Black Knight Butterfly Bush

Hardy in zones 5 to 9, 'Black Knight' is happiest in full sun and average, well-drained soil. It is not fussy about fertility, but sharp drainage — especially over winter — is the single most important thing for a long-lived plant.

Planting

1. Choose a spot with full sun (six or more hours); bloom and color fall off quickly in shade.
2. Make sure the site drains well. Butterfly bush resents wet feet, so avoid low spots that stay soggy.
3. Dig a hole as deep as the root ball and about twice as wide to loosen the surrounding soil.
4. Set the plant so the root flare sits level with the surrounding grade, then backfill and water in thoroughly to settle the soil.
5. Apply a 2 to 3 inch layer of mulch to hold moisture, keeping it pulled back a couple of inches from the stems.

Care & maintenance

• Water. Keep evenly moist through the first growing season while roots establish; afterward it is notably drought tolerant and only needs water in extended dry spells.
• Feed. A light feeding with a balanced, all-purpose fertilizer in early spring is plenty. Avoid over-feeding, which pushes soft leafy growth at the expense of flowers.
• Light. Full sun all day produces the most spikes and the deepest color.
• Prune. Butterfly bush blooms on new wood, so cut it back hard — to roughly 12 inches from the ground — in late winter or early spring before new growth begins. This drives vigorous stems and abundant flowers.
• Mulch & winter care. Maintain mulch year-round. At the colder end of its range (zone 5), leave the old stems standing through winter and prune in spring; a winter mulch over the crown adds insurance.
• Pests & disease. Generally trouble-free. Watch for spider mites in hot, dry weather and the occasional aphid; good air circulation and well-drained soil prevent most root and foliage problems.
• Bloom tips. Deadhead spent flower spikes through the season to encourage continuous rebloom and keep the plant tidy.

When does 'Black Knight' bloom, and for how long?

It flowers from summer into fall. Because it blooms on new growth and reblooms readily, removing spent spikes keeps fresh blue-purple plumes coming for months, often right up until the first frost.

Why isn't my butterfly bush blooming?

The usual causes are too little sun, too much nitrogen fertilizer (which favors leaves over flowers), or pruning at the wrong time. Since 'Black Knight' blooms on new wood, cutting it back in late winter or early spring actually increases flowering — just make sure it gets full sun and go easy on the feed.

When and how should I prune it?

Prune in late winter or early spring, before new growth starts. Cut the whole plant back hard, to about 12 inches from the ground. It blooms on the season's new stems, so this hard cut produces a fuller, more floriferous plant.

Does it need full sun or will it take shade?

Give it full sun — at least six hours a day. In shade you get leggy growth, fewer spikes, and weaker color. A hot, bright, well-drained spot is ideal.

Is it deer resistant and good for pollinators?

Yes on both counts. The aromatic foliage is usually left alone by deer, while the honey-scented, nectar-rich flowers are a powerful draw for butterflies, bees, and hummingbirds — which is exactly what earns the plant its name.

Can I grow it as a hedge or in a container?

At 4 to 6 feet tall and wide with a fast growth rate, 'Black Knight' makes a lovely informal flowering hedge or screen when spaced a few feet apart. It can also grow in a large container with excellent drainage; just be ready to water more often, since pots dry out faster than open ground.