Last spring I planted some Achillea Walter Funke and Millefolium terracotta in a new bed with the hope that it would self-seed and spread as far as it wanted. From everything I've read I was under the impression that Achillea was a pretty tough plant that could handle most conditions as long as it wasn't too wet, however none of the six or so that I planted last year seem to have survived the winter. Everything else in the same bed (grasses, Eryngium, Verbena, Sisyrinchium, Fennel, Alliums - generally drought tolerant species as far as I know) seems generally to be thriving, so I'm wondering what has killed these off that didn't apparently cause any problems to other allegedly less hardy plants. Are there pests that could have eaten the new growth, or are they more susceptible to UK winter conditions than most? I'm wondering whether I should I try again with these or put something else in that might thrive more easily? Any tips gratefully received!