If the yarrow has grown spindly, you need to cut them back and they will re-bloom. Also, it is probably time to "divide" your yarrow. Wait until after bloom to divide them. Dig up the clump. Divide it in half with a sharp spade and then divide in half again. Replant each division and water often and deeply to re-establish your yarrow.
If the yarrow has grown spindly, you need to cut them back and they will re-bloom. Also, it is probably time to "divide" your yarrow. Wait until after bloom to divide them. Dig up the clump. Divide it in half with a sharp spade and then divide in half again. Replant each division and water often and deeply to re-establish your yarrow.