this is quoted from http://www.flowers-plants-gardening-advice.com/har...
How to Dry Yarrow
In Cathy Miller's book Harvesting, Preserving, Arranging Dried Flowers, she speaks of of spritzing the heads before and after drying with clear acrylic spray or extra-hold hair spray to keep the pollen from dropping.
Although I have done this after they have dried, I have never done it before drying them. It sounds like a good idea, but not completely necessary.
Yarrow is so easy to dry, all you need to do is strip the leaves off and gather 5 to 7 stems into bunches. It's best to stagger the flowers to avoid damaging the shape of each head.
For a more open head, dry the stems upright in a vase with an inch of water that is allowed to evaporate.
Hang in a dry place with good air circulation.
Pastel yarrows such as A. millefolium, 'Cerise Queen' and 'Summer Pastels' are best to dry in selica gel in order to retain their color.
Drying Time is three weeks.