Lythrum Salicaria (Lythraceae) - This plant is another import from Europe that has flourished here. Everytime I saw it there were many monarch butterflies present.
But, as Katie Eiring wrote me:

"Although purple loosestrife does have a pretty flower, it is very important to understand that it is not a good plant to have around. It produces millions of seeds per year and will displace native wetland vegetation creating a monoculture and thereby degrade wildlife habitat and choke out waterways and wetlands. In many states, it is illegal to buy, sell or transplant any form of loosestrife (hybrids will also take on these invasive properties)..."

(There was also an interesting article in the August, 1999 issue of "Discover" magazine about this) Blooms late summer. (Photographed in WI)