How many plants under 200 watt (18000 lumen) T5 lights?

So I've found a couple T5 lights laying around at my house and from what I've read, they could be good for starting seedlings (they are 6500K daylight spectrum). I've never tried starting seeds with a grow light and thought it would be nice to give it a shot. I've researched a bit about grow lights on the internet, but I am still not totally sure about how many plants I can put to grow under them. From one source I've read that you should have 100w for each plant. If that is true, then I could theoretically grow 2 seedlings under them. The problem is (or maybe isn't), that the tubes are quite long (around 1.5m = ~5ft). I have 2 lamps with 2 tubes each. Each tube is 50w and gives 4500lm, so together it is 200w and 18000lm.

My questions are:
Do you think it would be possible to grow seedlings under these lights?
If so, how many seeds can I grow under them? If only 2, won't there be a lot of wasted light (given that the tubes are 1.5m (~5ft) long and the seed tray will be like 10cm (~4in) wide)?
Should I mount the lights like 4 tubes in a single row or 2 tubes in 2 rows?

Thanks in advance for any replies!

Yes, you can absolutely start seedlings under those. There's a lot of bad information floating around out there that suggests you need $200 full-spectrum lights just to start a flat of seedlings. I think in part this is because a lot of people talking about grow lights on the internet are coming from a perspective of indoor grow operations that need to bring plants through the full lifecycle under lights, which is considerably more difficult. Much less special treatment is needed for seedlings, who are only going through the very early stages of growth under lights before you bring them outside.

I think the exact number of plants is not important, it's more about the amount of area you can properly light. You want to put the lights very close to the seedlings (as close as you can have them without the heat damaging the plants), so the available light doesn't extend much beyond the footprint of the lamp. I typically start two standard 10"x20" flats per each two-bulb fixture because the sizes match up well. I've had great results with that, nice healthy plants with no stretching. I think if you mount all four in a row you'd have a little less wasted light spilling out the sides, and could maybe even squeeze in an extra couple flats, but either configuration will work fine, so do whatever is convenient in your space.

I am not an expert, so please take this with a grain of salt.

I first used some 2ft LED shop lights from Lowes. I'm not even sure what the lumens were. I put some white paper around the edges and had 1 light over a 1020 tray, which for me had about 40 plants in it. They did pretty well. I now have some 2ft grow lights, and I still do about 1 light per 1020 tray.

You can't just plan for wattage/lumens, you have to take into account the size of the light too. A super bright flashlight may have a lot of lumens, but the area it can cover is very small if you have it close enough to the plants. A generic 4ft shop light may not be as bright as some grow lights, but you can put quite a few plants under it because of the size. I would experiment, and in general not worry too much. Try to get it over the plants, close at first and raise it as they grow. Put the light about 4inches above the growing tray and see how wide the light is. That is about the space you have to put plants :)

Good luck. I'm going to be starting my seeds next month.

**Edit** Ian beat me to it :)

The ones I use are 32 watt for 1.2m bulbs so rather similar it seems. I find with mine I can space them about 10cm apart and they will over that area so yours would cover a band about 40-45 cm wide and around 1.4m long (I find the very ends do not give enough light)

I grow seedlings up to the first leaves but also tomatoes up to about 15cm tall. I wouldn't want to go any larger under that type of light, but for my purposes it's the cheapest way to do it.

Thanks for all the answers! It gave me encouragement, I'll give it a try.

I personally start 288 seedlings (4 trays X 72 cells) under 80w of cheap led lights (4 20w lights).  I have great results in my opinion.  I could also start 800 seedlings under the same setup (200 cells per tray X 4 trays), but they'd have to be transplanted to larger pots sooner.  200w of fluorescent could easily do the same assuming they can be spaced out evenly over the trays or plants.

Because my led lights are cool to the touch, I can pretty much put the lights as close as possible to the seedlings without fear of burning them.  I'd guess a fluorescent bulb could be kept within about an inch or so without harming the seedling.

My setup works for about six or so weeks, and then they need more light to continue growing.

Here's a link to a breakdown of what I use and the costs.            https://permies.com/t/174883/Seed-starting-plant-propagating-setup#1373121

I use those T5, twin tube lights you describe to start my seedlings, and have been using them for years. They work great for starting seeds indoors.

Denis Schwarcz wrote:


If so, how many seeds can I grow under them? If only 2, won't there be a lot of wasted light (given that the tubes are 1.5m (~5ft) long and the seed tray will be like 10cm (~4in) wide)?
Should I mount the lights like 4 tubes in a single row or 2 tubes in 2 rows?

I have two, twin light fixtures that I use side by side with about a 6 or 8 inch gap in between them. Under those, I have four 72-cell trays. My fixtures can be raised and lowered which helps as I do need to raise them a few times as the seedlings grow before I begin to harden off my seedlings outdoors.

Hops this helps!