Hugelkultur is a German gardening technique that involves building raised garden beds using decaying wood and other organic materials. It's a sustainable method that creates a self-sufficient and fertile growing environment for plants. The word "Hügilkultur" translates to mound culture or hill culture in English.

(From Wikipedia)

Hugelkultur

Hugelkultur is nothing more than making raised garden beds filled with rotten wood. This makes for raised garden beds loaded with organic material, nutrients, air pockets for the roots of what you plant, etc. As the years pass, the deep soil of your raised garden bed becomes incredibly rich and loaded with soil life. As the wood shrinks, it makes more tiny air pockets - so your hugelkultur becomes sort of self tilling. The first few years, the composting process will slightly warm your soil giving you a slightly longer growing season. The woody matter helps to keep nutrient excess from passing into the ground water - and then refeeding that to your garden plants later. Plus, by holding SO much water, hugelkultur could be part of a system for growing garden crops in the desert with no irrigation.

Caveats

I do think there are some considerations to keep in mind. For example, I don't think I would use cedar. Cedar lasts so long because it is loaded with natural pesticides/herbicides/anti-fungal/anti-microbial (remember, good soil has lots of fungal and microbial stuff). Not a good mix for tomatoes or melons, eh? Black locust, black cherry, black walnut? These woods have issues. Black locust won't rot - I think because it is so dense. Black walnut is very toxic to most plants, and cherry is toxic to animals, but it might be okay when it rots - but I wouldn't use it until I had done the research. Known excellent woods are: alders, apple, cottonwood, poplar, willow (dry) and birch. I suspect maples would be really good too, but am not certain. Super rotten wood is better than slightly aged wood. The best woods are even better when they have been cut the same day (this allows you to seed the wood with your choice of fungus - shitake mushrooms perhaps?).

Another thing to keep in mind is that wood is high in carbon and will consume nitrogen to do the compost thing. This could lock up the nitrogen and take it away from your growies. But well rotted wood doesn't do this so much. If the wood is far enough along, it may have already taken in sooooo much nitrogen, that it is now putting it out!

Pine and fir will have some levels of tanins in them, but I guess that most of that will be gone when the wood has been dead for a few years.

How to Build a Hügelkultur Bed

First, select a sunny spot that’s roughly 8 by 4 feet. (A bed built parallel to a slope is a good idea, as it will catch water.)

If the site has grass or is weedy, you’ll need to clear it down to bare soil. To suppress weed growth, mow and cover the area with cardboard or wood chips.

Now, dig out shallow pits, retaining the turf or topsoil for the tops of your mounds. Make the pit or trench 12 to 18 inches deep, keeping the same depth as the full length of the bed. Beds need to be narrow enough that you can reach the center; we suggest no more than 4 feet across.

Next, lay the woody material into the dug-out area, starting with large logs or downed trees. Add a layer of branches and twigs. A mix of hard- and softwoods is recommended. Avoid using woods that are slow to rot, such as locust, cedar, or redwood, or any that release toxins that inhibit plant growth, such as black walnut.

Like building a lasagna garden on top of the wood, top the layer with grass and grass clippings—nearly any kind of organic material—and pack firmly. If you have excavated turf, place it root-side up on the wood.

Continue to arrange the wood longitudinally and as tightly as possible. The pile can be as long and high as you like, but I suggest a 2- to 3-foot high bed as it’s easier to work with (and can last without water for 2 or 3 weeks). Some folks build them really tall, up to 5 or 6 feet high, but I would need heavy equipment to achieve that.

Then, water the layers well. “When it sprouts mushrooms, you know it’s wet enough,” says Tim Murphy, a gardener in Kingston, New York. Fill in any cracks or spaces with grass, leaf litter, and manure. “The tighter, the better,” he adds.

Finally, top off the bed with 2 to 3 inches of topsoil and a layer of mulch.

Maintaining the Hügelbed

If you build this in the fall, let the whole thing settle over the winter, and it will be ready for planting next spring.

In the first year, the pile will need watering as the wood breaks down. The rotting wood will also be using up nitrogen that would otherwise be going to your plants, so it’s recommended that you plant legumes the first year since they produce their own nitrogen.

Note that the greater the mass, the greater the water retention. Experienced hügel gardeners have found that if the beds are high enough, they don’t require irrigation at all after the second year. Steep beds also mean more surface area for planting; plus, their height makes harvesting easier.

Eventually, the rotting wood will hold water like a sponge, making the bed drought-resistant. The top of the bed will be naturally drier than the base, so you can plant things that need more water nearer the bottom and those that like it, which will be drier near the top. You can plant on the sides as well as on the top and bottom, increasing yields in a small garden.

A Living Sponge

In the first few years, the heat-producing composting process warms the soil in a hügelbed, providing a somewhat longer growing season. The decaying woody matter is a source of long-term, slow-release nutrients and helps to keep excess nutrients from filtering into groundwater.

The wood, acting like a sponge, stores rainwater to release during drier times. Hügelbed soil is self-tilling over time. As woody material breaks down, tiny air pockets open in the crumbling soil, allowing air to reach plant roots. In time, you can plant into the topmost layer of soil/compost, which becomes rich with beneficial microorganisms.

First-year hügelbeds can be big producers. Murphy reports a harvest of 120 pounds of cucumbers and 42 good-size pumpkins, as well as giant sunflowers, from two first-year beds. Murphy looks beyond the first few years, though: “These are serious, permanent raised beds. What you are building is a living, breathing sponge.”

The Benefits of the Hügelkultur Method

The rotting wood hosts beneficial fungi, bacteria, insects, worms, and microbial growth that create nutrients your plants can use. Over time, the mound will shrink as the wood rots, but you can always add more soil or compost to the top. You will have created an ecosystem in which the beneficial organisms will thrive.

Hugelkultur is popular with gardeners who have struggled with heavy clay and poor or compacted soil. It is a good way to build up a planting bed and turn woody debris into a garden.

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