Hügelkultur, "mound culture" in German, is, simply put, a raised garden bed. The centuries-old technique uses wood, leaves, and other landscaping debris that would otherwise end up in a landfill to build a nutrient-rich mound for gardening. The materials are layered and piled high, creating a tunnel-shaped hill that provides years of nutrients for itself through sustainable, natural processes. There's more than one way to build a hügel bed, and each method yields the fertile earth necessary to suit your needs.
Any biomass that can go into a compost heap is good enough for Hügelkultur. That includes logs, tree branches, wood chips, grass clippings, chicken manure, eggshells, coffee grounds, and cardboard. Logs decompose much more quickly in soil, so you'll need enough dirt to fill in the gaps. Be wary of using redwood or cedar, which can take a very long time to decompose, and allelopathic trees like black walnut, which inhibit new plant growth. Treated wood and grass clippings from treated lawns are also off-limits, as are dog and cat waste.
A hügel bed starts with the largest-sized logs and branches at the bottom. Fill in gaps with leaves, manure, and other biomass materials, repeating the layers as if you're building a lasagne. Water each layer and continue piling until the mound is the desired height. Keep in mind that your hügel bed will shrink over the years as the material decays. Finally, cover the heap with topsoil and mulch.
Hügelkultur is a type of permaculture. The technique mimics the earth's natural processes to sustainably increase productivity and efficiency. Hügel beds recreate the nutrient cycling that takes place when fallen branches and other organic materials accumulate on the forest floor. The soil becomes more enriched as the debris decomposes over time. It also prevents harmful greenhouse gases from entering the atmosphere with a process called carbon sequestration.
Where to build your hügel bed depends on the space you have available and what plants you plan on cultivating. Hügelkultur beds do well when they are about three feet tall, though building a smaller mound will not produce a weaker bed. If you're digging a trench, choose an area of yard roughly 6 by 3 feet to accommodate the fully assembled mound.
The real question is: how long will your garden take to fully mature once it's planted? That all depends on how you build your mound. A large bed is capable of supplying nutrients for your garden for more than 20 years if the base is comprised of only hardwoods. You can add compost to your raised bed, but hügelkultur creates an ecosystem within itself that can persevere for years.
Hügel beds support a thriving population of beneficial creatures like worms, insects, and microbes. The developing ecosystem breaks down biomass efficiently, helping gardeners manage heavy clay or compacted soils with ease. The decomposing materials also hold water like a sponge, which may eliminate the need to water your garden regularly. Expect the mound to shift over the years as wood and debris decompose at different rates. This means your hügel bed is also a no-till garden.
As the wood rots and breaks down in a hügel bed, it plays host to beneficial fungi as well as bacteria usually found in compost heaps. Decomposing material pulls a lot of nitrogen from the soil. Unless you plan on leaving your mound to "cure" for some time, you'll have to add compost to start planting right away. Choose plants that don't require nitrogen-rich soils during the first year. The soil will eventually heat up like a compost heap, possibly extending your growing season.
Which type of hügel bed is right for your garden? The traditional hügel bed requires a trench about one foot deep to start. Some gardeners build narrow mounds from there as high as six feet, which reduces compression on the material at the base of the pile. Tall heaps also increase surface area for gardening, an excellent option for large beds and fruit-bearing trees. Less ambitious gardens can get away with a shallow trench and a shorter mound but benefit from starting with nitrogen-rich composting material.
If you have a pile of dirt that needs to disappear, skip the shovels and build your hügel bed on the flat ground. You may have to prepare the area by leveling the soil and removing any debris that doesn't benefit the hügel bed. If your lawn is still intact, remove the layers of sod and set them aside as you heap the logs and twigs on the ground. Fill in the mound with surplus dirt and top with a layer of upside-down sod and mulch to finish.
An enclosed, raised hügel bed is perfect for properties with limited yard space, or to complete a finished look. Build a small enclosure with wood for a modest backyard vegetable garden. If you have a recently felled tree to dispose of, consider building a taller structure with corrugated metal or other scrap materials. Straw bales provide a rustic aesthetic as a hügel bed border, as do fresh logs.