A greenhouse within a greenhouse...

Posted 1/21/20 (Tue)

After getting a good response from last week’s article, here is more food for thought about greenhouses, no pun intended.

There are several ways you can grow garden produce even in the northernmost communities of North Dakota like Pembina, Neche, Walhalla, Westhope, Sherwood, Portal, Crosby and the International Peace Garden.

I don’t think anyone has ever tried this, at least not in North Dakota, and I’m not sure I want to spend the money to prove this theory would be effective.

But what would happen if someone built a greenhouse within a greenhouse and then grew plants inside the smaller greenhouse?

I think about this a lot because  on sunny winter days that could be as cold as 10 below zero, the temperature will often rise to 70 degrees in a passive solar greenhouse.

That heats dissipates quickly after the sun goes down, but if some of that daytime heat from the sun, could be trapped inside the smaller greenhouse, it wouldn’t get nearly as cold as it would inside the larger greenhouse.

Here’s an example. We all know how cold the wind is when it blows on a winter day. If you are inside a greenhouse, you are out of the wind, but still have to deal with the ambient temperature sans wind chill.

Now, if you have a second, smaller greenhouse, you’re first of all more protected inside of it, because you are technically, even further away from the wind.

A greenhouse is supposed to have two sheets of plastic, the outer layer and an inner layer, both separated by just an inch or two to act as an insulator.

If the smaller greenhouse was considerably smaller proportionately, that would act as additional insulation.

A double passive solar concept would trap more heat because there would be less to escape from the inner greenhouse, thus the night temperature inside would drop far more slowly.

Thick, R insulation could be used on the north and northwest sides to prevent further cold from entering because of our prevailing northwest wind.

That means supplemental heat would cost considerably less, even in the depth of winter, so it is possible that a moderate sized electric heater would do the trick inside the smaller greenhouse.

Still, another safeguard would be polycarbonate panels. In other words, if the inner greenhouse was built with “corrugated plastic,” it would trap more heat inside than a greenhouse would with traditional 6 millimeter plastic.

I’ve thought about this for a number of years because of something that happened in my own back yard.

In 2008 we had a 12 x 18 foot greenhouse that was built in Israel from polycarbonate and rubber. It was often quite warm in there during sunny, winter days, so much so that I could work in a T-shirt and that greenhouse wasn’t passive solar. I did have the floor covered in black, rubber mats, however.

Inside that little greenhouse, I used some old glass cooking ware my wife no longer wanted. I placed it on the shelf, on a black rubber mat, and attempted to grow plants under the glass, inside the polycarbonate.

It worked until it got extremely cold. Then the plants froze. Unfortunately, I didn’t have access to electricity in that greenhouse, so was unable to protect them other than with sunlight.

So what would be the difference if that glass or “corrugated plastic” was much larger in size and was inside a traditional greenhouse? Couple that with a hole in the ground to capture geothermal heat and now all you need is lighting to circumvent the short days of winter.

It would most likely function quite well if all the necessary precautions are taken and adhered to. It would certainly be a disciplined task.

The downside is the price to make this happen. Although it would all be a one-time expense, except for the electricity, it would still cost a sizeable amount.

But as people continue to become more serious within our state’s boundaries about growing fresh produce and herbs, something like this could be the answer.

The caveat is you can only grow certain items like lettuce, kale, spinach, onions, cabbage and carrots. You have to know what plants withstand cold better because they are the ones that will survive.

Interested growers could seek grants or money through the Agricultural Products and Utilization Commission (APUC) to offset the overall bill it would take to build such a structure.