Hi Elizabeth,
I've built a small hugel bed in Victoria, it's probably 3 years old now. It's maybe 14 feet long by 3.5 feet wide, dug down 2 feet and built up 2.5 feet above the surface.
Wood used was a mix of maple, alder, fir, logs and large branches, mostly reasonably rotten.
The soil from the excavation was by no means sufficient. Also tends to lose a lot of soil from the top in rain. If I bother with any more, I'd likely design it as a conventional raised bed around the hugel for longevity.
Despite addition of more soil and manure/compost to the top, it's been far less productive than other areas of the garden, and the wood inside does not seem to be decaying rapidly. Doesn't seem to do much in the way of holding moisture despite adding a LOT of
water during initial construction, several times. Given that the wood had to be carried up out of a nearby gully, log by log, it was a lot of work for the small size and poor results.
I've also seen hugels used in a food forest where the water table was on the high side in winter, @ Mossy Banks farm, North Cowichan. Planted in 2012, individual mounds for each tree/bush in the lower portion of the food forest. Mortality in these trees/bushes has approached 50% at this point; I would credit issues with the hugels for at least part of this problem, as I observed similar problems with insufficient soil on top, settling/washing away in rain... both exacerbated by the Muscovy ducks until they were fenced out entirely, as individual
fence circles for each tree did not protect the base of the mounds
enough.
So, in my
experience:
1) You'll need loads of extra soil.
2) Keeping a 4 foot wide bed 4 feet high sounds tricky.
3) I'd be hesitant to plant trees *on* the hugel due to stability/erosion issues. Maybe next to it would allow any moisture/nutrient benefits to be accessed by the
roots with less risk? Or do you need the height above the water-table?
At least with annuals, you can patch up the mound when you plant. With perennials, you have to keep on top of things to avoid losing trees when the soil washes away.
4) I'd be planting nitrogen fixers first and foremost, and I'd definitely want a cover crop ASAP to keep the bed from falling apart in the winter rains; clover? I must admit I did not do this when I first created mine, and hope it will help next time round. However, this *was* done @ Mossy Banks, and didn't seem sufficient.
I'd be very interested to hear how it works out, hopefully the larger size of yours makes it much more effective.
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