Say adieu to winter with a hotbed in the garden


Above-ground beds can be built up, then covered to capture the heat of the decomposing manure base. (Charles Herve-Gruyère)
March 12

The word is common enough: a hotbed of revolutionary zeal, a hotbed of culinary exploration, a hotbed of whatever is currently being fostered, nurtured and spread around. But its true origin is known only to gardeners with an eye to the past. A hotbed is a garden bed in which manure is collected and enclosed, then topped with about five inches of soil. The heat from the manure’s decomposition warms the soil for plants growing in cool weather.

There are various traditional styles of hotbed-building, of which the most elaborate was to dig a pit below the frost line with masonry walls and elaborate drains. But in essence a hotbed is just a heat source under a cold frame — which is a bottomless, glass-lidded, soil-filled box. The modern method is to bury an electric cable in the soil. But electricity is expensive, and the old manure trick is simple and more natural.

My friend Charles Herve-Gruyère, a permaculture farmer who raises vegetables in Normandy at his beautiful Ferme Biologique du Bec Hellouin, sent me photos of his hotbeds. Like me, Charles has rocky soil, so he chose to not dig down at all, but start at soil level. That made for better drainage instantly.

Inspired by an 1845 book on vegetable cultivation, Charles mixed fresh horse manure with horse manure composted at least one month to get a slow, even heat of decomposition. The horses had been bedded on straw, which is the best carbon-rich material to mix with horse manure when composting. (A nearby riding club was delighted to have Charles haul away its stall litter, and it’s worth seeking out stables and farms that will do the same.)

He stacked the material in long rectangular beds, tamping and watering as he went. He placed wood-frame boxes on top and filled them with soil. Instead of installing glass covers for the frames, he used wire hoops to support plastic film. Strings that crisscrossed these tunnels allowed the plastic to be pushed up or down and secured, to regulate the heat.

Charles built his hotbeds in mid-January in a winter climate not unlike Washington’s. The crops he sowed or transplanted were soon ready for harvest, with pea shoots, baby chard and baby kale coming in within three weeks. Radishes, turnips, carrots, beets, chervil and kohlrabi soon followed. Come spring, he’ll follow the cold-weather crops with summer ones such as tomatoes, which will sink their roots deeply into the beds’ rich substrate.

Building a hotbed transforms the cold frame on top into a little heated greenhouse, just by harnessing the work of bacteria. It gives you natural, compost-powered heat. There is some art to the process: If your bed is too hot or the soil layer too thin, you can cook your plant roots. It will be warmer at the start and then slowly cool down. Because the winter weather is colder earlier and then gets milder, the two systems are in tune. Best to let the manure underneath cool to 120 degrees before sowing or transplanting seedlings.

Does the manure smell bad? Only briefly, when you’re piling it on fresh. After that, it’s just composting beneath the soil, thus providing heat for green leaves and an early taste of spring.

Damrosch’s latest book is “The Four Season Farm Gardener’s Cookbook.”

Tip of the week

Winter jasmine, often confused with forsythia, blooms on and off through the winter but will finish with a floral flourish in early April. Old thick mounds can be rejuvenated by a hard prune after flowering. Use lopping shears to cut them back to just a few inches above the ground. Vigorous fresh growth will return, along with next winter’s flowers. Stems that have tip-rooted can be dug and potted up and given away.

— Adrian Higgins

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