Good advice and stuff I've thought about...we have mainly clay, parts of the property are not particularly well drained, and planting from pots can certainly created an amended base where it retains water (and dries out more quickly, too: double dragon). I've restricted planting from pots to the fall season where there's an opportunity for the roots to grow into the native soil even as the tree is going dormant. So far, with two years to judge, it's been relatively successful, and with as wet as it was this summer, I might have expected to lose some of the chestnut to soggy feet. Finger's crossed, but they still look healthy.
I did end up watering yesterday. The trees weren't showing frank signs of stress but there was some subtle drooping and browning. But the ground is starting to crack some from a month without rain. Crazy year. Well...every year seems to be crazy in one way or another lately.
Here is what I do for planting from RB2 containers into my heavy clay.
1) I use a tractor auger that is very slightly larger than the container I'm planting from.
2) I auger the hole between 2 and 3 feet deep.
3) I back fill the bottom of the hole with quarry stone (quarter size).
4) I add fertilizer at this point regardless of the time of year. I use the wrong end of the shovel to make sure it falls down into the stone.
5) I add a thin layer of clay (less than 1/2") just to protect the roots from contact with any fertilizer that doesn't filter down.
6) I add the tree. My total backfill level is such that the top of the medium will sit about 1" above ground surface level.
7) The spot I pick is not a low spot where ground water will drain into the hole, but I use the native clay to build a slight ramp from the ground level to the top of the medium. This also helps keep ground water from draining in.
8) I cover it with landscaping material and put more quarry stone on it to hold it in place and act as permanent mulch.
We get very wet periods in the spring where the medium can act as a pond. By augering that deep hole and backfilling with stone, any rain water that does enter naturally (not ground water drain) ponds in the stone below the root ball so the tree doesn't have wet feet. During dry periods in the summer, the landscaping material and stone mulch help reduce moisture loss without attracting rodent nesting. The reason the auger size is very slightly larger than the root ball is so that the lateral roots don't have far to grow to get into the native clay. The clay under the top few inches will retain water well.
If you have the ability to provide supplemental water in the field when it has been dry for a while, do so. Your clay will be dry and the promix will not hold water better than the clay. When it is dry, the clay will suck the water from the promix and the promix will dry out more quickly than the clay.
Thanks,
Jack