This greenhouse project is my attempt at lengthening my growing season and increasing heat units so I can grow heat loving plants in North-Central Idaho. We have an approximately 90 day growing season, and we’re at about 4100 ft in elevation. Our last frost date is June 15 and the first frost is usually September 15. The cool evenings and short season don’t exactly add up to ideal tomato and watermelon growing conditions. I aim to change that!
My chosen design for this project was a 16×24 hoop structure (already had it so why not?). The interior is dug down to make a level-ish workspace inside. We went down anywhere from two feet at the entrance to about a foot on the downhill side. Along the perimeter there are raised beds about three feet tall and three feet wide. These growing beds are the wood-core/hugelkulture style inspired by Sepp Holzer’s work in Austria, and brought to my attention by Paul Wheaton over at Permies.com., but I don’t want to get ahead of myself here. I’ll talk about those beds in more detail later on because I think they are integral to what I’ve been able to accomplish thus far with my little experiment (insert teaser here: gone from growing zone 6 to 8… with no heat inputs)
So for a little background information on what I was dealing with when I decided to tackle this project: the chosen building site is on the edge of one of our pastures, down hill from the main house. There is no shade in the area except for what is cast by a hill directly to the East. The ground slopes basically from the East to West with a slight cant towards the South (drops about six inches in ten feet). There’s about a foot of topsoil(sandy loam), with several feet of heavy clay subsoil underneath that. So all in all not to bad for raw materials.
The initial stages consisted of laying out the perimeter for the 16 by 24 foot hoop house, and then clearing out the sod. That’s right me and my boys were Sod Busters for a week or so! We did it the old school way to… all shovel work. I hadn’t gone back to the old place in CA to get the tractor yet so we had a bit of a workout on our hands.
The next step was creating the raised beds that would run along three sides of the interior. Once again me and the boys got busy with the shovels and started moving some earth. Initially we dug down about two feet deep and three feet wide. We dug a little wider than that on the North side to make room for that bed to extend beyond the perimeter to harvest moisture and berm up that side. The idea here was adding a lot of thermal mass on that North side to help keep some of the cold out and harvest the runoff from the roof for extra moisture to that bed. We also dug down another foot or so through the center of where all those future beds would be with the thought of holding even more moisture in the wood of the hugel beds. I guess at this point I might as well go into the details of how I created those wood core beds and what it is that really makes them sing in this application.
So the basic idea of the Hugelkulture or wood core bed (I’ll interchange the terms in this article) is soil on wood. That’s it, pretty simple huh!?!? Well yes and no, there seems to be quite a bit going on there, not the least of which is a powerful wicking of moisture from the ground up into the bed. Many people say that the wood acts as a sponge, and it does, but I believe the greatest benefit in moisture retention is in that wicking action. With that being said do you see why I chose to utilize something like a wood core bed in a greenhouse? In theory this type of grow bed should really help to alleviate the moisture spikes so prevalent in a raised bed/greenhouse combination. This has proven to be the case in two growing seasons in my greenhouse so far. Do you want to hear the exciting part? According to the “experts” a hugelkulture bed won’t even begin to see it’s greatest benefits until after three to five seasons… that’s right folks, it is just now reaching maturity! Last season I only watered those beds two to three times a week (which is already pretty ridiculous!) but as this thing starts to come into it’s own I hope to only water two or three times a month! How cool is that? You can find more in depth information about hugelkulture here: Richsoil.com.
So the basic construction of the bed goes as follows: layer of wood, layer of dirt/compost/hay whatever, then repeat as desired to get to the size bed you are trying to make, then top the whole thing off with soil/compost, plant your stuff, mulch, and then water it all. That’s it people… no rocket science involved here, just plenty of emulation of the systems God set in place for the creation of our food systems.
When I built these beds I went one layer (a pretty deep one) of rotten, and charred wood deep. On a quick side note I think it is preferable to have wood in various states of decay, as well as different species of wood in these beds, but really just use whatever it is that you have available. I used a bunch of old fir and pine fire wood, along with some wood from a slash pile on the property (most likely the same species of trees), and some rotten alder from the creek beds.
On top of the wood layer I put a nice thick layer of old chicken bedding (not composted) in direct contact with all of that wood. I did this to alleviate any possibility of all that wood (read carbon) sucking up all the nitrogen in the soil. Some people claim this is a problem, others state not. I tend to lean towards the latter, but I had the fresh chicken poo rich bedding so I used it. This addition should help the hugel bed mature faster as the wood continues to break down.
After the chicken bedding went a layer of all the sod the boys and I had taken out of the construction area. This went in grass side down to add more nitrogen to the mix deep in the bed. Once again, I don’t think this is critical for the function of the bed, but you want to make sure the grass is buried deep in the bed to keep it from sprouting where you are trying to grow your crops.
Following the sod layer went the top soil that was removed, along with some compost. That’s it wood core bed/hugelkulture complete. I obviously built vertical sides for this application, but you don’t have to do that to reap the benefits associated with the wood core beds. I have built several of these beds in conjunction with the remainder of our garden area without the vertical retaining walls. You just have a bed that looks like a mound rather than the traditional raised bed appearance.
OK, back to the greenhouse… To support the structure of the hoop I went down to the hardware store and picked up some galvanized fence posts. These are the kind that they use to make chain link fences, and it just so happens that for my high tunnel the hoops slide right into the inside of these posts. I drove the eight foot fence posts into the ground every four feet, got the transit level out, and cut the tops of the posts off level. From there I set the hoops, put in the support framing to hold the greenhouse film in place, and stretched the film.
With this design I am able to roll up the sides for ventilation in the hottest part of the summer (only need to do that for about a month here). This provides plenty of cooling to keep the plants from getting stressed. In the picture above I hadn’t yet put the end wall and door on, but that is in place now. On the end walls there are automatic vents that open without electricity(I believe they have some kind of wax that expands at a relatively low temperature and pushes a piston to open the vent). These work great and is all the venting that I usually need.
Now for the results so far…I have observed over two winters that the interior temperatures bottom out at about 14 degrees F. It only drops to 14 wether it is 10 degrees outside or -10! The lowest recorded temp I’ve seen so far is -14 F. That’s a negative folks, and it still stays 14 F inside the greenhouse! Don’t get me wrong that’s still plenty cold, but it is a sight warmer than -14. As with any greenhouse when the sun comes out it easily goes from 25F to 85 F in a relatively short time. For me the big deal is the 14 F… that puts me firmly in a zone 8 growing zone inside while being zone 6 outside. This is all accomplished with only one layer of greenhouse film and no outside sources of heat. It has been my understanding that a good rule of thumb is for each layer of glazing you apply to your greenhouse you should expect an increase of one zone. So with that being said this design has gained me two zones! I think there is plenty more I could do to optimize this system, and as I come up with tweaks I will update this page, but for now I’m looking at moving my starting bench and getting an extra hardy Arctic Frost Satsuma to plant in the open space through the center of the greenhouse. How freaking cool is that? My own fresh citrus at 4100 feet in Idaho with no heaters… give me a break!
I believe that the combination of the sunken walkway along with the decomposition in the raised beds is what is adding up keeping this area so much warmer than it would be otherwise. The mass of the Southern wall absorbing heat from the sun(that side of the bed has greenhouse film that rolls all the way down to ground level with a three inch air gap) in combination with the double thickness of the Northern wall losing less heat has got to be helping out as well. All in all it seems like a bunch of little things are adding up to pretty big results.
Aside from the temperature gains with this system I’ve noticed that the wood core beds retain water extremely well vs. a traditional bed or even planting directly in the ground in a greenhouse. I usually only have to water about once a week, then up to three times a week during our hottest month. This thing really is working well. The greenhouse application of hugelkulture may just be the perfect combination. My outdoor wood core beds do great. They heat up sooner, and provide interesting micro-climates for optimum growing conditions of different plants, but they do dry out fast up near the top and mulch is a bear to keep on them. The sheltered beds in the greenhouse just don’t have those problems, and even moisture is always a plus in your greenhouse. The one drawback that I have noticed with these hugel beds both inside and out is that there are a ton of mice, voles, etc. living in my beds right now. I didn’t notice this last year, so maybe it’s just a natural cycle. I sure hope so, otherwise I’m going to have a heck of a time with those vermin from here on out.
The first season I harvested my first ripe tomatoes since moving to Idaho! I’m not talking Early Girls here either, I grew pounds of Brandywine, Cherokee Purple, Goliath, you name it. It was awesome! All in all I think we got about 20 pounds of ripe tomatoes and about four times that amount in green tomatoes to can. In the second year I was busy building infrastructure for the garden in the spring so I didn’t bother starting any tomato plants that year. What I ended up with was about a dozen volunteer tomatoes along with several pumpkins, acorn squash, spaghetti squash, and a couple of butternuts. It was an absolute jungle in there. I did mange to get some watermelon, cucumbers, muskmelon, and chili peppers planted in there. All of them had respectable harvests. Like I said an absolute jungle! I wish I would have taken some pictures. If you are a person who likes an orderly garden it would have driven you nuts, but I couldn’t have been happier. There was just so much abundance with such a small amount of work. This year I’m planning on putting a lot more time and effort into my garden… I’ll keep you updated with the results so check back every so often.
Some updated pictures of the Greenhouse on May 1st 2015… still tucked in for the winter! You can see some of the integrated hugelkultur mounds extending beyond the greenhouse structure creating more microclimates and shedding cold air moving down slope. I even documented my first perennial holdover from last year… a nice little rosemary plant (zone 7)! I hope to add more perennials this season, maybe even that citrus I was talking about earlier.