Yarrow is an amazing plant.  Not only does it send up amazing umbels of gorgeous color year after year, the bugs love the tiny mini-blooms and the leaves give off the most wonderful herbacious smell.  
 
Once they are established, they can tolerate drought, are not fussy, and often give you a second flush if you keep them moderately watered and weeded through the summer.  They are almost entirely disease free and nothing messes with them (ahem, not even deer.)
 
Cut all the way to the base when cutting for your vase.  Avoid cutting in the hottest parts of the day.  Strip the leaves so they aren't in the water.
 
Plant this beautiful plant for its feathery fronds of foliage and tall stems of gorgeous color.  Get more plants for the best variety of color.  
 
Plant 8-12" apart and water well until roots are established.  Plants grow 2-3' tall.  Seedlings come in a 2.5" pot.
 
There is no refund or warranty for plants that die after purchase.