For years, I have been hemming and hawing for years about whether to invest in a high tunnel. Our area has very high winds and I see more local tunnels with the plastic down than up. Between the expense and the maintenance, it just has not made sense for us. Last year, a video by Huw Richards made me sit up and pay attention. He had worked with Jack First to create a course on hot beds. After reading Jack’s (short, easy-to read, affordable!) book, I dove right in. Hot beds seemed like an alternative worth pursuing.
(tl;dr, hot beds are an incredible way to extend the gardening season but I did lose one war of attrition with a clever field mouse)
Why hot beds? Hot beds are a very old technology that were heavily used in French intensive gardening. The idea is to create a hot compost pile, put a growing medium on top of it, and trap heat in with “lights”. Essentially, it’s a cold frame over a compost pile. This essentially creates a heated micro-greenhouse that allows seeds to germinate outdoors mid-winter as well as protects young or cold-sensitive plants from freezing weather. By fall, the material has broken down into compost and can be used throughout the garden.
Hot beds offer a few additional advantages. In maritime climates, you can garden in late winter and spring when water is readily available. Even better, the covers protect the plants from excess spring rain, cold saturated soils, and nutrient leaching. Because it is too early in the season for many insects to hatch, there are minimal issues with pests. I grew the best turnip greens of my life in a hot bed! If someone was determined, they could grow and preserve a huge amount of food in early spring and then back off during the drought months of summer.
The Setup: There are a lot of ways to set up hot beds. The core requirements are
some kind of container deep enough to hold compost and planting medium,
“Lights”, i.e. glass or polycarbonate lids that can be closed, propped open, and removed
space between the lights and soil
These can be purpose built or cobbled together with found materials. Jack has great examples of using found materials on his Instagram page.
Our first year setup was a bit off the cuff and we have made some changes for Year 2. We made rectangles out of straw bales and put a 4X8’ raised bed with a cold frame lid on top. The goal was just to get some height without having to spend more money on wood. (Mistake #1: This would have been a lot more affordable if we had taken the time to find some old local hay bales.) The straw did act as a great insulator, though.
Historically, the compost heap was made with fresh manure and straw bedding.We filled them with fresh manure and sawdust from the stable my horse is boarded at. Filling two 4X8’ beds required two fairly full loads with a dump trailer. It was a nice way to help close the ecological loop on my sporthorse activities. Importantly, I have grown vegetables in manure from our facility previously. While our hay supplier could change their herbicide program without notice, we have not had issues with persistent herbicides.
Interesting both Jack First and Charles Dowding are a bit down on trying to make hotbeds with manure and shavings or sawdust. In most of the United States, the available carbon for bedding comes from sawmills; we rarely see straw used in stables. Manure and sawdust is what we had and I thought it worked nearly perfectly. It got hot fast and stayed warm and microbially active well into the summer. For people without ready access to manure, Jack’s book goes into other non-manure methods of creating an active pile, including ramial wood chips, wool, leaves, and even old clothes.
Plants cannot grow into hot compost. They need a suitable, cooler substrate. This becomes a bit of a choose-your-own-adventure game. We had just moved and did not have much to work with on the new property. Trying to figure things out quickly in mid-February, I just bought some bags of “raised bed mix”. (Mistake #2: this mix was too expensive, too woody, and did not want to hold water well. We got away with it but I do not recommend this approach.) This year, we had a pile of four-way topsoil for landscaping projects and I will used that. In theory, you can use the compost from the previous year’s hot bed as your substrate but I haven’t tried that yet. I think some combination of actual soil, compost, and potting mix would get decent results. In short, you need a good six to eight inches of something plants want to grow in. It’s a bit of an interplay between having enough hot compost, substrate, and room for the plants. When possible, a deeper substrate is better. Tip: Put in more compost than you think. The levels will rapidly drop.
Ok, so cold frames on straw bales, hot compost in, substrate on top, close the polycarbonate lids. Check. Now we must be patient. It got HOT. It hit 160 within two days and stayed there. We just started a new, smaller hotbed and it immediately shot to 140. This is so NOT the time to plant. Young seedlings can get excessively leggy and be negatively affected by off-gassing ammonia. The bed must be allowed to cool off a bit. I gave it a week and probably should have given it more.
When to plant: In the Pacific Northwest, our limiting factor is light. Even with a warm envirorment, there is still not enough light for plants to photosynthesize well. Planting too soon also risks the beds cooling off before the risk of freezing has passed. I started in mid-Feburary. By the time we had reached the spring Equinox, the plants were ready to leap into action.
Direct sow vs. transplants: Scratching the seed-starting itch, I had started some transplants indoors but this was unnecessary. One of the great advantages to the hot bed is the ability to direct sow and get high germination rates in February. Anything I direct-sowed grew much faster than my transplants that had to adjust to the heat and humidity.
From there, it was almost too easy. A little bit of organic fertilizer and just enough water to get seeds germinating and keep things happy and we were off to the races. When it was warm, we vented the lids. When it was freezing, we kept them closed. We grew hakurei turnips, radishes, carrots, bok choi, tatsoi, lettuce, spinach, arugula, kale, spring onions, cilantro, beets, and more. Two 4X8 beds were the picture of abundance and we actually overproduced. For fast-growing vegetables, like radishes, we got two full plantings and harvests before moving on to heat loving crops. We had multiple nights that dipped below freezing without a hint of cold affecting the beds.
In early May, a couple weeks before our last frost, I transplanted dwarf tomatoes and direct sowed cucumbers and butternut squash. They all loved the warm start to the season, being protected from rain, and the nutrient-rich compost. In a Juneuary year when people’s tomatoes weren’t ripening, I had ripe slicing tomatoes by the end of July. Here in the PNW, I have never gotten a butternut squash to truly ripen before fall. Close but no cigar. Last year, my butternut squash were so ripe they were practically fluorescent orange at harvest. One plant oversupplied us with squash for the winter. Cucumbers? Off two plants, we had more that we could eat. More than our neighbors could eat. The early start and bottom heat did wonders.
Once the season wound down, we let the compost sit protected under the lids until late January. The worms had already moved in and were transforming it to vermicast. While the sawdust could (and will) break down a bit further, it is a wonderful byproduct that I can use throughout the garden. (stacking functions, FTW!) I currently have it covered and am using it to charge biochar along with a homemade blend of organic fertilizer.
So, where’s the rub? There were two main challenges. First, while this can be done by hand (and that’s how it was done for most of intensive garden history), it is much, much easier if you have a tractor. We started the year without a tractor and, following an unrelated injury, finished with one. If you are moving dump trailer loads of hot compost, a tractor makes it attainable.
Second…and this was the real headache….we had a problem with a field mouse. A wily little rodent realized there was a spa with a sauna and fresh salad greens in the middle of a frozen field. Can you really blame it for moving in? The problem is the manure in the compost heap. Manure can contain pathogens that cannot be allowed on food. Under organic standards, composts containing manure must be either
turned five times and maintained between 131 and 170F for 15 days OR
Incorporated into the soil a minimum of 120 days before harvesting any produce that has contact with the soil surface OR
Incorporated into the soil 90 days before harvesting all other food crops
Because this compost pile is quite hot but is not being turned, there is going to be some gray area. Bottom line, we don’t want pathogens on food, period. So, when a field mouse started tunneling through my hot beds like Scrooge McDuck, it created a problem. Thankfully it had tunneled in a way that didn’t get manure on the produce and I was able to reset the bed. But if any manure had come in contact with food, it would have had to go straight into the compost bin and been a waste of effort. This where using sources other than manure could be safer, though it might not have the same protective heat during a hard freeze.
Part of the problem was undoubtedly my straw bales. The mouse could easily climb the bale and burrow into the bed. This year, I have built one solid bed and measures will be taken as we get the garden ready this spring. Meanwhile, I’m letting the coyotes have a go.
Given that Year 1 was generally a massive success, it means I’ll change everything in Year 2 and risk a sophomore slump. The first change is converting my two big hotbeds back to cold frames. I think certain cool season crops might actually prefer a slightly cooler environment. We also have more overwintering broccoli and leeks than we are going to be able to readily eat as well as plenty of preserved food. No need to work harder than necessary! I will plant those out in March.
We have built a new smaller hotbed, 4’X6’ made of two raised beds stacked on top of each other. My hope is that the solid sides will make it a bit less inviting for any mice than the straw bales. My plan is to roll the dice on getting some very early potatoes going and see if there is enough heat to protect them from the cold. I was also a huge fan of the early turnips so will dedicate a lot of space to them. Some early lettuce is always nice. After they are out, I’ll whack in a couple of dwarf tomatoes, a couple cucumbers and maybe a pumpkin. Meanwhile, I am dreaming of that first early radish!
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