Acidic fertilizer will not work on your Buddleias.....but the better the soil, the better the plant.....which may give you better looking flowers.

Specimens pruned to a low framework at end of winter produce fewer stems with much larger inflorescences. Other than that, differences in size will be due to how well an individual specimen is doing and how large a particular variety makes them. 'Black Knight' probably falls into the small-medium range.

silverneedle, just out of curiosity, why are you contemplating buying this particular butterfly bush? I don't know what part of the country you live in, but butterfly bushes have become so popular in recent years with such a profusion of outstanding new cultivars that in most places south of Zone 4 or 5 one can't hardly walk five feet at a garden center without bumping into some kind of butterfly bush or other. And a lot of them are big-ish plants with blue-ish flowers. So there doesn't appear to be anything special about the one at Gurney's, and I don't see why you would choose to purchase an unnamed variety from a mail-order nursery that's mediocre at best, and borderline disreputable. Frankly, if it were me, and I was going to buy a blue butterfly bush through the mail, I'd rather pay more for a nice 'Blue Chip' from Plant Delights than a get-what-you-pay-for special from Gurney's. But it's up to you. As for its identity, obviously it's hard to say for sure from their description and photo, and that may be a red flag in and of itself. There are so many named varieties of buddleia nowadays that if they're not even bothering to promote a cultivar name for it, that ought to tell you something. wonbyherwitz mentioned a couple of the nicer blue-flowered butterfly bush varieties in the trade, and I would add a plug for my personal favorite, which is one called 'Ellen's Blue.' And for all we know, the ones that Gurney's is selling could be one of these varieties, but it's not clear. For what it's worth, from my experience in the nursery industry I would think it's unlikely that Gurney's is selling variable seedling butterfly bushes, as clematisintegrifolia suggested. Buddleias can definitely be grown from seed, but at least east of the Rockies, that's not usually the most efficient way of mass-producing them in a nursery setting. Their stem cuttings root so readily and grow so rapidly that that's almost always how they're grown except for breeding situations. So, rather than seedlings, I would surmise that Gurney's may have one or more big, old blue-flowered butterfly bushes that they use as stock plants and take a bazillion cuttings from every year. It could very well be that those old plants either don't have a cultivar name, or it's long-forgotten. So what they're selling may still be asexually propagated, but just not a named variety. But it's hard to know for sure. Have you tried calling Gurney's and asking them the identity of their butterfly bushes?

Have you seen Butterflies nectaring from them at lower height? I've had them in the back of my mind since I don't have space in sun for the larger ones including the "dwarf" English series. I have one true white dwarf in a container that's doing very well. Blooms were small but plentiful last year. Regular sized red one that I put in large container last year only has new growth on one side. It was a cheap roadside stand purchase just to have blossoms for Butterflies. If the Blue Chip is successful for drawing in Butterflies it would be a boon for me. I like to see photos from "real life" in addition to catalogue photos so I can get a better idea of what things really look like when planted. Thanks.