My North Dakota Cistern-Fed Solar-Heated Greenhouse

Although my house has no basement, it does (because of the depth to which frost penetrates In these parts) rest atop a 5 foot deep foundation wall. Which meant that it was a simple matter for me to build an underground cistern by [1] digging a 5 foot-wide by 5 foot-deep by 30 foot-long trench along the south side of the house, [2] laying up cement block walls on the three outer sides of the excavated area, and [3] pouring a 3 inch-thick cement floor the length of the trench.

I laid the cistern’s walls up out of 6 inch by 8 inch by 16 inch cement — not concrete — blocks. (Cement blocks are less expensive than the “aggregate” kind. Also, they’re a lot denser and therefore easier to waterproof.) The mortar I used to bind the blocks together with consisted of fine sand and cement in a three- or four-to-one ratio, mixed with enough water to form a “mud” that would stand up when put in a pile.

I’ve experimented with all kinds of goop, to waterproof the cistern’s walls and-after a lot of effort — I can’t say that I’m satisfied with any one method, since I can’t keep water in mine above the four-foot level. If I were to start all over again, I think I would first plaster the entire inner surface of the cistern . . . just as grandpa would’ve done in the old days. Then I could apply a coat or coats of epoxy, tar, emulsified asphalt, or whatever to further seal the tank.

Water is piped into my underground cistern via downspouts connected to the eaves troughs on the house. (It’s amazing how fast a little rain or snow on the roof will fill up a cistern. So, be sure — if you decide to rig a similar setup — that you put a bypass in the downspouts or figure out an overflow pipe arrangement of some sort. Otherwise you’ll have a wet greenhouse on your hands at times.) I remove water from the cistern by means of a small hand pump installed in the “roof” of the water tank . . . a handy combination to have not only for when I need to water plants, but in case of a fire.

I finished off the cistern with a floor (or roof, depending on how you look at it) of rough-cut, full-thickness 2 by 8 planks. As of this writing, the planks have been in place three years and show no signs of deterioration, even with all that water underneath. (The problem in my greenhouse — even in winter — has been excess dryness, not too much humidity.) As long as the boards stay dry, they should go another three years-or more-with no problem.

The Solar-Heated Greenhouse

After completing the cistern, I began work on the greenhouse itself. First I measured out the conservatory’s width (12 feet) and length (30 feet), then dug a trench a foot deep by a foot wide along this line and-using a modification of the type of form described by the Nearings in their book, Living the Good Life laid up a rock wall from the bottom of the trench to an aboveground height of two feet. (I used rock for this wall simply because it’s free for the hauling around here. You could just as easily substitute recycled cement blocks, cinder blocks, etc. )

If you plan to build a greenhouse similar to mine — but you want to lay the rock up by hand, without a form — I recommend you lay one tier of the stone at a time. (That is, start at one end of the trench and work your way clear around to the other before beginning the next layer of rock.) Lay old barbed wire, scrap iron, etc., in the corners to tie them together. And remember that when you’re using stone rather than cement blocks, your mortar can be somewhat weaker. Five or six parts sand to one part cement is plenty strong enough.

As for the “see-through” part of the greenhouse, half of my 30 foot-long conservatory consists of a Lord & Burnham pre-fabbed unit that I’d erected on the southeast side of my house some years before. (I moved this unit to its present location for reasons of increased heating efficiency.) I built the other half of the greenhouse from scratch using some recycled 4 foot by 6 foot windows (six panes of glass each) I got from an old house that was being torn down in town. These windows are supported on 2 by 4 framing at six-foot intervals.

The redwood benches in my greenhouse were constructed from a salvaged livestock watering tank. (These aren’t hard to come by. At farm auctions, you’ll often see a pile of lumber for sale containing the remains of cedar and/or redwood tanks. Of course, you should stick with either cedar or redwood, since other types of wood will rot out on you after three or four years.) For general use, a planter bench six inches deep will do fine. Otherwise — if you want to grow carrots or other deep-rooted crops — your benches will have to be 10 inches or 12 inches deep. And you don’t have to build the planters from scratch: You can use old dresser drawers, pails, or just about anything that’ll hold dirt.

Greenhouse Ventilation

The sun can really heat up a glassed-in room or lean-to on a clear day . . . even when the outside temperature is quite low. (My greenhouse, for instance, has been warmed to near 100 degrees Fahrenheit when the outside air was no more than 15 degrees Fahrenheit above.) For this reason, a vent system of some sort is an absolute must. The breather on my greenhouse is a little large-two feet wide by ten feet long-but it’s better to have one that’s too big than one that’s too small. (Don’t skimp on the size of your greenhouse’s vent, or your vegetables will cook right where they grow!)

Supplemental Greenhouse Heating

For supplemental heat in the winter, I’ve finally — after messing around with many different types of heaters — settled on using an Ashley wood stove. The Ashley is airtight and has a thermostat (both of which are essential features for keeping fumes down and maintaining an even temperature). Three or four logs will keep this efficient stove fueled up for more than 24 hours.

If you decide to go the wood stove route, bear in mind that a good chimney is essential. I tried just using a stovepipe at first, but-because the stove burns its fuel very slowly-ran into problems with creosote forming in the pipe. A brick chimney — preferably one with a smooth tile core-is best. (For proper draft, the chimney should be higher than any nearby roofline.)

I also find it useful in the winter to insulate the greenhouse by attaching a long sheet of plastic to the underside of the framing that holds up the unit’s lowermost row of windows. I don’t like to buy or use plastic, but—since I only need it four months out of the year — I do get quite a bit (three years) of use out of one 16 foot by 30 foot sheet . . . and it does give me prettier tomatoes.

What I’d ultimately like to do, I guess, is find some way to solar heat the cistern’s water during the summer, then recirculate the liquid through the greenhouse in the winter. I’m currently experimenting along this line.

The Greenhouse Perimeter Planter

Because the frost does go down deep here (five feet) in winter-and because the stone wall upon which my greenhouse rests only extends a foot below the earth’s surface — I’ve constructed a second rock wall parallel to and three feet away from the greenhouse, as high as the conservatory’s stone wall itself. The space between the two walls serves as a summer planter and, more importantly, helps to insulate the below ground portion of my greenhouse from frost.

In more southerly parts of the U.S., it may not be necessary to build an “insulating planter” of this kind: A couple of rows of baled hay around the greenhouse might work just as well, although the bales would have to be replaced every year. Remember, though, that hay not only retains heat but can retain a nice mouse population, too. Make sure your greenhouse is rodent-proof before you line its perimeter with hay, or you may have to share your produce with some little furry critters!

A Solar-Heated Greenhouse Pays for Itself in Food

Thanks to the extended growing season made possible by my combination cistern/ greenhouse, I’ve been able — over the past several years — to grow and enjoy a number of vegetables that I might not otherwise have been able to harvest. I’ve even grown fruit, to a limited extent. While in Arizona a couple of winters ago, I bought two Bonanza peach trees and a nectarine tree (all dwarfs) . . . and I now have peaches and nectarines on those trees inside the greenhouse. (Since the trees are only 5-1/2 foot tall when mature, they can stay inside the greenhouse-in pots or boxes indefinitely.)

During warm weather, of course, I do have to open the greenhouse’s vent and door so that the fruit trees (and other goodies) don’t get cooked. Otherwise, the little conservatory has been very easy to care for . . . and it’s paid for itself many times over in fresh edibles.

Now I’m planning a swimming pool which will be enclosed in a greenhouse-like building. Who knows? Next year at this time I may have a solar-heated, swim-in greenhouse. If so, I’ll let you know!