I live on rolling hills of decomposed granite, in an area that gets about 25 inch of rain a year, primarily in the months November through April. The summers are dry and hot, with little to no rain from June into October, and temperatures around 110 degrees and higher.
I was going to plant my elderberry trees in mounds of wood chip mulch, but today the idea occurred to me to plant them in pits. I'm thinking of pits that are three or four feet wide, and one to two feet deep. Probably 2 or 3 inches of mulch lining the pits, and branches laid across the top for shade.
My thought is that the pit would capture more water and would remain a few degrees cooler then the surrounding area during the hot summer months. By placing tree branches over the pits the root Zone would be shaded, again creating better temperature regulation during the summer months. Of course the mulch would both retain water and moderate soil temperatures.
The soil on our land is decomposed granite, so I really have to work to retain moisture and friability, and I'm thinking that this might be the best approach well the trees are still young and getting established. Eventually the pits will fill in from the biomass created by the trees, is they are known to drop a lot of leaves and a lot of broken branches.
Can any of you see flaws with this plan? Suggestions of how I could do it better? Would it be better to stay with my original plan of wood chips above-the-ground and planting through them?
I think I might be worried about waterlogging the trees, but since you are having trouble retaining water, this might be a good idea.
Another thing I am perhaps concerned about with using woodchips is the possibility of the woodchips retaining moisture too well and not giving any back to the tree until the woodchips decompose.
I'm thinking something like a pit filled with a mix of organic matter, dried leaves, cut grass, kind of like compost, mixed a bit with rocks or sand might do. Though, this makes me a bit worried about the possibility of gleying. Though, gleying be a benefit, because retaining water is currently an issue.
The soil tends toward sandy with some clay, but very little else beyond the thin layer of top soil, so it tends to be poroys. I don't think drainage will be too much of a problem, but that is some great feedback re: wood chips vs other fill.