I need a tall ground cover for Zone 3 and this plant I was informed does great for a ground in the Dakotas!! Need this...Judy G
I think your information may be a little faulty :-) Most sources list Perovskia hardy to only zone 5, possibly 4. And I would not consider it much of a groundcover. It is considered a subshrub in my area and doesn't really spread - a mature plant might reach 24-30" across, but that's it.
Nepeta, cat mint, may be a better choice - hardier, long bloom season, likes similar growing conditions and much more appropriate used as a groundcover. It spreads!