Answer: Sometimes a pest or disease problem particular to a species can build up when that species is planted repeatedly in the same spot. This is why vegetable gardeners often rotate crops according to plant family. Asters can be susceptible to downy mildew and therefore are often rotated to avoid problems.
This does not mean that an aster cannot do well in the spot one grew before, or even where they have been grown for years, it just suggests rotating to other flowers to avoid problem in the long run. I space is limited, I would suggest you go ahead and keep planting asters there as long as they continue to do fine!
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