I've posted a video by Paul Wheaton on how to build "hugelkultur" raised beds before. But with the tantalizing promise of vastly reducing, and possibly even eliminating, the need for irrigation, it seems like a topic worth revisiting.
Developed by Austrian hill farmer Sepp Holzer, hugelkultur at its simplest is a process of piling up logs, brush and other carbon-dense biomass, and then building up raised bed gardens over the top of those piles using top soil and compost. The theory is that the biomass slowly decomposes over time, feeding the plants above with nutrients and also providing a sponge-like layer beneath the growing substrate—absorbing and rereleasing water to the plants as needed.
As I mentioned in a previous article on hugelkultur, the process can be taken to what looks like relatively industrialized extremes—using diggers and earth movers to pile up biomass over huge distances.
Paul Wheaton's latest video explores the process in a little more detail, visiting a freshly built hugelkultur operation in Montana that, its owners claim, has never needed to be irrigated at all. True, the plantings won't look much like a garden to your traditional horticulturalist (one YouTube commenter said it looked like they were mostly growing weeds), but a closer look suggests this is a planting of edible polycultures that includes squashes and zucchini, radishes, lettuce and a whole host of other crops.
It would, of course, be interesting to know what kind of yields these folks are getting—and whether they are growing for a commercial operation, or for their own sustenance. Polycultures like this seem to be well suited to personal consumption—where you might want to picka lettuce here, a squash there. But I find it hard to conceive how they would work on a commercial level, where you need to have an efficient system for harvesting marketable crops at roughly the same time.
I'd also be interested to know whether there are problems with nitrogen robbery from the plants as the wood decomposes, and whether anyone has studied methane emissions from beds like these. (Anaerobic decomposition creates methane. Methane is a potent greenhouse gas.)
Anyone have any insights?