Answer: These plants do seem to do better with division about every three years, and they also appreciate a bit of compost or well aged manure as a top dressing when they have been in pace for a while. Although they are listed as drought tolerant and can survive with very little water, they also do better with ample water during the summer just as most plants do. These steps will help to keep them vigorous, as will deadheading or removing the spent blooms as they occur. Vigorous plants seem better able to withstand insect attacks and in general seem healthier overall. In my garden I have allowed them to self sow and then remove the oldest plants when they seem past their prime. Based on your description I am not sure if this is the case, but this plant is also occasionally affected by what is basically the same thing as "aster yellows". Unfortunately there is no cure for it except to destroy the plants. They are also occasionally attacked by various insects, but this is fairly unusual. With any luck, dividing them next spring will solve the problem.
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