Most people when they talk about hugelkuktur, are talking about mounds. Mound gardens in my area would dry out too fast, thus I focus on hugelkuktur pits, which I've shortened to hugelpit. The same general ideas are involved with pits as with mounds......a base layer of logs or woody debris overlayed with organic material and soil.
When I first started this project, I hadn't heard about hugelkuktur. I referred to this as my biotrash pit. I have numerous pits in the property, some natural and some manmade. This particular one beside the driveway is manmade and huge. The previous owner had excavated it out to get rubble to create the driveway, thus creating a long narrow pit, in places 6' to 7' deep.
Over time I filled the pit with cut up fallen tree trunks and limbs, covering them in dirt, weeds, brush trimmings, and any other biotrash I had on hand. Eventually I filled up the monster pit and planted bananas. But over time the material has decomposed, sinking down to leave a 3' deep pit. As a hugelkuktur pit, it is functioning very well, but I want to fill it in again, bringing it closer to driveway level.
Time for experimenting again ! 😀 Above, this is one end of the pit. I'm using cardboard in place of wood, but otherwise the method is the same. A layer of cardboard 2-5 sheets thick, then a layer of "brown" organic material, wet it down (this is when rain really helps out), then more cardboard, topped by a layer of fresh grass clippings or horse manure. Repeat.....over and over and over and over.
Above, a closer photo. I have access to a lot of cardboard which it basically ink free. I will also use cardboard that has black print, but I tend to avoid colored inks.
I took the above photo on a sunny day, so it's not so easy to see. But what it shows in the cardboard covered in a light layer of "browns" -- forest duff, fallen leaves, twigs, ferns, dry bananas leaves. I'll add a little moisture using a watering can (no hoses reach this spot) before adding more cardboard. Then I'll mow some grass and top it off with a 2" layer of clippings.
By the way, I'll also throw in rocks in order to give the newly created "soil" stability. Otherwise the bananas that will eventually be growing here will fall over. And as with the gardens, I'll also add some soil amendments as I have them : bones, coral sand, lava sand, biochar, urine, various manures.
I'm interested to see if cardboard will be an effective "water sponge". It's going to take a heck of a lot of cardboard to fill this pit. But once completed and planted with bananas, the test will come with a drought year. Will the cardboard preform as well as wood? Time will tell. The goal is to have all of my banana patches growing atop water retaining hugelpits.