When you read about planting in mounds or hills, usually what the expert said applies to small mounds only a few inches tall. In a garden with good soil and a level site, small mounds warm up faster than the ground level soil and give some vegetables like beans and sweet potatoes a head start. There’s a major difference between planting a mound garden and planting a row of hills. Garden mounds are a major piece of work and don’t make sense for everyone who already has a garden that does well. You might want to try one as an experiment, even so.
In this garden spot, where several months out of the year the water level holds slightly above or below the surface of the ground, mounds have become something like the Aztec people built in pre-Columbian days. The Aztecs liked swamps. One of the stories about the founding of the Aztec Empire involves an attempt at misdirection by local people the first Aztecs encountered, near what is now Mexico City. The residents didn’t particularly want this band of intruders for neighbors, so when the Aztecs asked where a good place to settle might be, the local folks sent them to a swampy place full of dangerous poisonous snakes (story continued below the “How To”).
How To Make a Garden Mound
- Stake out an area about four feet wide and as long as you care to make it — about 15 feet is a handy size.
- If you start with sod, you can just build the mound on top without digging up the base area. Turning the sod over loosens things up a bit and I always do it, can’t stop myself. Turning it upside down a spadeful at a time works pretty well. Chop bigger clods into smallish clods with the spade.
- Lay tree limbs and other coarse debris on the base layer. Break things up so the limbs fit neatly and make a layer of branches (up to two inches in diameter) that covers the ground.
- Dig soil out from around the mound and turn the sod upside down on top of the branches. Chop the clumps up and even the dirt out.
- Add hay, grass clippings or tree leaves in a loose layer two or three inches thick. Mix tree leaves with other debris if you can, since tree leaves might not decompose quickly by themselves.
- Dig more dirt from around the mound and make another layer. Repeat as many times as necessary to bring the mound up to about a yard in height. As the debris decays and the dirt settles, a three foot mound compacts to about two feet, the minimum needed for good drainage.
- Mulch the top of the mound with hay and leaves and hold the mulch in place with dead limbs and tree trimmings. Leave the mulch and brush cover in place until planting time. Worms will have converted much of the buried debris to castings by the time you seed the mound with vegetables.
Instead of leaving for better digs, the Aztecs were overjoyed to find a place just like home. They were a nation of swamp farmers, and poisonous snakes were one of their favorite foods. The Aztecs would have been happy here, I think. Probably anyone from the old mound-builder culture would have settled into this little spot without complaints. If you visit places like Cahokia or Angel Mounds, you see a lot of the same swampy agricultural challenges. In most of these places only the big residential or ceremonial mounds remain and everything else got drained and plowed under, but in some parts of South America the Aztec mound fields survive. The fields cover so much coastal ground that archaeologists argue about whether or not they’re manmade. Having built a few small ones myself and experimented with growing food on them, there’s no doubt in my mind the Aztecs built those runway mounds. It’s an extremely practical way to grow crops in swampy ground.
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Essentially, what a swamp garden entails is two or three feet of good soil above swamp level. Vegetables need rich, airy soil at least two feet above the water table. If a hole dug two feet deep in your garden doesn’t drain dry in under 24 hours, you’ve got major drainage problems. Here, a hole never drains in the wet season, so I face the same problem the Aztecs did. What you have to do is pile dirt on top of dirt. You don’t have to truck it in, you can use what dirt you already have. What you wind up with is a network of mounds about a yard tall and as wide as you care to make them, separated by deeper channels. I’ve dug an outlet from the channel system so the rainwater and groundwater doesn’t rise above the bottom of the trenches, but I can also plug the outlet and fill the system, taking advantage of storms in drier times.
Technically there’s no need to plow the original site or turn the sod under, but I spaded the original ground up anywhere to speed things up. The Aztecs didn’t, whatever vegetation was in their field originally just got covered up with dirt. The wild layer dies and becomes good soil. Here I built mounds with sod stacked upside down and mixed with subsoil from the trenches. That’s not great soil for the first year of planting, but it did grow some good vegetables. Where I’ve worked the soil for more than one season, vegetables are doing much better. This summer my tomatoes and daikon planted on older mounds with better drainage did exceptionally well, and neither one prospers in wet heavy clay. In the real swamp garden on new mounds, turnips and beets prospered but beans and okra struggled. Things get gradually better, though. We had never managed to get onions to do anything but rot, but this year a mound produced a healthy crop of smaller red onion bulbs than you’d find in a grocery, and next year I’m expecting improvement.
This winter I’m rebuilding the old mounds and working in more organic material. At the bottom I start with branches and twigs from the yard, using everything I’ve trimmed from the trees and bushes that’s less than two inches in diameter. The bottom layer adds some drainage where it’s need the most, raises the mound level slightly, and sits deep enough that it won’t rob nitrogen from the soil the vegetables use. Over the top I put a layer of soil just a shovel full thick and chop it up with the spade so it settles in between the branches. Smaller trimmings and hay I cut from the yard with a kama sickle (martial arts weapons nearly all derived from farm tools and they still work for that) go into the next layer, and as the mound gets higher I work in finer material like grass clippings and leaves. Putting a layer of mulch over the top protects the mound from erosion and gives the worms a bit of shelter. This is very similar to the French Intensive gardening method, but several times the height and the labor. Someday maybe the dirt will be good enough that I won’t have to do this every year.
I like the mounds. They grow food. I’m having success with things that never grew here before, and I like the way the garden looks, sort of like an old Civil War trench system. The mounds put planting and weeding at about waist level instead of foot level, which is lots more convenient. This year, even though most of the mounds were new, I grew enough vegetables to supply myself with food from early spring onward, with enough canned produce to get me through the winter without depending on the grocery for much more than bulb onions and potatoes. Next year I’m hoping to get a good crop of those. It’s a good feeling to walk past the vegetables at Kroger and realize I don’t need those, because I have the equivalent already at home.
I do wish the swamp water level was a little more dependable, because I can’t lay straw in the canals and collect the insect rice that the Aztecs grew. Some little bug laid masses of eggs on the wet straw, all about the size and shape of rice grains. I hear they were delicious.