Does it have to be E. Purpureum?? I have some really nice Eupatorium 'Glow' that is much nicer than the species. Dark purple stems and full flower heads that last longer. I germinated these 2nd generation this last year and they appear to come true. I have some seed if you like.
I am sending you an email with dtails. Following is a picture taken in late September here in zone 4a. Eupatorium fistulosum Atropurpureum 'Glow' seeds are available at www.hardyplants.com.
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