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Winter Got You Down? Curl Up in a French Hot Bed


You Bet Your Garden
Question of the Week © 2018 Mike McGrath

Note: Hundreds of Mike's informative articles are available (in alphabetical order!) right here at the Gardens Alive website. To find Mike's answers to your specific garden problem, Click here and find your topic (like Weeds, Worms, Rhododendrons...) in our complete alphabetical archive of Questions of the Week.

Winter Got You Down? Curl Up in a French Hot Bed

Q. Last year I had some success constructing cold frame boxes to keep my greens going into winter. I have read about hot bed gardening, saw that you mentioned it briefly in an archived question of the week, and was wondering if you have heard of any success using this method without the manure? I was hoping I could maybe accomplish the same results with leaf compost, but wondered if there might be some trick I could try to make it more successful.

----Victoria in East Greenbush, New York

A. Want to know how we choose the Questions we answer in depth every week? Its January. The official temperature in my little town was zero at 5 am this morning. My hands have been freezing since September. I have a burst water pipe somewhere inaccessible under my kitchen, which means that the main valve is temporarily shut off and I am flushing toilets with melted snow. (At least the power isn't out!) When we got this email, the words "French hot bed" just made me feel all warm and toasty….

So let the frozen minds among us warm up with the answer to the obvious question: "What exactly IS a "French hot bed"?

It's a clever technique that dates back to eighteenth century French gardeners who were too small-scale to afford a greenhouse but who wanted to grow lettuce all winter and get a head start on growing melons so that theirs would be the first to market in the Spring. I originally learned about the technique when I was the editor of ORGANIC GARDENING magazine (specifically in 1994) from a 1913 book called "Intensive Culture of Vegetables (French System)".

And Victoria is correct that manure is traditionally involved. Whether you were in England, Holland, France or those revolutionary "United States" across The Pond, manure—specifically horse manure—was plentiful and darned useful, especially in the winter when it was used to heat early greenhouses.

In many cases, big piles of it were simply placed in a central area of the structure, with the most frost-tender plants positioned close to it. Horse manure gives off a lot of heat as it breaks down, but was still useful even after it was fully composted and 'cool', when it was pressed into bricks that were then burned for more heat—maybe with some peat mixed in.

But not everyone could afford a greenhouse. They required a lot of physical labor to maintain, and were mostly the play-toys and prized possessions of the wealthy and privileged. Washington and Jefferson each had one. And captains of industry like Dupont and Nemours had several in which they would keep prized fruit trees and tropical plants alive and sometimes even producing over the winter.

The French even gave some of the earliest 'glass houses' plant-specific names: 'Pineries' were used to grow pineapples (not pine trees, which needed no such protection), and potted citrus groves were moved indoors into structures called 'orangeries' at the end of summer.

All of which is really neat. But didn't they smell bad inside?

Horses pulling endless carts and carriages through the streets; chamber pots emptied directly into gutters; no indoor plumbing; no real refrigeration--most of the world smelled bad in the 1700s.

Anyway, the much-smaller-scale 'hot bed' is about four feet wide by twelve feet long. The wooden frame that sits on top of the bed is about a foot high in the front and sloped to be slightly taller in the back. Its covered with two panels of glass side-by-side with a wooden divider in between (so you can work the plants on one side without making the other side cold).

Where's the manure? In a pit underneath.

As the weather started to get cool, 'hot-bedders' would excavate the soil underneath the bed-to-be to a depth of about two feet and replace it with fresh horse manure mixed with straw bedding. They would also typically add some aged manure to keep it from getting too hot. Then they'd position the cold frame on top, and cover the manure with several inches of good soil and/or compost to prevent direct contact between the manure and the plants that they would be growing.

Very important: Even cold frames without manure typically need to be vented on warm and sunny days to keep them from overheating. Hot beds even more so—especially early in the season when the manure would be really cooking and outdoor temps could still be warm. Back in the 'old days' gardeners could only manually raise the glass tops in the morning, prop them up with sticks and then lower them back down in the evening to avoid overheating. Today, gardeners can purchase automatic vents that raise and lower the glass for you based on the actual temperatures in the frame; a great investment.

Now, let's finally approach her actual poopy question

First, I think this is a great use for actual horse manure. If there are stables nearby, I'd seriously advocate for historical accuracy. Otherwise, shredded leaves alone would not be able to generate enough heat. And adding kitchen waste to the leaves would actually cool things down. But my classic 'hot compost' combination of shredded leaves and spent coffee grounds might work nearly as well as manure if you finesse the timing.

The heat from composting manure is self-generating and surprisingly long lasting. I would recommend getting the combination of shredded leaves and coffee grounds cooking for a day or two beforehand to make sure that burying them under some soil doesn't prevent their further composting from lack of airflow. Once they are cooking I think they'd do fine.

Or, if the area is near a grounded outlet, you can try a trick devised by the late Mel Bartholomew, who originated The Square Foot Garden method.

Yes, Mel made an electrified square foot hot bed; specifically, as part of a very special article he did for the February 1996 issue of ORGANIC GARDENING magazine featuring ten new "Square Foot" strategies.

To make his electric bed, Mel excavated a four-by-four-foot area about 18 inches deep, put down a layer of insulating plastic foam material, covered that with an inch of sand, then a greenhouse heating mat (these things are designed to handle dampness), then four or five inches of soil and compost.

Install some plants and a cold frame overtop and you've got some toasty roots. You could even run the mat on a timer to mostly stay on just at night.

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