Getting it to bloom

Why won't my Butterfly Bush 'Black Knight' bloom? (and how to make it flower)

Also called Butterfly Bush, Summer Lilac (Buddleja davidii 'Black Knight').

More about butterfly bush 'black knight'

About Butterfly Bush 'Black Knight'

Buddleja davidii 'Black Knight' · also called Butterfly Bush, Summer Lilac · flowering

'Black Knight' is a vigorous deciduous butterfly bush carrying long, arching panicles of deep violet-purple, honey-scented flowers from midsummer into autumn. A magnet for butterflies and bees, it thrives in full sun and ordinary well-drained soil, tolerates drought once established, and blooms hardest after a firm spring prune.

Plant type: flowering

Watch for — Failure to bloom: Usually too much shade or skipped pruning. Site in full sun and cut hard in early spring, since flowers form only on new wood.

The reasons butterfly bush 'black knight' isn't blooming

Almost every non-blooming butterfly bush 'black knight' traces back to one of these, roughly in order of how common they are:

Too little sun — most of these need full sun (or very bright light) to flower well; shade gives leaves, not blooms.
Too much nitrogen feed, driving lush foliage at the expense of flowers (very common with general or lawn feeds).
The plant has not been deadheaded, so it stops flowering once it sets seed.
Irregular watering — drought or waterlogging at the budding stage makes buds abort.
It is still too young or was checked by a transplant and is rebuilding before flowering.

Feeding butterfly bush 'black knight' a high-nitrogen general feed and growing it in too little sun — you get a big leafy plant and almost no flowers.

The fix — how to get butterfly bush 'black knight' to flower

Maximise sun. Give butterfly bush 'black knight' the sunniest spot you have — for most bedding and fruiting plants, more direct light directly means more flowers.
Switch the feed. Move off high-nitrogen feeds and use a higher-potassium "bloom" or tomato-type feed as it comes into flower.
Deadhead regularly. Remove spent flowers often to keep it producing more rather than stopping to set seed.
Water consistently. Keep moisture even through budding and flowering — drought-then-flood swings make buds drop.

Light and feeding do most of the heavy lifting here. Dial in the spot with the light guide for butterfly bush 'black knight' and get the feeding right with the butterfly bush 'black knight' fertilising schedule — the wrong feed (too much nitrogen) is one of the most common silent reasons a healthy plant makes leaves instead of flowers.

Bloom season and what to expect

Butterfly Bush 'Black Knight' flowers across its growing season (mostly summer) and, kept fed and deadheaded, can bloom for many weeks or right up to frost.

Post-bloom care so it flowers again

Deadhead, keep feeding lightly, and many will rebloom; collect seed from the best plants at the end of the season if you want to grow them again.

For everything else this plant needs day to day, see the full butterfly bush 'black knight' care brief and its watering schedule — a stressed, badly watered plant rarely has the energy to flower at all.

Butterfly Bush 'Black Knight' blooming — frequently asked questions

Why won't my butterfly bush 'black knight' flower?

Butterfly Bush 'Black Knight' blooms on the season's growth given enough sun, warmth and the right feed — there is no cold or photoperiod trick, just good growing conditions and a bloom-leaning feed. The most common reason it is not happening: Too little sun — most of these need full sun (or very bright light) to flower well; shade gives leaves, not blooms.

How do I make butterfly bush 'black knight' bloom?

Give butterfly bush 'black knight' the sunniest spot you have — for most bedding and fruiting plants, more direct light directly means more flowers. Move off high-nitrogen feeds and use a higher-potassium "bloom" or tomato-type feed as it comes into flower.

When does butterfly bush 'black knight' normally bloom?