Answer: Perennials for clay soil are much fewer in number than those that demand good drainage. The problem with clay is that it holds moisture and this tends to rot out the roots of all but the toughest of plants. If you decide that you'd like to amend your clay soil, here's how you can do it:
Spread 4-5" of organic matter over the entire planting bed and dig it in to a depth of 8-10". Plant your plants, then mulch over the bare soil with another 1-2" of organic matter. This will help slow evaporation, moderate soil temperatures and suppress weeds. At the end of the growing season, dig this mulch into the soil and and another 1-2" of organic matter over the top of the soil. After repeating this process for a few years, your clay soil will be transformed into good garden loam. Now for the plants:
Acanthus will accommodate itself to the heavier soils although it doesn?t make them any hardier in the wintertime (if you can?t grow them in regular soil, you can?t grow them in clay)
Achillea will grow anywhere.
Aconitum or monkshood is a classic lover of heavier soils.
Alcea or Hollyhocks surprisingly grow well on clay soils.
Amsonia should do well on clay
Anchusa is good once established and protected from slugs (slugs like damper soils and they love anchusa).
Anemone are excellent perennials for clay soils and will thrive and flower well on clay. The taller varieties are excellent.
Asters will grow anywhere. Clay or concrete does not matter to this plant.
Bergenia should be fine.
Brunnera macrophylla should do well although I?m not sure about the newer hybrids ? you?ll have to trial them.
Campanula the taller species do better than shorter, so do try C persificolia. You?ll find variations between varieties. ?Loddon?s Pink apparently does well but ?Telham Beauty? is variable.
Chrysanthemum (Shasta Daisies) do well enough although they will be shorter-lived on clay than in well drained soils.
Coreopsis verticillata is the best in clay. Forget all others.
Doronicum spring bloomer, one of the earliest of yellow daisies.
Digitalis or foxglove ? scatter the seed and let nature take its course.
Echinops make good clay soil plants. These things will grow anywhere but in a closet.
Geranium: most of the geraniums will tolerate clay soils.
Helenium are very showy daisies and they will grow handsomely on clay soils.
Helianthemum or perennial sunflowers will grow nicely.
Heliopsis is another of the perennial sunflowers and it too grows well.
Hellebore surprisingly enough are listed as surviving quite nicely.
Hemerocallis are great on clay. They do take a little longer to become established but can?t be killed.
Heuchera are great perennials for clay soils and this is a relief given the amount of semi-shaded clay around.
Iris germanica or flag iris will grow on heavier ground although many of the species iris will not.
Kniphofia will tolerate clay.
Hope this short list is helpful.
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