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And 'Cowhouse Green' seems to grow well for you. Here it is struggling, and I must move it this year so that I would increase better. Now it has had only one flower for several years. It grows in a dryish shady spot, maybe more sun or more moist would be better?
I know that light intensity can influence the depth of yellows, bugt what about pH?
Leena, ´Cowhouse Green´ doesn´t grow well, if at all, in the heavy clay of my garden. Yet potted in the soil of my compost heap, it does very well. The conclusion may be that it likes rich and somewhat more aerated soil. Nowadays I grow it in pots into a nice clump and plant it out in the borders then.