Butterfly bush

Identification and origin

Buddleja davidii belongs to the Scrophulariaceae, the figwort family. It is native to China, from the center of the country to the southeast, and to Tibet. The name honors the missionary and naturalist Armand David (1826-1900).

It is a deciduous to semi-evergreen, bushy shrub with long, arching stems that reaches mature size in one or two growing seasons. In gardens it usually grows 6–12 ft tall and 4–15 ft wide. The young stems are square in cross-section and covered in white hairs. The leaves are opposite, lance-shaped, and pointed, up to 10 in long; dark green to gray-green on top, they are white and woolly underneath.

The tiny, tubular, honey-scented flowers are lilac with an orange throat in the species. They are packed into dense spikes at the tips of the current year’s shoots, 4–12 in long. Bloom begins in summer and continues into fall if spent spikes are removed. Rich in nectar, the flowers attract butterflies, bees, and other pollinators; native butterfly caterpillars, however, cannot feed on the foliage.

Cultivars vary in size and in flower and leaf color: ‘Black Knight’, with very dark purple flowers; ‘Royal Red’, magenta-purple; ‘White Profusion’, white and very hardy; ‘Pink Delight’, true pink over silvery foliage; ‘Nanho Purple’ and ‘Nanho Blue’, more compact; ‘Harlequin’, with cream-edged variegated leaves. Dwarf series such as Buzz stay around 3.5–4 ft.

Light

Plant it in full sun, with at least six hours of direct sun a day: that is essential for heavy bloom. It tolerates part shade but flowers less there. Avoid very windy sites and deep shade: the long branches can snap in summer storms, and wind rocking the shrub loosens its roots.

Watering

After planting, keep the soil moist but not soggy for about eighteen months while the shrub establishes. Check by touch, especially in dry weather: the soil should neither dry out completely nor stay waterlogged. Once established, butterfly bush is drought tolerant and generally needs no extra watering. During long dry spells, though, watch for spider mites.

In containers, do not let the mix dry out, and set the pot on feet to help it drain. In every case, excess water is the enemy: roots rot in cold, waterlogged soil in winter.

Potting mix

It adapts to almost any soil: clay, loam, sand, chalk, or shallow rocky ground, acidic to alkaline, with a preference for a pH of 6 to 7. The one essential is good drainage. Amend heavy clay, or plant on a mound or in a raised bed, or even at the foot of a house wall. Each year, from late fall to late winter, spread an organic mulch at least 2 in deep to hold moisture.

On fertile soil, no fertilizer is needed: the mulch of compost or well-rotted manure is enough. Poor soils call for moderate feeding.

Temperature

It is hardy in USDA zones 5a to 9b and withstands lows of -4–5 °F. In the coldest parts of that range, stems often die back to the ground, but the crown resprouts vigorously in spring. Late frosts can damage young leaves. It tolerates heat, humidity, coastal sites, and salt. It is not a houseplant.

Repotting

In the ground, it can be planted at any time of year, though spring is the best season. Dig a hole as deep as the root ball and two to three times as wide, mix compost into the soil you removed, then firm it in and water well.

Dwarf varieties do well in containers, in a peat-free potting mix blended with loam-based compost, plus slow-release fertilizer for the first summer. Repot them every two to three years; they eventually need a container about 2 ft wide. After that, give slow-release fertilizer each year, or liquid feed during the growing season.

Pruning

It blooms on new growth, so a hard annual prune gives the best display. Cut all stems back to about 12 in above the ground, to a low framework, in late winter or very early spring, before new growth starts. Avoid pruning in fall or winter, which raises the risk of cold damage. Pruned this way, the shrub stays around 6–8 ft.

Cut off spent spikes before seeds form: the shrub looks neater, bloom lasts longer, a second flush of flowers can follow, and you prevent unwanted seedlings. Because cut stems can resprout, do not leave prunings lying on the ground.

Propagation

Cuttings are the usual method and root readily. Take softwood cuttings in early summer, or semi-ripe cuttings from midsummer on. Hardwood cuttings can be taken from fall to midwinter; they produce plants ready to set out after about two years.

Seed works too, but the shrub already reseeds heavily on its own: a single spike can produce up to 40,000 fine, winged seeds, spread by wind and water, that stay viable in the soil for three to five years. Pull up unwanted volunteer seedlings.

Common problems

• Self-seeding and invasive behavior: it escapes from gardens onto vacant lots and railway banks and crowds out native plants. It is listed as invasive in several regions, and Oregon banned the sale of unapproved selections, except sterile cultivars. Deadhead, or choose sterile or low-seeding varieties such as ‘Miss Ruby’, ‘Miss Molly’, or the Lo & Behold series.
• Spider mites: they build up mainly on drought-stressed plants in dry weather.
• Aphids: curled, puckered leaves with pale or yellowish patches.
• Other pests: capsid bugs, caterpillars, weevils, and nematodes, especially in sandy soils.
• Downy mildew: in cool weather, when foliage stays wet for long periods.
• Root rot: waterlogged soil, especially in winter.
• Honey fungus, leaf spot, and viruses: less common.

The same symptom can have several causes: always consider exposure, drainage, watering, and recent weather. Avoid insecticides, especially during bloom, to protect beneficial insects.

Toxicity

No toxicity has been reported for dogs, cats, or people. The foliage can, however, irritate sensitive skin, so wear gloves when pruning.