Deep Dive: From Hot Beds to Hot Dams
New research by Austria's agricultural experiment station Zinsenhof on the age-old method of generating heat from manure: How it works and is it worth it for larger home gardens or market gardeners?
How can early tomatoes and cucumbers be harvested in a four-season climate without the expensive and environmentally unsound practice of large greenhouses heated by fossil fuels? This question has been driving trials at Austria’s agricultural experiment station Zinsenhof since 2010. In March, beds in unheated polytunnels become hot “dams” or mounds to get a head start on the season. Their work seems to have potential for “low-energy” vegetable production in northern market gardens.
What is a hot bed?
Anyone who has passed by old-fashioned piles of fresh manure has observed steam rising up from them. You can literally see the heat production and at some point long ago people started to capture that heat to extend the growing season. The principle is simple: microbes break down organic material and generate heat in the process. In recent years, hot beds have become popular again thanks to older and younger gardeners alike, most notably influencers Charles Dowding and Huw Richards in England. I highly recommend this excellent article by Brett Gallagher, a market gardener in the Czech Republic, describing how they heat their greenhouse in spring with manure. My focus here is on a commercial application of creating hot dams for growing tomatoes, peppers and cucumbers 6-8 weeks earlier with the dam or hilling technique.
Hot beds are also still used in Austria by some “old-timers” such as Alex Wanas, who heads “Grün Reich” (a play on words, meaning “green rich/realm”), an education facility for apprentices in horticulture and floristry. They use 1x1 m (roughly 3x3 ft.) compost bins as hot beds.
Fresh horse manure is layered with garden soil. The material is tamped down several times during the process to ensure it is packed firmly. Or, as Brett Gallagher describes it:
I dance on the turds like I’m making wine to pack it down, especially in the corners.
Too much compaction is not ideal, however, because microbes need oxygen to do their work. Once full, the bed is watered thoroughly. Within days, the temperature rises to 60°C/140°F in the center of the bed, emanating enough warmth to act as a heat mat or floor heating for anything on the top as well as raising the ambient temperature of the polytunnel.
During the first two weeks, the hot bed (and hot dams) emits ammonia that can damage seedlings, so it is wise to wait with sowing, planting or setting seedlings on top. After that first phase of high heat, the bed can be planted or used as a natural, zero-energy heat mat for transplants, which is how Charles Dowding uses it. An equally important advantage is having nutrient-rich composted manure when the decomposition process is complete.
Hot bed technology in commercial production
Hot beds were used until the 1950s in commercial vegetable growing as well as home gardens. In the post-war era, large-scale greenhouse production overtook the traditional, labor-intensive hot beds and cold frames spanning across fields. At about the same time, transportation technology, including refrigerated trucking and air freight, helped to centralize production. Energy-intensive greenhouses soon took over, concentrating commercial vegetable growing in regions with amicable climates. In Europe, that is Spain and the Netherlands; in the USA, it is California. Warm-weather crops were soon available year-round, wherever you lived.
Those winter tomatoes in heated greenhouses have a price, however. Up to one-third of the price tag at the supermarket covers the cost of fossil fuels. Hidden costs, but nonetheless costly, are the CO2 emissions from those fossil fuels. It is estimated that winter glasshouse tomatoes cost ten times more in CO2 emissions, and most of that is due to heating, not transportation.1 (We do not eat fresh tomatoes in winter, because they taste horrible, but that is another story.)
Apparently, consumers were and are willing to pay the price for tomatoes in winter. Because labor-intensive but energy-extensive hot beds are extraordinarily energy efficient and require minimal resources, the practice remains alive in some home gardens. Cold frames fit in wonderfully with today’s “upcycling” trend: a wooden frame can easily be built out of pallets and an old window repurposed for the cover. For home gardeners willing to make the effort, cold frames are relatively affordable and can easily be used as hot beds in the spring. My cold frame is multipurpose, serving as a cold frame in spring, a tomato bed in summer and a salad bowl in winter. It is far superior to my polytunnels in warming the soil on cold winter days and protects frost-hardy greens very well.
Back in the day, both warm and lukewarm frames were used. Warm hot beds were filled with 60 cm/24 in. of packed horse manure, whereas lukewarm frames had a thinner layer of 30 cm/12 in. of fresh manure or other organic material such as composted manure, leaves or kitchen waste. Once the organic material had warmed up (usually 3-4 days) a layer of garden soil was spread on top (20 cm/8 in.). The frames were protected by glass windows. Straw mats or carpets provided additional protection on nights below the freezing point. In the morning, the cover was removed and the windows propped up to allow for air circulation and temperature moderation.
Obviously, that meant a lot of labor in commercial production, but once the infrastructure was in place, cold frames could be used as hot beds year after year. Lettuce, spinach, radish, carrots could be sown in January, and as of March the beds would be planted with tomatoes and other vegetables worth “forcing” (an early term for season extension). As you can see in the historical photos below, the frames were usually facing south-southeast at an angle of 15-30 degrees to make the most of the winter sun - still best practice today.
The Hot Dam: old technology in a modern context
(One could say, “Hot damn, a hot dam!” but I shall refrain.)
Over the last 14 years, researchers at Zinsenhof have developed and continuously improved a low-energy system for planting out tomatoes in early March in unheated tunnels. The agriculturaly experiment station is located in Lower Austria, about an hour west of Vienna, in a four-season climate with low temperatures comparable to USDA Zone 7a/6b. They use a hilling method (Dammkultur) - also used in field growing and some market gardens in Europe - to create hot dams the length of the tunnel.2
There are two basic ways to create hot dams: on the surface or in a trench. The surface mounds are simply built up with organic material on top of the soil, 80 cm / 32 in. wide and 60 cm / 24 in. high. The trench method requires removing 15-20 cm / 6-8 in. of soil for trenches some 80 cm / 32 in. wide. The trenches are then mounded up to a height of 60 cm / 24 in. with organic material, including manure, straw, wood shavings, compost, and amendments such as horn powder and lime granulate. The soil is replaced on top of the hills and the tomatoes are planted on both sides of the mound at the base. Three rows of lettuce are planted on the mound to maximize the bed space.
I should note here that the transplants were all grown in a glasshouse on heat mats and under artificial light, so energy use is still in play. Tomatoes and peppers are sown in December, lettuce in January and cucumbers in February.
Zinsenhof’s experiments have shown that surface mounds - those made on top of the soil without trenching - produce more heat, probably because the entire surface area of the mound is above ground and improves access to oxygen. In general, the temperature inside the mounds quickly reaches up to 55°C / 130°F, but the root zone of the plants remains a comfortable 17- 20°C / 62-68°F, perfect for growth. A further advantage is the ambient heating from the bottom up: Although one March was especially cold, with night temperatures well below zero, the plants survived thanks to the warmth emanating from the dams.
In Zone 7a, where planting out tomatoes in unheated tunnels usually happens in May, hot dam techology makes it possible to plant tomatoes, peppers and cucumbers 6-8 weeks earlier than in an unheated greenhouse. First harvests are also earlier, obviously: cucumbers and tomatoes by mid-May, peppers by mid-June.3 For consumers accustomed to year-round tomatoes and cucumbers on supermarket shelves, this might not seem significant. For the growing number of market gardeners producing local vegetables by hand, it could be a revolution, giving them a slight bit more competitiveness in a world dominated by centralized, year-round production of summer crops in heated glasshouses.
Over the years, the researchers have experimented with a wide range of material, including a variety of fresh manures (horse, sheep, chicken, cow) and organic material, from straw to hemp seed press cake or meal. One key learning: just like hot beds, the mounds must have a critical mass (at least 40 cm / 16 in. wide) to ensure sufficient heat is generated. As with composting, nitrogen-rich material is essential and the dams must be watered well to activate microbial activity and generate heat.
Zinsenhof always tries new mixes of organic material, with the goal of consistent warmth over several weeks. Their method comes closer to composting than hot beds, because there is relatively little fresh manure involved. While there is no one “recipe” they use, here is one example. Note that it includes surprisingly little manure and several amendments:
3 cubic meters shrub chippings
1 cubic meter wheat straw
1 cubic meter wood shavings
1 cubic meter sheep and pig manure (50:50)
0.5 cubic meter compost
25 kg horn chips
25 kg horn powder
6.5 kg lime granulate
This batch was for a trench or mound of 14 m / 45 ft., 80 cm / 32 in. wide. One cubic meter equals 1.3 cubic yards, one kg equals 2.2 pounds.
The researchers note that all manures work well and they have used horse and cow manure as well. Obviously, a positive side effect of using hot beds or dams is that the organic material left over after decomposition is nutrient-rich compost. In addition, the process of decomposition produces CO2, which plants need to thrive. (Large glasshouses frequently pump in CO2 to accelerate growth.) A key point: the tunnels must be well-ventilated to ensure good air circulation; the humid environment can cause fungal issues.
What I’m thinking about
Context is everything, as I’ve noted before, so the question is: Do hot beds or hot dams make sense for me?
If I were going to use a hot bed in my small demonstration garden, it would be in the cold frame, a 6x1 m / 20x3 ft. wooden frame. It isn’t very high, so I would probably pack in horse manure and then raise the frame sufficiently off the ground to add soil on top and help with making oxygen available for decomposition. Then I would let it cook for a couple of weeks before planting into it. This would be at least a solid day’s work - picking up 1.2 cubic meters of horse manure from friends, spreading it, packing it, lifting the frame, topping off with soil and watering in. It would cost very little beyond the hard labor and give me a no-cost heating mat for transplants and a head start with tomatoes.
If I wanted or needed that, which I actually don’t.
What I’m more seriously thinking about is creating hot dams in one of my hoop houses (polytunnels). A colleague has implemented Zinsenhof’s hot dams in hers and is very satisfied with the results. She did indeed start picking tomatoes and cucumbers in May in Bavaria, which is impressive. Thanks to the dairy farm she lives on, she has easy access to manure and straw.
In my case, I would have to source fresh manure because we have no livestock. My friends with horses compost the manure throughout the year in large batches, but I would also need fresh manure. I have spent hay that I could mix in, but I would have to source wood shavings or spend a day cutting back and chipping shrubs (a task on the to-do list anyway). I would also add alfalfa meal and lime granulate to my mix.
Based on Zinsenhof’s work, here is my recipe for a single 15x1 m / 50x3 ft. bed in my tunnel:
1 cubic meter shrub chippings or wood shavings
3 cubic meters spent hay
1 cubic meter fresh horse manure
1 cubic meter composted horse manure
6 kg alfalfa meal (€50)
25 kg horn chips (€50)
6.5 kg lime granulate (€15)
Note: I use Euros because that is my currency. The exchange rate to US dollars is nearly 1:1 as I write this.
The amendments make sense in any case, although the amounts used for one bed are unusually high in these recipes, but the other important cost is labor. This project would add at least 2 days of (hard) work to collect and transport materials and another day or two to move, spread and mix it. Expenses would total €115, plus a gift for my horse-loving friends and gas money.
Would it be worth it to start tomatoes, peppers and cucumbers 8 weeks earlier? Perhaps, but in my case, the main value would be in the nutrient-rich soil that would result in the process. One dam would be sufficient for keeping flats of transplants frost-free, and, once completely decomposed, enough to provide nutrient-rich organic material for the other two beds in the tunnel. Distributing it to the other two beds would mean another half day of work.
The extra two days of labor to essentially make compost on-site would indeed save money. In my region, a minimum order of carbon-rich, organic compost is 5 tons (6.25 cubic meters) at €100 per ton, plus delivery costs to total €650. On the other hand, I would “only” need 4.5 cubic meters for all three beds in the tunnel, so that €650 also includes enough material to top off beds elsewhere. The actual cost of compost for the one tunnel is €468, plus the amendments - without the added bonus of planting earlier.
Because the amendments make sense with or without the question of hot beds, I’m going to leave that expense out of the equation and just focus on the classic question: Make or buy? Are the extra two days of labor worth it to get a head start and save €468 on compost? Two days works out to an hourly rate of €23.40, which in market gardening isn’t bad, but it also isn’t great. If the actual work took longer (highly likely), it would quickly become slave labor. That said, for smaller market gardeners like me with modest budgets or with smaller sales volumes overall, it might be a good business strategy.
There is yet another cost factor in the hot dam method: the transplants. In order to grow strong tomato and pepper plants during the darkest days of the year to transplant in March, energy costs for artificial light and heat mats have to be included, as well as the extra labor for growing the transplants in less than optimal conditions. Further, if this equipment isn't already available, it would have to be purchased.
In sales terms, it means selling 46.8 kg / 1,031 lbs. of tomatoes to recover the expenses and extra labor. For one bed, that is equivalent to four weeks in high season. For market gardeners with enough time and access to materials, it might pay off, especially since it means becoming more competitive with greenhouse growers and bringing cucumbers and tomatoes 6-8 weeks earlier than usual to market, while also producing nutrient-rich compost. However, based on my yields, the sales from those first weeks gained by early harvest would all go toward recuping the expenses. The costs of growing transplants starting in December aren’t even included here.
That said, Zinsenhof trials also include lettuce on top of the mounds with harvest end of April. At a price of €2 per head, that is a turnover of €270, which might be welcome extra revenue for some. In my case, it is not, because I sow lettuce in October for overwintering and harvest heads at the end of April anyway.
What appears to make economic sense (and does, if compared to fuel-heated greenhouses) might not be the greatest idea after all. Small year-round growers like me usually have their tunnels in production year-round. In March, mine are full with lettuce, spinach, Asian greens, purple sprouting broccoli and a myriad of greens for salad mix. At most, I would be willing to sacrifice one bed per tunnel to create hot dams for keeping transplants warm and producing nutrient-rich compost, but then I would also be putting the other crops at risk due to the ammonia emissions. The hot dam technique would require me to completely change my planting schedules and sacrifice one whole tunnel for starting summer veg early. If I didn’t love winter veg so much, I probably would consider it.
For home gardeners with large gardens, a traditional hot bed is still a smart strategy, especially for growing frost-sensitive transplants. Those with complicated set-ups with artificial light in dark corners of their homes could reduce their energy consumption and have stronger plants.
Over the weekend, a special case arose that makes me want to do at least a hot bed: A friend, whose garden mysteries have become bestsellers in the German-speaking world, needs zucchini in May. Her first mystery involves a Hugelbeet (Hugelkultur), a corpse and a bumper crop of zucchini, but the filming takes place two months before zucchini has reached its high season here. I might give it a go…
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