there 'native' habitat is woodland really. they don't like to get over dry . not certain about the clay. sorry I cant be more helpful.

Might be too wet for them. They do like the soil to dry out somewhat after setting seed. That is what happens in a woodland setting. Once the trees get their leaves, the soil does dry out.

Soggy clay soil, not ideal conditions, they need a moist but well drained soil… after they've died back this year I would suggest lifting them and putting some course grit in the bottom of the hole and replanting. Ours grow in a shrubby area where there is a lot of humus but the soil is still free draining.

I have them in clay improved with grit. They get the sun at this time of year, but are shaded later in the season. But they never get waterlogged

Really sound advice from all of you. Many thanks.

I had a feeling that I was doing the wrong thing and I shall certainly add grit to the soil. I might do this straightaway as they only went in yesterday.