Black Knight butterfly bush

Sun

Full sun; aim for at least six hours of direct sunlight daily.

Water

Water regularly during the first growing season. Once established, water deeply during prolonged dry spells; avoid persistently wet soil.

Soil

Well-drained soil, from sandy to loamy or improved clay; tolerates a broad pH range. Winter drainage is especially important.

Height

6-10 ft (1.8-3 m)

Spread

4-8 ft (1.2-2.4 m)

Flowers

Summer into autumn; long, tapering spikes of deep violet-purple flowers with small orange eyes.

Foliage

Opposite, narrow, pointed grey-green leaves with pale, softly hairy undersides; usually deciduous, sometimes partly retained in mild winters.

Growth

Fast; often makes several feet of new growth in a season.

Care needed

Medium

• Pollinators
• Deer resistant
• Drought tolerant
• Heat tolerant
• Low allergen
• Fragrant

Northern hemisphere months. In Australia, shift them by six.

Difficulty

Easy

Toughness

Very tough

Pick your zone, country, light and whether you have pets.

1. Planting day

   1.6 ft (0.5 m)

2. After 1 year

   4 ft (1.1 m)

3. After 2 years

   5 ft (1.7 m)

4. After 3 years

   7 ft (2.3 m)

5. After 5 years

   8 ft (2.4 m)

   full size

About 3 years to full size.

From a typical nursery size, at an average rate for this kind of plant. A rule of thumb, not a forecast.

Cats and dogs

Safe

Invasive in

the US, Canada, the UK and Australia

In the garden

Spreads vigorously: plan to contain it.

Growth rate

Fast

• Water deeply through the first growing season, allowing the soil surface to dry between waterings.
• Plant where water drains freely; avoid hollows that stay wet through winter.
• Apply a thin layer of compost in spring; rich feeding usually produces excessive leafy growth.
• Mulch the root area, keeping mulch clear of the woody crown.
• Remove faded flower spikes before seeds mature, and pull out seedlings promptly.

Pruning. In early to mid-spring, after the worst frosts, cut the previous year's growth back to a low framework about 1-2 ft (0.3-0.6 m) high, cutting just above healthy buds. Remove dead stems once new growth shows which wood has survived; avoid hard autumn pruning in cold climates.

Early spring

Check for emerging buds and prune once the worst frosts have passed.

Spring

Plant in a sunny, well-drained position; water in thoroughly and mulch.

Late spring

Remove dead wood that has failed to sprout and check nearby ground for seedlings.

Summer

Water young plants during dry weather and remove fading flower spikes.

Late summer

Take semi-ripe cuttings if you want plants identical to the parent.

Autumn

Remove remaining seedheads before they shed seed; leave major pruning until spring.

• Place toward the back of a sunny mixed border, allowing room for its arching branches.
• Pair with silver foliage and pale flowers to make the dark purple flower spikes stand out.
• Use as a summer-flowering focal shrub in a gravel garden with good drainage.
• Where it is not invasive, include in a nectar border alongside plants that feed butterfly caterpillars.

Garden styles it suits: Cottage garden, English country, Pollinator garden, Sensory garden, Dry gravel garden and Blues and silvers.

Plant it with: Lavender, Catmint, Russian sage, Yarrow, Purple coneflower and Lamb's ears.

Wildlife: Flowers supply nectar for many adult butterflies, bees and other insects. It is not a significant caterpillar food plant for most native butterflies, so include locally appropriate larval host plants as well.

• Winter dieback: exposed stems may die in cold weather; wait for spring growth, then remove dead wood. Protect the root area with mulch without burying the crown.
• Root rot in wet soil: improve drainage or move the plant to a raised position; do not keep watering saturated ground.
• Spider mites in hot, dry weather: look for stippled leaves and fine webbing; rinse leaf undersides and avoid broad-spectrum insecticides.
• Unwanted seedlings: remove flower spikes before seed ripens and pull seedlings while small; deadheading reduces spread but does not make the plant sterile.

Not known to be toxic to cats, dogs, horses or people; no part is recognized as a significant poisoning hazard. It is an ornamental, not a food plant, and eating plant material may still cause stomach upset.

Where it spreads. Buddleja davidii spreads aggressively on disturbed ground, riverbanks and railway land in the Pacific Northwest and parts of the eastern United States, southwestern British Columbia, the United Kingdom, and parts of southeastern Australia, including Victoria and Tasmania. It is subject to restrictions in Oregon and Washington. 'Black Knight' produces viable seed and is not a sterile alternative;

• Alternate-leaved butterfly bush (Buddleja alternifolia)

  Has alternate rather than opposite leaves and small lilac flower clusters along arching, previous-year stems in late spring or early summer.

• Orange ball tree (Buddleja globosa)

  Produces round orange flower heads rather than tapering purple spikes, usually in late spring or early summer.

Buddleja honours the English botanist Adam Buddle, davidii honours the naturalist Armand David, and 'Black Knight' refers to the cultivar's unusually dark flowers.

Propagation. Take softwood cuttings in late spring or semi-ripe cuttings in summer, rooting them in a free-draining medium under humid conditions. Seedlings do not reliably retain the cultivar's dark flowers, so use cuttings to reproduce 'Black Knight'; do not propagate where local restrictions prohibit it.

Accepted botanical name

Buddleja davidii

In the US and Canada

Introduced in Canada, Hawaii, the lower 48 states and Puerto Rico.

Native range

China North-Central, China South-Central, China Southeast and Tibet.

Measured

Soil pH 7 to 7.5. Average height 300 cm.

Sources: GBIF (CC BY 4.0), USDA PLANTS and Trefle (CC BY 4.0). The care and design notes are LandscapeGPT's own.