Y - Yes, it's Yellow
My yarrow is "Moonshine," and it's one of the original plants in my Butterfly Garden, planted in 2017. Of course, it's been dug up and divided twice since then. Yarrow is SO easy to propagate from divisions and chunks you might mistakenly break off when you prune it. I've had all my divisions bloom the same year they are transplanted. I hope some of my seedling yarrows will mature, they are different colors. I also have some young white yarrow and Summer Berries.
I have LOTS of flowers that bloom in yellow, but none other than yarrow whose names begin with Y.
Which brings us to Z.