Question
Is this true for most perennials?  I also have day lilies etc.
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Followup To

Question -
Once the black eyed susans flowers turn brown, do I cut down to the ground or can I just leave it alone and it will take care of itself?

Answer -
Hi Linda,
Thanx for your question.  You can cut the flower stalks off now if you like.  Let the rest of the plant live until the first frost then you can remove all of the dead material and even cut it down to about 2 inches above the soil.  I hope this helps.
Tom

Answer
Hi Linda,
Thanx for your follow-up. The same thing applies to daylilies.  I have a ton of them and I had a cool experience getting off on all of their beautiful blooms this past summer.  Take care and let me know if you have other questions.
thx,
Tom