HI Jan I am no expert on yarrow plants.but from what i have read about the plants they require full sun and well drained soil they need to be watered at least one time a week in the dry season water at the roots not on the folage.the yarrows only bloom in June then again in september.make sure you cut all spent blooms after they dry up even though part of the plant might still be blooming just cut the dry ones.the older plants need to be devided about every 3 years and planted at about 18 to 20 inches apart.there are many different varitys and colors of yarrow. The taller ones need to be planted at the back of the flower beds.the yarrow blooms are short lived flowers but will atract all kinds of butterflys.also check to see if there might be some kind of insects on your plants.here are a couple of links you could check out on the yarrow plant.
http://www.thegardenhelper.com/Achillea.htm
http://www.blackanddecker.com/outdoor/plantcaredetail.aspx?Doc_ID=2061
hope this helps. your friend in gardening.mike57

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Jan