Hillside Hugelkultur and Swaling, pt 3 : Bring in the rot, bring in the funk

You find "ideal" hugelkultur wood under piles of wet leaves on a forest floor, or partially submerged in slow-moving water, or under similarly damp, rot-producing conditions. One of the lovely things about a hugel-system, however, is its great flexibility and "fault tolerance." Pretty much any wood that isn't directly poisonous to detritovoric life will do fine, and the kinds of wood that are rot-resistant (cedar, fir, walnut, osage orange, black locust, teak, treated lumber) have much better uses than rotting. In fact, if you happen to have a bunch of walnut, osage, or teak lying around that you are thinking about using for hugelkultur, please stop, because every time you think about misusing precious wood like that, an angel gets put in a woodchipper.

Photo (c) HighCountryPosts.com.
Even after chipping, angelic mulch
retains a visible glow, as seen here.

There are lots of excellent woods, though, many of which are common and easy to find literally lying around precisely because they aren't super durable, rot-resistant, or valuable. In fact, it's easier to just list the ones you should avoid. Unless you find them already decaying, I would avoid oak, cedar, cypress, or fir trees. However, it's a little more involved than that. Only the heartwood (core wood) of a tree gets the rot-resisting treatment; the outer layer of living sapwood resists rotting by virtue of being alive. So, the outer layer of any tree will generally rot pretty well, and younger, thinner branches / trunks / etc will be mostly sapwood, and therefore rot easily, so go ahead and dump them on the pile.


Since a moisture level over 20% or so must be present in wood for fungus to live, you need to use wood that's either fresh-fallen, or that stayed in moist conditions. To help the process along for some of the dryer pieces I had, I cut the top off of an old 55 gallon drum and built a large-scale compost-tea maker with the bubbler from an aquarium. Actually, at first I just put all the wood in the drum with water, but unsurprisingly that starts to stink after a bit due to anaerobic growth, which is the opposite of what I want anyway. So a cheap aquarium bubbler keeps the water oxygenated, and adding some compost to the vat creates a nice environment for getting the wood saturated and also colonized. None of this is essential, of course, but it seemed to help, and meant the wood would leach less moisture from the soil as it "charged" initially, reaching equilibrium with soil moisture (to avoid both dry dirt and dry wood, I'll also water everything a lot.)

So now I'm starting to stack the wood. It's all deadfall, especially easy to get since Hurricane Irene knocked down a lot of trees and branches. I also laid down dead leaves and other detritus. To build the mounds, I'll just continue to stack more branches, larger stuff on the bottom. I'll intermix it with dead leaves, the dirt I dug out initially, and aged manure. The aged manure comes from a farm rescue, where it is produced continuously as a side effect of taking care of lots of abandoned ponies, donkeys, and so forth. They pile it up on the edge of the farm, where it ages and breaks down into excellent rich soil amendment. It's not a necessary part of a hugelkultur mound, but given that it's not only free, but I'm helping dispose of what would otherwise be a "waste" product, and I can stop by the farm on the way home from a weekly out-of-town appointment anyway, it's hard to come by a more perfectly permacultural way to stack functions and multiply effects.

This is the first rough laying of the lower mound (the hillside will be two mounds, eventually.) You can see the large pieces of wood used to form the bulk of the lower level, as well as the sod I cut off the surface initially, which I flip grass-down and lay on top of the bigger trunks. As that material dies, it will create a pulse of available nutrients, particularly nitrogen compounds, to aid in decay of the big woody pieces. See my post on soil nitrogen for way more information on this topic than you probably wanted. The hose snaking through is a homemade drip-irrigation hose; it involves finding a discarded old garden hose, then drilling lots of tiny holes in it. For now, it's saturating the lower soil level before I add more stuff on top.

 Here's the first mound, construction complete. I've deliberately swaled the top edge of the mound, the create a path for excess runoff. It also provides a holding place for excess water, giving it time to absorb into the mound below it. Now, if you're particularly eagle-eyed and continuity-oriented, you'll notice that this mound doesn't run all the way to the end of the retaining wall. In fact, the area towards the retaining wall hasn't even been dug up. In reality, I did not do the extra digging to extend the mounding all the way until after this mound was complete, but that reflects not a careful and deliberate plan so much as an opinion on mound size which changed after I'd finished the first mound. Since this was my first hugelkultur work, I'd been conservative about dimensions, but as soon as I finished it, I realized it would quite nicely work if extended all the way to the far corner, rather than being unmanageable, and so that's how I would have done it to begin with were I to do the whole thing again.

It works! Water continues to behave in a manner consistent with my previous experience. At a low rate of flow, the mound simply swallows all water produced. Higher rates eventually saturate the immediate environment, fill the trench with water, and are absorbed over time. Higher still rates create exactly what I'd hoped, which is an small overflow stream moving away from the sidewalk and steps. Later on, this swale will empty into a depression at the corner of the retaining wall, a bowl-shaped area around a dwarf cherry tree, which will appreciate the water. I'll also be planting winter rye all over the swale, mounds, and so forth. Winter rye functions extraordinarily well for groundcover work. Scatter it in the late summer or fall, and it will solidly establish itself, resisting -30 F weather in the winter. In the spring, till it under or chop&drop it, before it sets grain heads, and it won't spread itself but will provide an excellent nutrient pulse after protecting your soil all winter long.

Here you can see the winter rye after a few days on the left, and after about a week on the right. The speed with which it germinated and grew I take as a good sign for the health of the new soil we're engineering. You can also clearly see the my seed scattering and raking did not produce uniform distributions, so I'll follow up with more seeds in the bald areas. You can also see in these pictures that the far edge has now also been dug up and hugel-ized, so the mound extends all the way to the far wall. These pictures also show, if you look carefully, how I laid pieces of scrap lumber along the bottom of the swale. This will provide solid footing, so that I can access all parts of the mound's crop cover, and it also helps to ensure that the swale doesn't became a miniature swamp if there are particularly torrential rains. The lumber will, eventually, break down, but as it's kiln-dried construction wood, it will remain relatively solid in this climate at least through next fall, by which time the mounds themselves will be solidly established enough that the earth will be much more resistant to erosion, becoming a mud puddle, and so forth.

Finally, this is the top mound, after its construction a week later. Same process, same results. Now, both mounds are seeded, swales are constructed, and on the top right of the photograph, you can see the new dwarf cherry that's below the curve of the hillside, at the corner of the retaining walls. All of the swales drain into its basin, where the hugelkultur mounds around it and its own thirsty roots deal with water overflows. At this point, it's just a matter of letting the mounds "cook" during the winter. The rye grass and the earth itself provide enough insulation that the deeper logs will stay above freezing for quite a while, allowing bacterial and fungal and other small life to begin turning logs into soil wealth. By the time spring rolls around, this area should be ripe and ready for planting. Also, having now observed for a while, I will be slicing the plastic wrapping off of the top of the retaining boards, because it is holding moisture against the wood. At the bottom I think it still serves a net benefit, keeping the wood from constant soil contact, but the top should be open to allow evaporation to keep the timbers dry.