Season extension?

edited November 2013 in Horticultural Crops
Anyone out there doing season extension in rigorous climates? I'm in Alaska, have been trying it on a small scale to figure out what we can do effectively once we are on our land full time in spring of 2015. I am very interested in learning what others are doing.


  • I manage an educational hoop house in Michigan's upper peninsula, where our average snowfall is 200 inches and many areas never go fewer than 30 days without frost, though we're no Alaska. With gothic-style hoop houses, internal support for snow loads, and another layer of interior protection (frost blanket over the rows inside the hoop), we're able to produce March - December.

    This year, we're working with putting compost piles inside the hoop to maintain higher internal temperatures, mitigate the nighttime temperature dips, and also to provide workable soil that's easily accessible for the first round of planting in late February/early March. Growing Power in Milwaukee uses a high-carbon compost pile in each corner of the structure for this purpose. It's high-carbon (wood chips, leaves, etc.) to minimize methane off gassing. We're using a moderately nitrogen-heavy mix and have been testing for methane and CO2, but have found that we are not exceeding safe levels with a pile as warm as 130-150 F.
  • Hi Louise,

    I work on NCAT's small-scale demonstration farm in Butte, Montana, where I'm experimenting with growing beyond our typical 75-85 frost-free days. I've found that MFC's suggestion of doubling up layers of protection works really well-- sometimes even tripling the layers, being careful to pay attention to light availability, and, on sunny days, ventilation. I love the idea of compost heating, but haven't had the resources to try it yet.

    I've also found that utilizing thermal mass to take advantage of the sun's energy after the sun is gone is very effective (though depending on how far north in Alaska you are, it might not work all winter). Milk jugs painted black, filled with water, and placed under low tunnels covering greenhouse and hoop house plants was an effective low-cost method. I was also able to grow cold-tolerant crops (eg lettuce, onions, herbs) all through the winter in an unheated greenhouse attached to the south side of our building, since the building's mass moderated the greenhouse's temperatures.

    Additionally, there's a traditional underground greenhouse technique called a walipini, which uses the constant temperature of the earth to keep the air relatively warm. I haven't tried it myself, and it looks pretty labor-intensive to construct, but it seems to be inexpensive in terms of materials costs and might be worth checking out-- the Benson Institute has a good overview of its principles and design and construction considerations: . Perpetual Green Gardens in Bozeman, MT, have built a somewhat fancier version of this to grow tropical fruits, using local geothermal as an additional heat source: for a brief explanation.

    Lastly, if you haven't seen them yet, ATTRA has a number of publications on season extension that have been helpful in my experiments; "Sustainable Season Extension: Considerations for Design," "Season Extension Techniques for Market Gardeners," and "Compost Heated Greenhouses" come to mind. All are available on the main website, and the latter two are free to download.

    -Zoe Carlberg, ATTRA specialist
  • Thanks, Zoe. I am delighted to learn about Perpetual Green Gardens. I see a road trip coming up. The Benson Institute site is definitely worth viewing.
    Consider the ability of Inca farmers to feed a relatively large population, at high altitude, and quite far south latitude. Anyone know good reference material about Incan farming techniques?
  • from what i have been reading over the years, composting for greenhouse heating is not a viable method. while a compost pile does produce heat, most is deep inside the pile. generally you end up with about a foot of insulative cover on a compost pile. hence the need for a long stem compost thermometer. if you extract too much heat either by turning or using heat pipes you can easily shut down the composting process and stop making heat. all the figures i have seen over the years indicate the continuous heat output of compost is very limited. some of the highest numbers i have come across are in the area of 5000btu per hour from a metric ton of composting manure. but, this was mechanically agitated and aerated and it only lasted for 21 days. another project came up with just shy of 1000 btu per hour from a ton of composting wood chips for 6 months. non of the info i have read has been from small 1 ton compost piles, these numbers were adjusted to per ton for ease of comparison. so to do a simple comparison. the max heat you can get out of an electric heater plugged into a standard US wall outlet is around 5000 btu per hour. so roughly equivalent to 1 ton of rapidly composting (21 day cycle) well managed manure or 5 tons of slower composting (6month cycle) wood chips. now for the bad news. here is a simple calculator to determine the required btu per hour to keep a room heated so many degrees above ambient temp. this link shows a 30x100 green house heated only 10 degrees (f) above ambient this will be a low estimate of the need since a green house is nowhere near as well insulated as a poorly insulated "room" since even the best green house is ALL windows. if you have not looked at the link, it shows about 50000 btu per hour to keep it only 10 degrees warmer. that is a lot of compost. the conclusions from this study published by attra here suggest the need for 13.5 cubic feet of compost to heat 1 square foot of greenhouse space. so think along the lines of filling your greenhouse 13.5 feet deep in compost and you would have a compost heated green house. hope you have a tall greenhouse or a basement under it. they also pointed out you would then also be creating 6 times the optimal CO2 and 50 times the optimal nitrogen as ammonia. this does not sound like a feasible method to put in place for greenhouse heating. even 1/6 of that for optimal co2 is a green house filled over 2 feet deep in active compost. now i can see if you have massive amounts of waste that you are already composting there is a possibility you could use the heat and some of the co2 to your benefit in a greenhouse. but i am not even sure the cost of the infrastructure to move the heat and co2 from the compost to the green house would be competitive with other means such as solar or ground loop heating

    food for thought
  • Hi Kenny,

    Thank you for this clear analysis of the actual effects of modifying from the "straight up Eliot Coleman model" of season extension. Another concept that I see thrown around a lot is the "Walapini", burying the structure below grade so some extent, with the idea that there is some kind of heat pump of sorts down there below grade ready to warm the structure for free. My sense is that if we were only talking about cutting in to south facing slopes, there might be some kind of advantage to be found. But that basically cutting into the grade is simply lowering the grade, no advantage. And that anything that cut off even the slightest sun angles in northern climates in winter would be a high price to pay for any possible gain.

    I am mainly looking at all of these modifications from an opportunity cost lens. In other words, if it costs the same amount of money to build a second passive structure as to modify the first one to something that needs active management and inputs, what are the opportunity costs in each scenario. And since spinach and brassica salad mix and kale are pretty happy in the cold, and return about the best one can imagine right now at the market, why modify a structure to grow less cold hardy plants that might not even fetch the same prices. That, and we have not even gotten into the fact that light seems to be at least equal to heat in the winter equation.

    This winter has certainly been more challenging than any in recent memory to growers here in SE Michigan. But I have seen many happy kale, spinach, parsley, root crops, cilantro, salad mix and other plants tucked under a single layer of remay in single and double layer hoophouses of 3000 sf or more (even a bit of lettuce when under poly row covers). My recent design efforts have been on exploring ways to reduce the amount of steel as a percentage of the square footage of covered space, and designing mobile on a human scale that does not require equipment or other great efforts to move structures. I think we will need a lot more poly, and smarter systems to make some next leaps into the hand-in-hand issues of lowering production cost and growing to "medium-scale".

    I think we will ultimately find examples of heat and light modifications that make sense, especially if it results in a bit more reliable production during the deepest of winter months, but I have yet to see one that looked like it would be better than an additional dollar spent on more hoophouses. Seems that an animal/vegetable cohabitation might be in order. Anything that you have come across? Jeff McCabe - Nifty Hoops
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