Answer: It is most important to select plants that are adaptable to clay soil because, when you think about it, the plants roots will need to extend far beyond that original hole.
Modern planting methods indicate that loosening the soil in a wide area and down somewhat deeper than the root ball is critical in encouraging the roots to grow well.
Watering is also critical. The soil should be kept moist but not soggy in an area slightly larger than the roots -- roots will not grow into dry soil.
You may find that a raised bed area is helpful if you try to grow shallow rooted plants such as azaleas; the reasoning here is that the soil they need (acidic, humusy, full of organic matter, moist yet perfectly drained) can be provided on top of the clay base. If you do this, be prepared to water carefully as needed every season.
I'm sorry I do not have a "simple" answer for you.
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