Dear friends, We are thinking of putting in Siskyou pink gaura in a bed dividing our lawn from our neighbor's to the east. One catalog states that they grow 4' tall; another, that they grow to 18". The first catalog states nothing as to their longevity; the second says that the plants blossom so profusely, that they need to be replaced every 3-4 years. Any ideas? Ron Van Nuys, CA