Thank you for the question. Don’t fertilize perennials now because the plants need to shut down for fall and winter. You don't want to encourage any new growth late in the year which might be damaged by cold. Most perennials are not heavy feeders and too much fertilizer can lead to too much vegetative growth that gets floppy.
It's best to fertilize perennials in the spring to give growth a boost and supply energy that can be stored for later in the season. The other advantage of fertilizing early is that plants are small and foliage is less likely to be burned from the fertilizer. A well balanced fertilizer like a 10-10-10 is good. Read more about the different ways to fertilize and when to do it here:
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