up production and save you money on seeds.

How far away should I place seedling lights?

You’ve got to keep two goals in mind: the amount of light you’re giving seedlings, and the coverage of light that you’re giving seedlings.

1) Amount of light:

We can measure the useful light to a plant (PAR—photosynthetically active radiation, or the parts of light that the plant can use to photosynthesize) in micromoles per meter squared per second ( micromoles/sec/m² ) . The goal here is to deliver 120–150 micromoles/sec/m² of PAR to your seedlings .

Some people will use a PAR meter to figure out how much PAR they’re getting to their seedlings and will adjust the distance of their lights based on that measurement. Don’t worry—you don’t need one of those if you’re using LED bars or fluorescent bars. Just remember the following distances.

LED bars (like Phillips): 8–12 inches away from seedlings

T5 fluorescents: 5–6 inches away from seedlings

2) Coverage of light

The next question is coverage. You can usually just eyeball this to figure out how far apart your lights need to be to deliver the same intensity of light across your entire seedling tray.

Again, we’ve outlined two simple rules to follow if you’re using the most common seedling lighting types:

LED bars: 8–10 inches apart from each other

T5 fluorescents: 4–5 inches apart from each other

Stick to these rules, and your seedlings will be ready to transplant in no time.

How long should I keep lights on my seedlings?

One convenient thing about artificial lights is that you can set them to turn on and off by themselves using timers.

But how long should you keep light on your seedlings?

Balancing light and electricity costs

Growers are going to need 12–18 hours of light per day.

Your place in that range depends on the costs of two things: real estate and electricity. Which one is inherently higher for your facility?

This gives you four situations.

1) If the cost of real estate is higher, then you want to move things through your space quickly. This means that you will be paying for more hours of light (18 or so) to move crops along.

2) If the cost of electricity is higher but you have plenty of space, then you’ll want to reduce lighting a bit (12 or so hours). This means that your crops will grow slower.

3) If both costs are equally high like they are at our farm here in Laramie, then you’ll find a balance. We give our crops about 14 hours each day.

4) If both costs are equally low, then go ahead and run them fast, at about 18 hours a day. (Just remember to give crops everything else that they need as well, like airflow and CO2, or they won’t be able to use all that light.

Don’t forget to harden your seedlings before transplant!

Remember to harden off your seedlings before transplanting them! Hardening seedlings involves hardening them to things like temperature and humidity, to reduce plant shock. It also means hardening them to light.

To harden your seedlings, place them in the main growing area for a few hours a day in increasing periods over about a week.

Need light help with the rest of your farm?

Light is one of the most important factors for growing healthy crops. Being a modern farmer like you comes with both a blessing and a curse: on one hand, light technology has never had so many quality options. On the other hand, choosing a lighting type can be a very overwhelming task.

To help you cut through the noise, we wrote The Modern Farmer’s Guide to Artificial Lighting. This guide explains the factors of a good light, how LEDs work, and some special considerations that indoor farmers may have for lighting.

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I am trying to develop Hydroponics/Aquaponics and Hydroponics fodder in Bangladesh conditions to make it cost effective with the participation of local farmer. Hydroponic/Aquaponics and fodder products is not popular in Bangladesh and competitive with ongoing agricultures ( not commercially viable option with seasonal crops). Bangladesh University of Agriculture, Maymanshing. Bangladesh Agriculture Research Institute trying for the same but so far to not so cost effective, not farmers friendly and not simplify to easy access for mass farmers (family farmer), rather it is replication of Westar system and Lab oriented.
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All the above will generate and making ground for hydroponics/Aquqaponics cultivation but, at a snail’s pace growing popularity of organic vegetable in urban areas. I am now trying to develop manpower and appropriate technology for our condition in cooperation (providing me lab and space) with Khulna University and also involving community for cultivation as do it yourself (DIY) approach. If I develop and standardized in Bangladesh condition then it must be a good productive agriculture system in water and land scar areas of Bangladesh, additional introduction of Aquaponics in urban city is a must to considering energy-water-environment- poverty- nexus of our country. The growth of urban population in Bangladesh is alarming compeer to any country of the world. Aquaponics is the highest productive food system would be cultivate in of season that’ are the beauty of the system (Farmers may get high price of vegetable) Aquaponis may go beyond season (which is my thrust area beside construction cost), for which it able to meet additional income of poor and future food security without impedes nature as water requirement (water footprint) is 95% less and productivity is 3 to 5 time higher and across seasonality that goes beyond existing agriculture and aquaculture practice, for which it would be good in coastal areas, where fresh water is scares. I am also trying to introduce Cow’s urine (Organic compost mater) in village grassroots small agriculture farmer with low-cost greenhouse.
Alone with urban Roof top- Aquaponics with which would improve health and sanitation in city environment and Aquaponics with Urine is the excellent high quality option for disable persons, house hold woman and children work with.
By 2050, we need to figure out how to not only feed but also nourish the three billion new people who will be joining the seven billion of us who are already here on the planet, and the year 2050, nearly 80% of the earth’s population will reside in urban centers. Applying the most conservative estimates to current demographic trends in Bangladesh is more adverse. And we need to figure out how to do this as effectively, ethically and as environmentally sensibly as possible.
Kindest regards
Rezaul Haq
[email protected]

How are the red/blue diode color combinations better than white light?

Hi Jack, plant pigments absorb lights at very specific wavelengths. Chlorophyll a and Chlorophyll b, the two main photosynthesis pigments, absorb red and blue lights.
You can learn more about how plant use lights and different types of artificial lighting in this free eBook:

I can’t open the link you posted. Page not found 🙁

Hey Nicola, sorry about that. Could you specify which link? I’ll get the correct one in there asap!

Great info here! Would you mind elaborating on the importance of hardening (first time I hear about it…), how much shock is there generally, and how does that affect operation / any tips?
One other thing I’m having trouble figuring out is how do you crunch the numbers on cost of real estate vs electricity then derive the number of hours of light?

Thank you very much!

Hey Edmund, sure. If the environment where you start a seedling and the environment where it is moved to mature are very different, then the plant might not have the necessary structure and defenses to deal with differences is temperature, airflow, light, etc. For instance, if a seedling is grown in a system that is sheltered and the air if very still, then it will not have the cues to grow a thick stem. If it is moved into a system with a lot of airflow (or outside where there is wind), its thin stem won’t be able to handle the change. Hardening a seedling is just breaking the transition from one environment to another into small steps so that the plant has the time to get ready for its new environment.

I am in the start up phase of setting up a vertical indoor micro green production facility and I am confused about the lighting systems that would work best for micro greens:

1. LED grow lights from Phillips (expensive)
2. T5 Florescents
3. T5 LEDs

From what I have read all 3 of them will work for micro greens, but which one is more cost-effective, t5 florescents or t5 LEDS. Any help with regards to this would be most welcome

Hey Atif – great question. At the seedling stage, plants typically only need 100-150 um/m2/sec. Fluorescents would provide plenty of light in this case. Just make sure that you have enough airflow to help with the heat!

I’m new at this. Starting veggies and flowers in my basement. I gave grow mats and 2 led light boxes. From reading I see the seeds need light for 12 to 18 hours a day. How about the heat mats. How long do they need to be on? Water daily? The one system I bought has self watering system.

Heat mats generally only need to be on until seeds have germinated. Daily watering is a good place to start. The media needs to stay damp during germination, but not waterlogged or soggy.

To give seedlings enough light, you'll need to keep LED bars 8–12 inches away from seedlings and T5s 5-6 inches away from seedlings. Learn more!