Hahaha, Jennifer, I always forget that some regions actually get rain! If your garden soil is rich, another thing you can do for yarrow and other Mediterranean plants like lavender, many Salvias, etc, is create a "lean" bed — one with sandy, even gravelly, well draining soil and limited humus (compost, not manure). It's difficult to incorporate some of those hot-climate plants into the richer beds without suffering floppiness, root rot or shortened lives.
I have the opposite problem here; it won't survive if it needs lots of water and nutrients. I have to create amended beds for veggies and hungry ornamentals.
Last year's Hidcote lavender and Moonshine yarrow did really well together!
