Yarrow does not like to be too wet. Can you describe the area where you have planted it? Did it come up ok, then decline, or not come in good at all?
I really can not help because my garden kinda grows "on its own", but my yellow yarrow is getting out of hand. It is in full sun and I do nothing for the wonderful plant except harvest it and let it dry. On the other hand, I have pink yarrow and it does terrible. They are right beside each other and the pink one barely blooms and will not dry well. Do you know anything about the pink variety. I got my yellow yarrow at Lowes on the clearance rack two years ago and it was dead looking and I didn't think there was any hope for it. It seems all of the plants I buy on the dead looking clearance rack have exceded their abilities.
I grew yellow Yarrow out by my veg garden in full sun and along the fence line both grew crazy and had to be thinned out yearly. Let mother nature water it.