Answer: Once frost has killed back the foliage, cut it at soil level and remove it from the garden. The roots of your asters are perennial and will live through the winter. New foliage and flowering stems will appear in the spring. You can mulch the soil over your asters with compost, aged manure or leaf mold to keep weeds from sprouting in your garden, and to protect the roots of your plants. Nothing else needs to be done once you've removed the debris and spread some organic matter over your garden plot.
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