maire-aine,

In years past, I noticed goldfinches feeding on the cultivars of fern leaf yarrow (achillea filipendulina) that went to seed before I deadheaded. I never saw them on any of the cultivars of common yarrow (achillea millefolium.) I'm in a much different climate zone from you as I recall. Here in the hot, humid, and droughty southeast USA, without deadheading, the fern leaf yarrow, in particular, seems not to fare so well in subsequent years, getting really scraggly and woody much more rapidly, and needing dividing to keep it going more frequently. The common yarrow doesn't seem to care much, except that I notice that some colors of some cultivars are prone to rebloom if deadheaded (mostly the dark pinks and burgundies and the whites in "Colorado Mix," but not "Cerise Queen" or the naturalized white common yarrow, nor the"Summer Pastels" or "Apple Blossom" cultivars of of common yarrow.

What happens in your area?