This is our second summer with our geodesic dome greenhouse, and I’m doing something a little different this year. A common problem in small greenhouses is regulating the heat, either too much or, as is often the case in Alberta, too little. Thermal mass is a great way to do smooth out those fluctuations, and we have a large water tank that we use to help with it, and also small milk jugs full of water right next to plants to absorb heat during the day and release it at night. Last year, we had a crazy freeze happen in September, and I knew I needed to do more to store heat inside the greenhouse. One idea I had heard about is a Subterranean Heating and Cooling System.

Basically, a SHCS uses a fan to pump the hot, humid air inside a greenhouse underground through tubes that will use the soil as a big heat sink. During the day the hot air goes underground where it cools, transferring it’s heat to the soil, and when it exits the tubes the air functions as a cooling system inside the greenhouse. At night, the effect is the opposite; the air inside the greenhouse cools quickly, but the air coming out of the tubes in the ground will be warmer with the stored heat from earlier in the day. Here in our part of Alberta, even in the middle of summer, it isn’t uncommon to have night time temperatures around 10C or less and the air coming out of the SHCS should be warmer than that. Yes it is complicated, but the science is sound and I know it works. At least that is what I had to keep telling myself as I dug down 70 cm to bury the pipes! Whenever I tried to describe the system to someone they looked at me like I am crazy. As I was digging I wondered if I really was. Crazy.

If you want to see a thorough analysis of this kind of system complete with temperature graphs, here is one built in Scotland: www.permaculture.co.uk/sites/default/files/Solar_Greenhouse_Article_Web_Ed.pdf

Another advantage of this kind of system is that it will warm your soil up quicker in the Spring. Here are some observations reported in the above article:

“By March 21st the average greenhouse soil temperature at a depth of 76cm (30in) was 16° versus 6.5° in the soil outside, representing stored heat of around 49kW hours.”

Those are some pretty good results!

Given the circular shape of our greenhouse, and the fact that I didn’t want to dig up the whole thing down to that depth, I only used five tubes, two along each side of the outer ring of the greenhouse, and one coiling in the centre section.

Basic layout of the greenhouse

Where the tubes are buried

Digging down to 70 cm means I was past the rich, black soil and into the clay. I tried to keep the clay in one pile to pack back down around the tubes, but it inevitably got mixed in some with the other soil. The first two tubes I excavated all the soil and clay under the growing bed, but after that I just removed the soil and dug smaller trenches in the clay where I wanted the tubes as I was running out of space to pile the soil up.

lots of digging to get this far

Trenching

Andrew helped me dig the centre area

Centre section

The plenum, where the fan will blow down into the tubes

The completed system with the buried blue can, and a black can on top:

The fan is on top of a second garbage can to get it up higher into the warmer air. The black can will also absorb some heat on a sunny day which further heats the air as it goes underground. On day when the temperature was 23C (73F) outside, and inside the greenhouse it was 32C (90F), air coming out of the tubes was only about 16C (61F). The large black thing to the right is a water tank with loops of a black garden hose for the water filter. It also helps absorb some light energy and radiate it back at night.

There is a lot more I could have done for this project (insulate along the outside of the trenches, solar powered fans, more tubes, etc), but as this isn’t going to keep the greenhouse going in the winter in a 3a hardiness zone, and I was simply running out of time to dig. The five tubes will be enough for us. I’d like to add a temperature controlled switch for the fan, but for now it is simply on a timer.

We already had to rely on it one night as it got down to -3C (26F). Naomi took frost cloths and covered the plants, and the heat being pumped up from underground and blowing under the frost cloths kept everything warm enough. This should be a big help near the end of the growing season.

Fall Update:

Early one September morning our outdoor thermometer said it was 0.8C outside. The greenhouse was 6.7C. The subterranean heating/cooling system was doing it’s job! I have a thermometer on a string that goes way down in one of the tubes. The air coming from underground this morning was still 10C.

With early October underway, our growing season is pretty much over now and we are so thankful for the harvest that we have been enjoying!

We really wanted to have a greenhouse to help extend the shoulder seasons and for protection from frost, but we didn’t expect how badly we would need it with snow and heavy frost in early September.

Frost blankets, two extra garbage cans full of water, and a heater running to keep everything alive through the cold snap! We knew this cold would be over in a couple days and we could expect several more weeks of warmth for the growing season if we could just get through this.

The gardens outside buried in early September snow.

Everything in the greenhouse survived and the plants in the raised beds (strawberries, lettuce, beets, carrots) came out fine. We pulled up the onions right away after the snow was gone, and they are drying in the basement. After the cold snap, we had several more weeks of our growing season.

With another heavy frost forecast for October 1st, we decided to pull the plug, or at least the harvest. We felt that it wasn’t worth trying to fight -6C (21F) in early October as there wasn’t going to be much more of a growing season. That night we emptied the greenhouse to let stuff ripen indoors. The greenhouse has been a wonderful addition to the yard, and we have been able to grow so much in there. For the last month Naomi has spent lots of time processing the food and trying out delicious new recipes. Lots and lots of canning.

The harvest from our own yard and greenhouse includes: cucumbers, grapes, chili peppers, bell peppers, banana peppers, jalapeno peppers, roma tomatoes, cherry tomatoes, peas, carrots, lettuce, spinach, beets, onions, shallots, strawberries, raspberries, choke cherries, potatoes, and lots of herbs (sage, rosemary, thyme, basil, oregano, coriander).

We also had a row of potatoes, onions, beets and carrots out at a friends farm. Our neighbor gave us two big bags of apples from their tree. From friends’ yards we harvested nanking cherries, raspberries, saskatoon berries and red currants.

We recently planted 4 blueberry shrubs and have some saskatoons to plant, so hopefully we will have that to add to future harvests, and we expect to see our three different apple trees start to produce too.

Bell, chili, jalapeno

A giant cucumber that was hiding in the greenhouse. And Chantel.

Choke Cherries

Picking choke cherries

Picking choke cherries

Three berry syrup -nanking cherries, red currants, choke cherries

Three berry syrup

A row of potatoes, beets, carrots, and onions at a friends farm

Big beets

Onions, potatoes, beets, carrots

Potatoes from the yard

and from the farm

Apples from our neighbours

Grapes

Beefsteak Tomato

Roma Tomatoes

The last strawbereies

Big peppers

Picking Saskatoons at a friend’s acreage

Raspberries, Nanking Cherries, Saskatoons

Strawberries and raspberries on my cherrios

Roma tomatoes

Roasting tomatoes to make sauce

Some of the late summer harvest

Did somebody say salsa?

Wearing gloves to avoid the burn

some of the herbs in the greenhouse

Onions drying for storage

Jalapeños

Chilies

The final harvest from the greenhouse

The last day for growing in the greenhouse

He makes springs pour water into the ravines;
it flows between the mountains.
They give water to all the beasts of the field;
the wild donkeys quench their thirst.
The birds of the sky nest by the waters;
they sing among the branches.
He waters the mountains from his upper chambers;
the land is satisfied by the fruit of his work.
He makes grass grow for the cattle,
and plants for people to cultivate—
bringing forth food from the earth
Psalm 104:10-14

Last Fall we purchased a geodesic dome greenhouse from some friends who were moving (they have a series of write ups about it on their blog, including how to make one of your own). They were very generous in selling it to us, and we have really enjoyed owning it.

The dome is 5.4 meters (18 feet) across, and about 3 m (10 feet) high in the centre of the dome with a 3v frequency design. It is very strong, and can easily support my weight on top, and will shed snow and support the snow load in the winter. We live in a 3a hardiness zone, and this close to the mountains we can get very late frosts in the Spring or early ones in Fall, and theoretically could get a freak snow fall at any time. Hail storms are also a problem around here during the summer. All this makes a greenhouse a very valuable addition to our gardening as it can really expand a very short growing season.

Before we could move it, the greenhouse had to be carefully taken apart, cleaned, and re-stained. We prepared the ground for it on October 26  by taking up the sod and tilling the earth.

The next day it snowed… and that snow stayed until April.

We kept the wood framing inside all winter, waiting for some nice weather to come. Once the snow melted, the base was levelled to prepare for the dome. We put concrete blocks under it to keep the wood off the ground.

This is a time lapse video that I made of the construction back in May. Don’t be fooled by it. Despite the four seasons seen in it, this was done over the course of an evening and a Saturday in mid-May.

Greenhouse Time Lapse from Troy Johnstone on Vimeo.

Putting the skin on – a special greenhouse quality, UV resistant poly.

Digging the hole for the water tank. The tank is on the north side of the greenhouse, right next to the door (just off the right side of the picture). By burying the bottom of the tank, it gives it the strength to hold in the immense water pressure. The purpose of the water tank is to provide some thermal inertia, if you will. It absorbs heat during the day, and helps keep the greenhouse warm at night. I have some water hoses that circulate water around the base of the plants. In the spring, that water hose can be more than 10C warmer than the air around it at night.

  
Painting the water tank. Ellie made her own old-timey bonnet.

To keep the greenhouse from overheating, there are four self-opening windows, two down low and two near the top of the dome. As the openers get warm, the hydraulic cylinder pushes the window out. We mounted a fan that is controlled by a timer on one of the top windows to force more hot air out the top. Once the sun shines on the greenhouse for awhile, the windows start opening up like petals on a flower.

Greenhouse set up and first plants in the ground! This photo was taken on May 22, 2014 – a bit of a late start because of the construction of the dome. The tomatoes were planted a bit too close together… live and learn.

The greenhouse fits in nicely in the back half of the backyard. The house had some large perennial flower beds when we bought it, and we have tried to keep them up, but we are slowing changing some of the landscaping over to foodscaping – landscaping with a purpose.

We had the greenhouse set up for about a month before we left on a 3.5 week trip to Ontario and New Brunswick! Thankfully, Naomi’s parents were able to keep an eye on the greenhouse in our absence. Here is how it looked when we left (June 23). The milk jugs are there to absorb heat during the day and radiate it back to the plants at night, which can get pretty cool this close to the mountains (between 7 and 13C, or 44F to 55F), even in July and August.

And when we got back on July 17:

We hung up some frost cloth on the south side to block a bit of sun and help cut down on heat a bit. On a cool day it can still get over 30C (86F) in there, and a hot (for here) day of 30 C it gets around 37C (97F) inside.

When we left for our trip we had one plant coming up that we didn’t recognize. It just had two big leaves, and looked similar to the broccoli that we had growing in there. We figured we would leave it there and see what it was when we got back. Well, we were surprised to return the next month to discover a giant canola plant! Not sure how that seed got in there…

The greenhouse is an important part of our gardening. Inside we have:

Bell peppers

Chili Peppers

Banana Peppers

Cherry tomatoes

Lots and lots of tomatoes. The ones above are roma tomatoes.

Peas

Cucumbers

Garden fresh herbs – rosemary, oregano, 2 types of sage, 2 types of basil, and thyme.

We are very thankful to have it to protect the crop from nasty weather as the greenhouse has already withstood many rain and hail storms. The plastic isn’t really tight and has enough give in it to bounce off most hail, but if it gets extreme it will probably get shredded. I have seen hail around here strip the vinyl  siding off of a house..

It is too hot right now, but later we will try planting some lettuce in the greenhouse so we can hopefully have fresh greens, hopefully right into mid to late fall.

We have already been enjoying eating quite a bit of fresh food from our green house, and are looking forward to the harvest to come. Along with the rest of the gardens, we have had some very tasty and nutritious home grown food!

Update: Yes, it can handle the snow.

Update 2: Check out our Subterranean Heating and Cooling System!

Basic layout of the greenhouse

Where the tubes are buried

Also, a photoshere image of the inside of the dome.