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I am currently growing some tomato and chili seeds under a commercial grow light and I’ve recently bought some LEDs which I intend to use to build bigger grow lights. After some consideration of how exactly I’m going to set everything up and photographing the plants every day I’ve come to a rather obvious conclusion. 99% of the light is not hitting the little seedlings but rather the dirt around them.

After this profound insight I started looking around for grow light system which only illuminates the plants by using LEDs very close to the plants and automatically adjust both light level and the height of the fixture but I have been unable to find any such thing (probably searching for the wrong terms). Does anyone here know of such a system/project or do I have to design everything from scratch? Feels like something which should exist since the majority of the cost of running many grow lights is the electricity bill and not the cost of the light itself.

  • the problem is as the plant grows the lighting will need to be able to move with the plant. normally indoor grow lights only light up the top canopy also note that the photons must hit/pass through the chlorophyll at 90° angle so that it has enough distance to slow and capture the energy from the sun – seedelicious Feb 20 at 16:29
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It is true that if you plant one seed in a six inch pot and let it emerge then for a while your light is illuminating one small plant in a whole load of brown dirt, and it is easy to conclude that light is being wasted and maybe you need to use a lens to focus the supplementary light directly onto your lonely seedling.

Commercial growers take a different approach. Greenhouse bench space is at a premium since likely it is being expensively heated. Let's say you decide to sow 1,000 seeds each week from January to March in order to bring on enough product to sell. The grower will use an automatic seeder to put the seeds in plug trays. These trays are the size of regular plant trays containing say 12x4-paks which would produce 48 plants in total, except that the little pockets (plugs) are quite small, enough to get one seed going. As you can imagine the result is lots of germinating seed uniformly spread over the tray. After germination and a few days growth all you can see is green leaves and no growing medium. Light (and space) is used to maximum efficiency. Once they have filled their plugs it is easy to move them into larger containers where they have room to grow.

Home gardeners do something similar, planting their packet of a dozen seeds into a small seed tray and then pricking the seedlings out once they get to a moveable size.

So when the seedlings are in seedling size plug trays, instead of wasting space and light illuminating dirt you crowd the seed trays under a few lights, and only switch on the other lights when the plants get large enough to need them. So rather than using one huge light with variable output focused on one seedling you perhaps need many smaller lights, using the appropriate number to get the illumination needed and to use much smaller pots in the initial stages.

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  • Great answer :) Is the replanting of the seedlings done by hand or automated by machines? Basically I want an automated system from seed to harvest :) not doing this on an industrial scale, just a hobby project, but I do like to automate things. – Beacon of Wierd Feb 20 at 0:08
  • By hand. I don't think the capability of machines to grasp the seedlings has yet achieved anything like comparability with human transplanters and the dexterity of their sensitive fingers. – Colin Beckingham Feb 20 at 0:21
  • You could probably prepare the larger destination pots/trays with dimpled surface automatically. – Colin Beckingham Feb 20 at 0:24
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Commercial hydroponic setups use white plastic on the floor to recycle light for the plants.

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You can't find it because it's not necessary; when seedlings naturally germinate outdoors, the sun does not send out a single ray focused on each seedling, it lights the whole area, including the soil and surrounding vegetation if there is any. Your grow lights will do the same thing, whether you decide the soil gets 50% or 99% of the light. So long as there is sufficient light to the growing area for the right length of time, at the appropriate distance, your seedlings will grow. assuming other requirements such as warmth and water are met.

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You need light (or better heat, so IR light) also on soil. The temperature of soil trigger most plant behaviour. Note: Seedling do not need much light, until they have already one true leave.

You forgot an other simple method: aluminium foils. Instead of directing light to the seedling, you can use less light, and aluminium foils around the plant, so that most light will bounce and reach the seedling.

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