This post is about providing light for vegetable seedlings.
Years ago, I did that by hanging a fluorescent shop light just a few inches above the plants. That’s a pretty common way to do it, and any number of reputable internet soources will tell you to do that.
As I get back into the business of starting plants while the days are still cold, I decided to re-think that. And, as it turns out, the world has moved on. What was the epitome of cheap, efficient lighting 10 years ago (linear fluorescent tubes) is now an energy-wasting extravagance compared to the latest generation of LEDs.
But after working my way through watts, lumens, and lux, and looking at cheap LED retrofits for old fluorescent tubes, in the end, the best and cheapest solution was sunlight.
This post is about constructing a simple window-hung greenhouse from a clear tote, a pool noodle, and some clear packing tape. Ten bucks, ten minutes, and a suitable window, and you’ve got the perfect temperature-controlled spot for raising your seedlings. There, a couple of hours of sunlight provides your plants with more usable energy than an entire day spent under closely-spaced fluorescents.
Here’s my journey, from fluorescents to sunshine via LEDs.
The starting point is an elderly 4′ two-tube fluorescent shop light. I’ve had it so long that I’ve forgotten when I got it. It looks absolutely no different from any other shop light: white, metal, and poorly built. But it works as well now as it ever did. It uses two 40 watt T12 tubes, and with the losses from the fluorescent ballast, probably consumes about 85 watts.
(T12? Just when you think the U.S. system of units could not get any goofier, something will come along to prove you wrong. Fluorescent tubes are measured in eights-of-an-inch diameter. Hence, T12 is an old-fashioned fat fluorescent tube that’s 1.5 inches in diameter.)
The first thing to give me pause is the amount of electricity consumed. I’m supposed to run that about 16 hours a day, in order to provide adequate light to my plants. That adds up to 1.4 kilowatt-hours (KWH) of electricity per day. Just to grow about two square feet of seedlings. If I need those lights on for a month, that’s about 40 KWH per month.
It’s not the cost of that that irks me. That’s about $6, at the rates I pay. It’s that it seems like a ridiculously anti-environmental thing to do, as a byproduct of trying to have a greener garden.
To put that in perspective, I’m pretty sure that’s more than all the rest of the lighting in my house consumes. And that’s enough electricity to drive my wife’s plug-in Prius about 200 miles.
All that, just so I can have my tomatoes a few weeks earlier. Seems kind of self-indulgent. Surely I can do better,
Next stop was an LED retrofit for those T12 fluorescent lamps. And here’s where I got my first eye-opener. A decade ago, there wasn’t a whole lot of difference in efficiency between linear fluorescent lighs and LED lights. But now, the off-the-shelf LED replacement “light bulbs” produce just about twice as much light as fluorescent bulbs, per unit of power consumed. About 120 lumens per watt, compared to maybe 65 lumens per watt for standard T12 fluorescent bulbs.
So I bought a couple of plug-n-play LED retrofit “tubes”. Some of those require you to rewire the light fixture, but others are straight-up bulb replacments. These are the exact size of a T12 bi-pin fluorescent, and, in theory, if you’re lucky, you can just literally swap the fluorescent tubes with the LED “tubes” and you’re done. It’s a bit wasteful, in that your’re still heating up that old (and now useless) fluorescent ballast (the gizmo that lights the fluorescent lamps). But it’s sure easy, and it’s sure cheap. A replacement for a 4′ T12 cost $8.50 at Home Depot — less than buying new T12 tubes individually.
Unfortunately, that easy upgrade didn’t work for my elderly fluorescent fixture. I’m not sure what gave that away faster — the erraticly flickering lights or the strong odor of burning plastic. If light bulbs could scream, these would have. (Although, oddly enough, I tried them later in a more modern fixture and they still work. Just not in the light fixture I have available for growing plants).
Time for yet another re-think. I could switch to the type of retrofit LED bulbs that require re-wiring the fixture. (So-called “ballast bypass” bulbs.) That’s not hard to do. I could just chuck the old light and buy a new one,. But instead, I got out a light meter and started assessing the situation.
Lumens and kelvins and lux, oh my.
Lumens measure the rate at which light energy is being produced. A typical “60 watt equivalent” compact fluorescent light produces about 600 lumens. A four-foot fluorescent tube produces about 2600 lumens.
Kelvin, in this case, provides a measure of the color of light that a light bulb produces. For archaic reasons, it’s called the “color temperature”. Just as you would see heating a piece of metal in a flame, a low color temperature means a reddish light, higher temperature means a blue-ish light. My understanding is that vegetable seedlings are happier with a higher-color-temperature bulb.
Finally, lux measures how brightly a surface is illumnated. One lux is one lumen of light energy flow, spread over a one-square-meter surface. Moonlight illuminates the ground at about 0.2 lux, direct sun at noon on a clear day is about 100,000 lux.
And a light meter, held 3″ from those T12 fluorescent tubes, registers about 6000 lux.
That’s pitifully dim, compared to sunlight. Direct sunlight slanting in my windows, in the afternoon, at this time of year, will easily illuminate the floor at a level of 20,000 lux.
That explains why you have to leave the lights on 16 hours a day if you want to grow seedlings. You need all that time so that the dim light of the fluorescent bulb can deliver something approaching the energy in a few hours of sunlight.
Greenhouses, cold frames, lazy gardeners, and dead plants.
There’s one obvious solution that doesn’t require any artificial light at all: Use a greenhouse or a coldframe. But in this climate, at this time of the year, that either requires heating (and so uses energy), or it requires a fairly attentive gardener.
Leave your delicate plants out on a particularly cold night and you can freeze them, cold frame or not. Leave them out on a particulary sunny day, and in a warm spell you’ll roast the plants to death.
I’ve learned over the years that coldframes just don’t work for me. One way or the other, I’ll forget to move the plants inside, or forget to prop the lid open, and the next thing you know, my seedlings are dead.
What I really need is a heated and air-conditioned greenhouse. That would be foolproof. But that would take a lot of energy to run, not to mention the cost of constructing it in the first place.
Or would it?
Hang a greenhouse out your open window.
I find that sunlight slanting in my windows is not quite adequate for growing my seedlings. For one thing, I have little south-facing window area. For another, they all spend a lot of energy leaning toward the light. And my windowsills are narrow, to boot.
But some people solve this problem by building a little exterior greenhouse, and attaching it to the interior of the house. In particular, they connect it to the heated and air-conditioned air inside the house. That way you get abundant sunlight for the plants, but the greenhouse can’t freeze or overhead, regardless of the outside weather. And it’s cheap: All the conditioning of that greenhouse air is done by tacking a tiny little additional load onto the house’s existing HVAC system.
After looking at commercially-built units (too expensive), and at plans for constructing such units out of wood (too complex), I realized that I already owned the primary component of the perfect exterior through-the-window greenhouse. It was a 56 quart clear tote.
As with my crude solar tomato dryer, if you need a clear box, it’s kind of nuts to spend a lot of time and money building one. It’s a lot cheaper and easier just to buy one. Sterilite, in particular, makes a huge range of clear plastic totes.
And so, open the window, wedge the tote firmly in place with the opening facing the inside of the house, and use a bit of tape and some cut-up pool noodles to fill the gaps and firm up the installation.
Here’s the result, which took me all of maybe 10 minutes to put together:
To orient yourself, you’re looking at a standard double-hung window. The bottom sash has been pulled all the way up, and a couple of clear plastic totes fill that gap, bottoms facing out. The pinkish things are cut-up pieces of pool noodle.
And the punchline? That’s the light meter reading showing at the top of this post. At 9:30 AM, on a clear spring day, the illumination level in that clear tot is already over 30,000 lux. That, plus the fact that the spectrum of sunlight is just what plants need to see, means that three hours in the greenhouse provides far more usable light energy to my seedlings than 16 hours of sitting under those fluorescent tubes.
And, unlike an exterior cold frame or greenhouse, the air temperature in this window-attached unit will stay at or near room temperature.
In the end, my failure to retrofit my ancient fluorescent shop light ended up generating a far better solution for growing my seedlings., And, yeah, it looks a bit redneck. But it only has to be there for a month or so. For 11 months of the year, that will just go back to being a normal window. And the totes will go back to being totes.
The only thing missing in the ten-minute version of this is insulation. And the obvious solution there is bubble wrap. So I’ll add that to the interior as soon as I can lay my hands on a big-enough piece of it. That ought to give roughly an R-2 level of insulation, not hugely different from the window itself.
