October finds many of us slogging back into our garden to slap at mosquitoes as we rip out the tired remnants of our May and June plant buying sprees.  The Georgia Perennial Plant Association offers the perfect insect-free alternative activity: to hear one of the most engaging horticultural speakers around who is guaranteed to teach you something you didn't know - Dr. Patrick McMillan, the Director of the Juniper Level Botanic Garden.  Dunwoody Nature Center, North Woods Pavilion, 5343 Roberts Drive, Dunwoody, GA 30338.  Free to Georgia Perennial Plant Association members; join for $35.  www.georgiaperennial.org.