Hugelkultur is a permaculture design method where compostable organic matter is used to create a raised bed. This technique goes back hundreds of years in Germany and eastern Europe. Recently, Sepp Holzer has done a lot of research and made some modern improvements on the old method.
Because of a hugelkultur bed's ability to conserve water and enrich the soil, some people have referred to hugelkutur beds as the ultimate raised bed design. If you want to learn more about it, Paul Wheaton's Hugelkultur micro-documentary contains a lot of helpful instruction.
Some people have reported immediate returns on hugelkultur beds, but it is generally agreed that the full effects of the bed may not be apparent for three to five years after creation and planting.
The natural composting process that takes place on the forest floor is the basis for hugelkultur. These beds consolidate this natural process in a confined area, making the process more intense and focusing its effects on a smaller area.
Popular materials for filling hugelkultur beds include tree branches, sticks, and any other organic materials like leaves, stems, and twigs. The organic material absorbs the water the seeps down into the ditch, and acts as a sponge to keep the water within the soil for a prolonged time.
The abundance of moisture and organic material creates a long and highly active decomposition process. The heightened organic processes warm the soil and increase fertility, replenishing the soil with nutrients over time.
Beds built in this way are not only beneficial for the plants that are planted directly on top of them, but also for the plants that are located nearby.
The most basic hugelkultur beds are simply logs piled on the ground, covered with smaller organic material, and then covered with soil. This technique is likely to work well in wet temperate climates, but not so much in hot dry climates.
"The recipe [for hugelkultur] is wood and brush, covered with soil." - Paul Wheaton
A more modern approach is to dig a massive ditch in the ground, and construct your bed within that ditch. This approach allows for better water retention and a more protected environment for natural decay to take place. This construction method is recommended for hot and dry climates. How deep the ditch should be depends on your climate. The dryer your climate, the deeper the ditch. In very dry climates, the deeper the ditch is dug, the better the hugelkultur bed will be likely to perform.
While piling soil on top of logs is the original method, better results have been demonstrated by creating alternating stacks of various materials. Beds that stack wood, brush, straw, and soil have been reported to work very well and produce results as early as the first year.
You can plant pretty much anything on a hugelkultur bed. The biggest concern is to avoid using plants that will receive too much water and nutrient after the logs within begin to break down. Depending on your climate, this may not be a big concern. A bed in a very dry climate is likely be relatively dry. Beds in very wet climates can become boggy during wet weather.
Many people have reported success with planting fruit trees and perennial edibles on hugelkultur beds. And there are countless examples of thirsty crops like pumpkins, cucumbers, and tomatoes thriving in this environment.
Don't construct your hugelkultur beds on contour to retain water, as in the use of berms and swales. If a hugelkultur bed becomes saturated with water for a long time, the logs within can become bouyant and break free of the soil above.
If this happens you may have to deal with runaway floating logs, an onrush of flood water flowing downhill, or both. Build any berms from compacted and intensively planted soil. You can have both features near each other, but don't rely on hugelkultur beds to function as berms.