Hi Andy, no problem, just wish I had more in the way of a solution!
What Peter is describing is exactly the pitfall I saw in action last summer.
The fact that your hugels are 3/4s soil seems hopeful; I think it was probably more like 50-50 originally, in the hugels I saw with trees in them... the one I built, about the same. More wood than soil available!
Still got bobcat access? If you can do more earthmoving, what about adding more soil mounded directly against the hugel, perhaps half as high, on the south side, then plant the trees into that? Might be more stable than the cups you describe. With the cups, they may fill with water, and then overflow, and erode... groundcover, groundcover, groundcover!
Plus, if the hugel is 4' high, and you dig a 1.5' hole at the halfway mark, you're nearly back at ground level!
If you do plant the trees directly into the hugels, maybe you could tweak the placements so each is next to, rather than right above, the concentrated wood piles?
I take it there isn't any more elevated area that could accept some of your most flood-sensitive trees, so that more suitable ones could be planted in the floodiest parts?
Andy Roo wrote:As for irrigation, drip is out because there is very little water pressure on site so I am leaning towards running a hose with a splitter to a couple different locations and filling one 5gal bucket per tree and dumping it on every week. A lot more work but not sure if there's a better option.
I would suggest coming up with a better irrigation plan if there is even the slightest doubt that someone will be able to water them as described. Once per week in a bad august drought sounds optimistic for young trees. Better options... Well, you could add infrastructure to address the water pressure, a reservoir and pressure tank sorta deal... The water in the pond here is pretty mucky, can;t be used in a drip system without major filtering, so the orchard uses sprinklers. Watered in segments to keep the plumbing and electric requirements down, off a pressure tank fed by a pump in the pond.
Andy Roo wrote:I cannot excavate a pond for drainage (draining the wetland area would be impossible in this case)
That's unfortunate; note that while draining the wetland would be impossible, and quite possibly undesirable anyhow, even a smallish pond can have a localized impact on the water table. I certainly can't quantify this, though... A pond could also help with the irrigation issue as above... and provide soil to mound against the hugels...
I guess the bright side is that once you get through the first few years, however you do it, the hugels will have become defacto swales with excellent soil, and should be a great place for trees!