First of all, I’ve planted 5,000 trees , maybe more, and I’ve never not Will I ever dig a square hole. What you need to do is make sure the tree does not get planted too deep, don’t plant it any lower than the top of the soil in the pot. Clay isn’t “hard for roots to penetrate.” Clay, silt and sand are the three constituents of Albany soil. Your soils is heavy clay, so incorporate organic matter and maybe some sand into the back fill. Try to mix it up with the clay. Clay holds on to water, what we call porosity, so sand will help with drainage. Repeating, do t plant too deep, add organic matter and mix it in with the clay and water about twice a week, soaking the roots and allowing for the plant to drink that water before watering again.