Answer: What a sad situation! You've correctly described what would happen if you dug a hole, added potting soil, and planted; you will have created a situation similar to planting in a huge container. However, without adequate drainage, some plants will die a premature death. There are some plants that actually like growing in clay soils and their roots are strong enough to penetrate deeply.
Besides being heavy when wet, and impossible to work when dry, clay soils also incorporate very little air, which can suffocate roots, and they can remain waterlogged for prolonged periods of time, often drowning plants, or at least exposing them to root rotting conditions.
Some of the plants that actually thrive in heavy clay soils include Aster (especially Michaelmas Daisy), Crocosmia, Helenium, Inula, Vibrunum 'Lanarth' (Japanese Snowball), Weigela, Rudbeckia, Boxwood, Spiraea, Forsythia, Hemerocallis, Monarda and Prunella. While these plants won't actually change the soil conditions, they will provide some wonderful spring and summer color!
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