Plant Review: Buddleia ‘Black Knight’

by Andrew Keys on July 14, 2010

Ladies and gentlemen, Buddleia ‘Black Knight’ is in full bloom.

Buddleia ‘Black Knight’ was the first plant I ever bought for a garden, and a decade-plus later, when we purchased our first house, Buddleia ‘Black Knight’ was the first plant I sought out.

I have four now, three of which form the focal point of my front garden the second half of the growing season. I’ve grown other cultivars, but ‘Black Knight’ is by far the most vigorous, the most long-blooming, the most fragrant I know. I try to deadhead to keep the flowers coming, which they will do until frost — but I usually get distracted and let them go after a while. Last year I deadheaded once a week for a few weeks, but it was warm, and the Knights bloomed their heads off even after I stopped.

Buddleias‘ chief failings — aside from the fact they’re highly invasive elsewhere in the U.S. and U.K. — are their tenderness and lack of presence year-round. Where they’re marginal, I believe planting vigorous cultivars like ‘Black Knight,’ siting correctly, and mulching are key. I’m in Zone 6A, but in a lowland near a river where cool air tends to pool. My ‘Black Knights’ are planted on the warm southwestern side of my house. After frost, I leave their canes on until I get antsy in March, and then I chop them almost to the ground — and I mean it. Off with their heads! Last year I even tried leaving a bit, thinking I might have taller plants. They were having none of it and returned well below my cuts, so I pruned to where growth started. Oh, well. I guess I’ll have to settle for 7-foot plants. Darn.

But speaking of pruning, this is Buddleias‘ other problem: for a few months, after I prune them, there’s no there there. What to do? Gradually I’m mixing in other plants to pick up the slack. I planted what I hoped to be an earlier-blooming wishlist plant, Spiraea douglasii, in back (though it seems not to bloom as early as expected) and have begun to add earlier bits and bobs in front, like Baptisia Twilite Prairieblues™. So we’ll see.


  • COLOR, COLOR, COLOR: ‘Black Knight’ has a most excellent purple flower. Other than that, it’s a bit of a plain Jane green, so you’ll see this points to my personal affection for the plant.
  • SCENT: I don’t know what it smells like — Vanilla? — but I love it. I need to go smell it right now.
  • LIGHT: I grew Buddleia ‘Nahno Purple’ in part shade in the South, and transplanted one of the Knights to a part sun location this year, but generally Buddleias want all ur lights.
  • WATER: Established Buddleias are drought tolerant plants.
  • INVADERS! – As I said, Buddleias are a horrible invasive species in other parts of the U.S. and the world. Check local listings.
  • LOW-MAINTENANCE: Post-frost, cut the canes to a very low framework before new growth emerges in spring. If you feel like it, deadhead to prolong bloom. That’s it!
  • DESIGN TIP: That hard pruning will ensure your Buddleias have a lovely vase shape, and it’s nice to plant them where you can enjoy that from above when they’re in bloom, say near a second story window you pass regularly.
  • FAMILY TREE: Buddleias are in the gargly-sounding family Scrophulariaceae. Probably their best known relatives are mullein (Verbascum species). Fascinating!

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