Clay soils can be wet and cold making it difficult for some plants to grow. Over the years I have had soil like this so here is a collection of plants that I find have grown well in these conditions.
Campanula lactiflora 'Loddon Anna'
Height x spread: 120cm x 90cm 9cm
Cirsium rivulare 'Atropurpureum'
Height x spread: 120cm x 50cm 9cm
Geranium x magnificum
Height x spread: 60cm x 60cm 9cm
Thalictrum delavayi 'Hewitt's Double'
Height x spread: 120cm x 90cm 9cm
Crocosmia × crocosmiiflora 'Emily McKenzie'
Height x spread: 60cm x 50cm 9cm
Persicaria amplexicaulis 'Rosea'
Height x spread: 90cm x 90cm 9cm
Phlomis russeliana
Height x spread: 90cm x 90cm 9cm
Ageratina altissima 'Chocolate'
Height x spread: 90cm x 75cm 9cm
Lythrum virgatum 'Dropmore Purple'
Height x spread: 102cm x 60cm 9cm
Hosta 'Honeybells'
Height x spread: 75cm x 60cm 9cm
Ajuga reptans 'Black Scallop'
Height x spread: 15cm x 40cm 9cm
Geum rivale 'Album'
Height x spread: 30cm x 45cm 9cm
1 of each (12 plants) requires a space of around 90cm wide x 120cm long. 2 of each (24 plants) requires a space of around 90cm wide x 240cm long. 3 of each (36 plants) requires a space of around 90cm wide x 360cm long (All measurements are approximate)
Clay soils in sun or partial shade
March to September