Buddleja davidii

South Australia's Plant Selector gives full sun, clay, loam or sand, acid to alkaline, and moderate supplementary watering, and ticks drought among what it tolerates. The Missouri Botanical Garden says it does poorly in wet, poorly drained ground and becomes sparse and flowers less out of full sun. HortFlora calls it deciduous; Weeds of Australia says it may lose some of its leaves in winter and calls it semi-deciduous. North Carolina State Extension says it flowers on new growth and is best pruned in early spring, and the Plant Selector that it can become straggly if not maintained. American references report spider mites. North Carolina State Extension gives seed or stem cuttings for propagation and USDA zones 5 to 9.

The Plant Selector lists it for habitat and ornament and calls it a great plant for attracting butterflies, and the Missouri Botanical Garden adds bees and a fresh cut flower. Missouri adds that it usually does not make a good single specimen shrub. Growers mostly list it by cultivar, and HortFlora treats several of the names on those lists, among them Black Knight, Royal Red, Nanho Blue, Harlequin, Variegata and White Profusion. Weeds of Australia names other buddleias that have recently naturalised in Australia.

The weed standing is the thing to know: Weeds of Australia regards it as a significant environmental weed in Victoria and an environmental weed in Tasmania and New South Wales, and says it spreads by seed carried by water, wind, vehicles, animals and dumped garden waste. The table above shows the sizes growers list. Stock that crosses a state border moves under the state and territory biosecurity rules.

Weeds of Australia, a Queensland Government fact sheet, says the flowers and fruit are potentially poisonous to humans, that dense stands can block access to rivers, and that the shallow roots can let stream banks erode in floods. It records no declaration by any state government authority. The Australian Plant Census lists it as naturalised in Western Australia, South Australia, New South Wales, the ACT, Victoria and Tasmania, and as doubtfully naturalised in Queensland.