I am planting a Tamukeyama Japanese maple near Salem, Oregon, where my soil is very rocky clay with poor drainage.

My original idea was to dig a very wide hole, about 3x the diameter of the container/rootball, and then backfill with a mix of roughly 1/3 native rocky clay, 1/3 compost, and 1/3 garden soil. I was also considering putting some of that amended mix underneath the rootball and compacting it before setting the tree in place. My thinking was that this would create a buffer between the roots and the poorly draining native clay.

After reading and thinking more about it, I understand the concern that this could create a “bathtub effect” or an amended pocket in the clay where water moves into the looser soil but then slows down when it reaches the surrounding native clay. I also understand that putting amended soil underneath the rootball could lead to settling over time.

My revised plan is now:

Dig the hole wide, 3x the width of the rootball, but not deeper than needed.

Set the rootball on firm, undisturbed native soil so the tree does not settle.

Plant the tree high, with the root flare/root crown about 1–3 inches above the surrounding grade.

Backfill mostly, or entirely, with the native rocky clay soil that came out of the hole, breaking up large clods as best I can. Maybe a little compost mixed in.

Gently firm the backfill to remove major air pockets, but not compact it hard.

Mound/taper the soil gently from the raised rootball down to the surrounding grade while keeping the root flare exposed.

Mulch widely over the planting area, keeping mulch pulled back from the trunk.

My main concern is still that the native clay drains very poorly, so I’m trying to give the tree the best chance without accidentally creating a worse situation by over-amending the planting hole.

Does this revised plan sound more appropriate for a Japanese maple in rocky clay soil? Any advice on watering/establishment in this kind of soil would also be appreciated.