French intensive gardening, also known as "double-dug raised bed gardening", is a
productive, sustainable method that has been around for a
very, very long time. While the French got credit, the Chinese have been gardening in deeply
cultivated raised beds like this for millennia - with no loss of fertility. (See Farmers of Forty Centuries by FH King)
Why is French Intensive Gardening Called "Double-Dug?"
What is meant by "double-dug"? It basically means that you dig down into the soil at two different levels, loosening it and incorporating compost. The first dig is when you loosen and actually remove (or move over) the top spade-depth's worth of soil (about 10 inches deep). The second dig is when you loosen the soil beneath that level with a digging fork, going down another 10 inches, and again incorporating compost. (See video demonstration above.)
By loosening the soil very deeply (18" or so) and incorporating a lot of compost, the soil gets fluffed up and raised above its surroundings by a foot or more. This fluffy, loose, deep and friable soil now allows plant roots to grow straight down, rather than going down a few inches, hitting a hardpan, and turning sideways where they compete with each other for water and nutrients.
Hardpan is a compacted layer of soil (a bit like concrete) that is actually created by rototilling or walking on the soil. It's counter-intuitive, but just below the depth of the tines on the rototiller, your weight, plus the weight of the machine, is compacting the soil. It's easier for plant roots to turn sideways than penetrate the hardpan.
I used French intensive gardening for many years because compared to wooden raised beds it is cheap, sustainable, preserves natural soil ecology, and if properly mineralized and the soil microbes are fed and happy, is capable of producing very nutrient-dense vegetables. (I no longer garden this way, but that will be explained farther down.)
Advantages of a French Intensive Garden
Uses less water, compost and fertilizer than many other methods of gardening.
Uses less space per given yield than many other methods of gardening.
Easy weeding. Because the soil is loose and friable, weeds are at a disadvantage, and can be more easily pulled up by their roots.
Works to benefit natural soil ecology, which in turn benefits us through more vitamin and mineral-rich vegetables.
Inexpensive. You do not need to purchase lumber, commercial potting soil, or a rototiller. It uses no gasoline and takes no trees or non-renewable peat.
French intensive gardening as described here is sustainable in the big picture. (Framed raised bed gardens rely on lumber and commercial soil mix which must be imported from off the land and be replaced periodically.)
Because of the depth of the soil bed, plants can be grown closer together than other methods, creating a "living mulch" that retains nutrients and water.
By the second year, French intensive gardening takes less labor, because soil ecology is optimized and earthworms and other soil organisms do much of the work for you.
Disadvantages of French Intensive Gardening
This method is a LOT of work the first year. Double-digging raised beds is hard work, especially in clay soils or if you have a lot of big rocks. (But if you water a day or two beforehand, it will make it easier, and things are easier in subsequent years because each year the soil is looser and more friable, and can be single-dug or broadforked relatively quickly.)
You must have the right tools to double dig. Ordinary hardware-store variety tools do not hold up to double digging. I've seen more than one fork handle snap. You need a forged, square garden spade and fork, available from Clarington Forge/Bulldog Tools. I’ve been using the same tools for over 35 years and they’re still going strong. (They used to have a cool video on their home page of how they're made, in a factory that's been cranking them out since 1789, in Lancashire, England - where my parents are from, but it seems to have disappeared now.)
After decades of gardening this way, I learned that while deep plant roots gather water, most of their nutrients come from the upper soil regions, so the argument that you can plant closer together because roots can go straight down for minerals is not accurate. At any rate, my worms and soil ecology now do all the hard work for me, and I give my plants much more room than I used to so they can spread out for optimal light and maximal nutrients.
The Worms Do the Work Now
After many years of broadforking in compost, worm castings, and actively-aerated compost teas, growing cover crops, and mulching with duck-manured straw, my soil is now so loose and friable that I don't have to fork it at all. I can stick my hand in the soil, and come up with earthworms. The work I did over the years has restored the land to its natural, undisturbed, healthy ecosystem, and it does the work for me now.
Don't give up! Nurture your soil macro- and micro-fauna, its microbes, and be gentle.
And here's a great read:
The Intelligent Gardener by Steve Solomon.
To learn to make framed raised beds, check out these related articles:
Raised Bed Garden Designs A gallery of 12 raised bed garden designs, with descriptions of construction techniques and materials
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