Hugelkultur is German for a garden bed with large amounts of buried wood for biomass. Logs of all sizes are piled and covered with several inches of dirt. Vegetables are planted on the resulting mound. The decaying wood provides long-term plant food and water retention.
Almost all of our vegetable garden, fruit trees, and hedges are planted on modified hugelkultur beds. Ours are underground instead of above ground, to retain a level landscape. Depth is typically 2 ft, and for a tree transplant, diameter is typically 3 ft, with a center sand mount for the tree's taproot to anchor in. Below a depth of 6-8 inches in our soil here in Central Florida is yellow sand (lacking significant organic matter) which we remove and utilize elsewhere. The plant material we add is mostly freshly cut small branches and leaves of trees, with larger branches comprising about 1/4 of the total material placed at the bottom. In hugelbeds for annual fruiting plants, such as tomatoes/peppers, we've found the need to add dead leaves, to reduce the nitrogen ratio of the resulting compost, for normal fruiting yields.
We have read that fresh cut plant material buried should decompose before planting in it. However, experience shows both perennials and annual plants grow vigorously right after planting in such a hugelbed, in which time the fresh cut plant material cannot decomposed. We suspect the fresh cut plant material is releasing nitrogen in a form readily uptaken by the growing plant, perhaps in a gaseous form or another.
After the bulk of the compost macronutrients are consumed, typically in a year, the hugelbed continues to provides humus, or organic matter, with myriad benefits. The water retention of the hugelbed is particularly valuable for us because rainwater drains fast through our sandy soil. Further nutrients are added by compost amendment on top, but also by companion plants or cover crops. See below the preparation of a hugelkultur bed for our cashew tree planted from a one gallon pot.
Topsoil is piled to the left of the hole exposing the yellow sand under the topsoil layer. In the foreground are removed tree roots. Sand from the hole is in the wheelbarrow. We utilize the sand where we need it.
The topsoil is about 6 inches deep and the hole is 2 feet deep, and 4 feet wide. It's hard to see in the photo, but we built a sand mound in the center so the tree's taproot can anchor in sand.
The heaviest wood goes in first so the less-nutritious biomass can help trap nutrients from above as rainwater flushes down. The wood comes from larger trees that we want to prune to expose sun to the garden.
The smaller branches and leaves go in next. The fresher the better. This deciduous rain tree just leafed out about a month ago. We use oak for blueberries since we read that it creates the most acidic soil, else we use whatever trees we need to prune. We've been using rain tree, oak, camphor, cherry and crape myrtle. We also throw in vines and even ferns sometimes for extra nutrition. Whatever is growing that seems to be imposing the most on the garden.
Next we backfill some of the topsoil. So we add about 4 inches of biomass, 2 inches of topsoil, and 5 gallons of water, and repeat 4 times, so it's like making lasagna. We omit the water if we're expecting adequate rain, in which case we tamp more. Settling isn't a concern, because we expect to build the soil level with compost and mulch, except in the center where these can fuel fungus on the trunk, so we tamp the center sand mound well, and we plant to ensure the soil line is no higher on the trunk than the highest root.
This final layer is a big fresh cluster of giant buds that sometimes fall out of the giant oak trees. Notice the sand mound in the center is built up along the way so it's like a lasagna donut.
This is the final layer of topsoil and the tree is placed in that. It's hard to see, but there's a final topping of water on there.
Last step is a thick layer of mulch. We make sure the hugelkultur bed stays moist, but not wet, through its entire depth. During a three month drought, a hugelkulture bed may stay moist for two of those months, then we have to replentish it. In a 6 month drought soil at 2 feet will need a couple of deep waterings.