Buddleja is invasive from seed, not from roots. B. davidii can produce millions of tiny, lightweight, winged, wind-distributed seed that can be broadcast far and wide by a windstorm. One flower cluster can produce upward of 40.000 seed.
Luckily, from what I have read, seed produced during summer are not released until the following spring, so trimming the plant back hard in the fall not only encourages heavy bloom the following year (they bloom on new wood), but effectively controls their invasiveness (as long as the clippings are properly disposed of).
I understand there is a wide variation in the fertility among different cultivars. I don't know about the 'Honeycomb' variety. I know it is a hybrid, and I remember some species have no wings on their seed, and seed of some hybrids have single or deformed wings and are consequently relatively non-invasive except possibly in a localized area.
I have two dwarf "seedless" varieties, the 'Tutti Frutti Pink' and the 'Heavenly Blue', and they do have a nice fragrance. For a while I couldn't decide which of the two was the more fragrant. Interestingly, I finally realized the most fragrant one was whichever one I smelled first. Apparently some desensitization took place quickly, such that my first sniff smelled really nice, but after that the fragrance seemed really faint.
