Well, they will need regular watering to become established, but after that I just leave the yarrow alone -- it makes it on rain water only.
I don't know what varieties of yarrow you have. I have the tall Parker's -- it stands up straight about 5 feet and has the yellow flat flowers, the moonbeam is like that but shorter. I have a red one that I love -- Weser River Sand stone -- beautiful color -- and stand up.
Some of the others -- Credo, Paprika, Terra Cotta -- tend to sprawl when they are in bloom. Eventually I cut them to the ground when the sprawling drives me crazy. I think that all of them should be cut to the ground after blooming. And yarrows need to be divided every few years.
Good luck
Susan

I agree with the above..they only need additional water the first year (till established). After that, I leave them alone.
I have mine planted in a wildflower area and they are thriving. The bed is in full sun, plus it is a dry, lean bed.
I do add mulch and compost in early spring.

Tammy

Ours came with the house. I do nothing to them -- no extra water (and we've been in a drought for several years now), no compost, fertilizer or mulch, really. In fact, now when I move bulbs around and have to pull up some of the yarrow to do it, I don't even bother replacing the plant carefully, I just set it back roots down. For the past two years I've had to rip out patches and trade them, otherwise the whole bed will be overrun. Before I pulled up the landscape cloth, it was pushing up through it. So I don't think you need to give it any special attention. It may not be as invasive in your zone.

I thought that the lady at the garden store said that the Yarrow Achilles would bloom all summer. If youcut them back to the ground can they still grow and bloom the same year?