Highest wattage of fluorescent lights for plant growing?

April 19, 2015 3:34 PM

I am currently growing pothos in a long planter in my bathroom with two four-ft. "grow light" fluorescent bulbs in a regular two-bulb fluorescent fixture. This is almost a joke, because...

pothos barely needs any light anyway. I'd like to grow some more interesting plants, maybe something that flowers (but not African violets -- the grow light cliche'). And since the shelf the lights are above is pretty high up on the wall, I'd like the plants to be pretty tall.

I've been reading about fancy high-intensity lamps that come with "ballasts" and get very hot and you need a fan to cool them off and the whole thing scares me. On the other hand, some people seem to do well with regular fluorescents, but I'm not clear about how they're doing it.

So my question is: what is the highest intensity fluorescent bulb I can put in my "regular" fixture without burning my house down? Note: I get virtually no natural light in this room.

posted by DMelanogaster to Home & Garden (7 answers total)

Best answer: What type of fluoro tubes are we talking about? Standard 4ft T12s that are 40W, or one of the fancy high output T5s? If they're T12s, you're not going to be able to grow anything with those. 40Ws spread over four feet is basically zero energy for the plant. The T5s, you might be able to, but only 2 of them isn't much (which makes me suspect its a standard T12 fixture). Again, assuming a standard T12 fixture, the max wattage is almost always 40Ws per bulb. And again, you're not going to be growing anything with T12s - it just isn't feasible. You'd be better off with an array of CFLs + reflectors for them.

The typical "grow light" set up is a high intensity discharge ballast that can run both metal halide and high pressure sodium bulbs. Starts at 150Ws, goes all the way up to 1600Ws. 150, 250, 400 you can probably get away without needing a fan, depending on what the size of your space is and how well otherwise-ventalated it is. 800W, 1600W HID bulbs - yeah, you need a fan for sure.

Honestly the best thing, even though this isnt your goal, is to go on some marijuana grow forums and get your research from them. They've already got that shit down to a science :)
posted by ish__ at 4:12 PM on April 19, 2015 [3 favorites]

The fixture will typically have a marking somewhere on it indicating the max wattage for which it is rated. Unfortunately, this may be a sticker that is long gone. What size (T5, T8, T12) and wattage are the current bulbs?

You can grow seedlings and some plants with fluorescent tubes (though you'll do better with a dedicated fixture that includes 4 or more 54W T5 bulbs). Generally, it will be hard to achieve flowering without brighter light (though you might get there with African violets and some other houseplants).
posted by ssg at 5:02 PM on April 19, 2015

I don't know much about the technical details, but I use one of these setups and my plants seem happy in it. I have a variety of cacti, an African violet, a miniature Phalaenopsis orchid, and a couple of other plants. The only thing that I've tried without success is rosemary.
posted by jkent at 5:30 PM on April 19, 2015

You don't typically have a choice about what wattage bulbs to install in a given fluorescent fixture. Four-foot T12 bulbs are 40 watts, four-foot T8 bulbs are 32 watts, etc. and they aren't interchangeable.

The only grow-light growing I've done was starting vegetable and herb seedlings. I used basic 4' T8 shop light fixtures, but the plants were placed only a few inches below the bulbs. I don't think it would work to grow tall, light-hungry plants below such lights, because most of the foliage would be too far from the light source.
posted by jon1270 at 7:16 PM on April 19, 2015

Orchids may be an option since they grow in filtered sunlight naturally, plus they love humid environments. See if there's a hobbyist club in your area - they often have great suggestions.
posted by mightshould at 9:56 AM on April 20, 2015

I grow plenty of interesting tropical plants in my basement with T12s, primarily Anthuriums (which bloom just fine, thanks) but plenty of other things too. In my case, the trick is to put the plants on adjustable-height shelves so that when plants grow too tall for the space, the relative locations of lights and plants can be changed to accommodate. MeMail if you're interested in discussing further.
posted by Spathe Cadet at 11:41 AM on April 20, 2015

You could always go CFLs.. you can construct a multi-bulb fixture based on the space you have and use something like the below, which would get you 210watts (or 420w if you get 4 bulbs). I think there are CFLs that go up to about 150watt, but they get more expensive per watt the bigger they get.

2-pack 105watt CFL bulbs (Amazon).

These will get hot, but not too hot - not really any need for cooling other than perhaps a small fan to circulate air. You can keep them extremely close to the plants (a few inches off them) without much issue - and indeed, you have to, as the CFL light doesn't penetrate like HPS.
posted by mbatch at 4:51 PM on April 20, 2015