Year Round Gardening with Passive Solar Greenhouses

Year Round Gardening with Passive Solar Greenhouses

Getting started with growing your own food can seem pretty daunting at first – figuring out what grows in your area, how to plant things, what to put where – there are a lot of things to work out in the beginning. However, one thing that always makes things easier is having a nice, warm space to get seeds started and protect plants from frost before you plant them in the spring.

Greenhouses can extend your growing season, and keep you from having to purchase baby plants from a nursery. With a warm, sunny, protected place to get plants going, you can get things going earlier, and harvest even more food.

Author Mike Oehler in his underground greenhouse.

Of course, greenhouses only go so far. These are essential big glass bubbles, designed to harness as much sunlight as possible and provide a frost barrier for delicate young plants. The problem is, glass isn’t the best insulator, so your climate has to be fairly stable for it to be suitable for plants.

If you have temperatures that still dip below freezing in the spring, you may have to rig up some additional heat sources in your greenhouse, or even wait to put plants out there. Some greenhouse kits make it easy for you, but more often than not, insulating and providing additional heat sources comes at an extra cost.

Your best bet for really extending your growing season without paying a big fat electric bill in the spring months is to instead build an underground greenhouse. These structures not only give you the chance to get your own seeds started, but can extend your growing season to year-round, even in the most unforgiving climates!

Now of course, when we talk about underground greenhouses, we’re not talking about a cave with plants in it – these things are far from dark, and can let the light in via a glass roof or a south facing set of windows, depending on what you’re working with.

First, let’s think about how heat storage works in an underground greenhouse;

  1. The sun comes up, and shines through the windows into the earth walls and floor of the greenhouse.
  2. The earth acts as a thermal mass, holding onto the sun’s heat and helping regulate the internal temperature of the structure even after the sun goes down.
  3. As the air in the greenhouse cools, the earthen walls and floor slowly release the heat of the day.
  4. Over time, this creates what is known in some circles as annualized thermal inertia, in which an annual cycle of heat flow is created within the greenhouse.

These structures work in pretty much any climate, with a few modifications. That’s the thing with a dug-in greenhouse – there is no one size fits all kit, you have to make adjustments according to your climate and location.

For instance, if you have an extremely rocky plot of land, digging down is probably more trouble than it’s worth. Additionally, if there are things in your soil that you’d rather not churn up, like from local mines, it’s nice to take some clean top soil and instead bury a greenhouse on the surface with bermed walls.

Adding a Heat Source

If that big hunk of earthen thermal mass isn’t quite enough for what you’re growing, electric heaters aren’t your only option. Using a rocket mass heater, you can heat the inside of your subterranean greenhouse extremely effectively, with very little wood to boot.

Rocket mass heaters are a simple design that works so effectively by utilizing the heat from the exhaust, rather than just the fire itself. You just run the exhaust piping all over your greenhouse, encasing it in a mass if you like (maybe a pretty cob bench!), and then run the end of the exhaust pipe out of the structure.

Be careful with rocket stoves though – these things pack a powerful punch, and can quickly heat up a room. Use them sparingly, and always keep an eye on how hot things are getting in there.

Creating Humidity

Having a heat source can quickly dry out the air in a greenhouse, so if you plan to supplement your underground greenhouse with a rocket mass heater, it may be a good idea to start thinking of ways to add moisture back to the atmosphere in there.

In a lovely book about a couple’s journey to off the grid living – Off On Our Own – the author Ted Carns details their own dug in greenhouse, and how they actually put a hot tub in the room, built right into the floor – talk about cozy!

You could even go so far as to integrate these systems further, not only using the rocket mass heater to warm the greenhouse, but running the heat source around a barrel or two of water for your hot tub for an off the grid hot water heater/greenhouse heater/humidifier triple whammy.

Core Design Elements

While every setting is different and will likely demand a different set of specifications, there are a few things that are pretty standard across the board you’ll want to keep in mind when creating your underground greenhouse plans:

Always capture southern sun – You’ll get the most sun exposure from the southern path of the sun, so make sure to plan windows accordingly if you’re not doing a transparent roof. South facing slopes are ideal for building underground greenhouses.
Insulate the roof – as long as you’re not using it to capture sunlight. If you are, plan on losing quite a bit more heat than you would otherwise. Remember, heat rises, and transparent materials aren’t exactly insulative.
Use the roof to capture a water source – Regardless of your window placement, your roof can be an excellent way to supply a water source right on top of your greenhouse. You can either do a simple rainwater catchment gutter system, or if rainfall isn’t prolific enough, create a radiative rooftop airwell to capture condensation instead.

Redirect cold air – To keep the plants happy and your internal temperature stable, it’s wise to build a cold sink into your greenhouse as well. This is basically a narrow channel dug in to the greenhouse, often below a walkway, that allows cool air to sink, and keeps it away from the plants.
Leave your walls bare – don’t cover them in anything but earthen plasters or cob. Remember, you’re capitalizing on that mass to hold onto the sun’s heat – hiding it underneath wood paneling isn’t going to do you any favors. Seal it if you must, but let it be a big, dense wall of dirt.

Hands On Learning: How to Build One

I wish I could give you some step by step tutorial on how to create one of these, but in truth, all I know is what I’ve read in books and spoken to permaculture experts about in the past. There are no comprehensive tutorials detailing this process on the web, and it’s still a pretty fringe concept, even for dedicated gardeners.

However, I do highly suggest reading Mike Oehler’s The $50 and Up Underground House – his designs incorporate a greenhouse on the south facing side of his home designs, and the concept is very much the same.

There are also a few video resources on the subject – this “Greenhouse of the Future” one is particularly well done, rich in information, and the end result is a beautiful earthen jungle room that’s one part sanctuary, one part food production machine.

Otherwise, your best bet in finding an education that covers underground greenhouses, airwells, and rocket mass heaters is going to be taking a professionally taught in-person Appropriate Technology course, like the one we’re about to have with Tim Barker here at Wheaton Labs in western Montana.

The course not only covers passive solar heating and greenhouses, but will also feature special rocket mass heater instruction by the master herself, Erica Wisner. This course will take place this summer, so if you want a spot, get it now.

Other than that, if you can’t make it to a class, it’s time to start playing around in SketchUp, rent an excavator, and get to work. Though figures vary by climate, these greenhouses have been known to maintain an internal temperature of 40 degrees in 20 degrees below zero weather – without a heat source.

How long is your growing season? Would you produce more of your own food if you had the growing season for it?

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