Butterfly bush

Light and exposure

Buddleia wants sun, as much as possible. Give it a spot in full sun, sheltered from prevailing winds if you can, though it copes very well with windy spots once well rooted. The less light it gets, the shorter and thinner its flower spikes.

In the garden, the foot of a wall that catches the sun for much of the day, or an open bank, suits it beautifully. It also does very well planted on its own in the middle of a lawn, as long as you give it room, a mature plant easily reaches two to three metres across in every direction.

Watering

Buddleia is not fussy about water. In the first year after planting, water it regularly to help it settle in, especially through dry summers. Once well rooted, it manages perfectly well on its own and copes with long dry spells without flinching.

In open ground, a good deep soak now and then always beats light, frequent watering, which encourages roots to stay near the surface. In a pot, which is fairly unusual for this shrub, keep an eye on the compost in summer, but above all avoid overwatering, which rots the roots far more reliably than drought ever tires the plant.

Soil and repotting

Buddleia is not fussy about soil, it grows just as happily in poor, stony ground as in ordinary garden soil. The one genuinely non-negotiable point is drainage, soil that stays waterlogged all winter will eventually rot the roots.

That tolerance is exactly what explains its spectacular success on railway embankments, wasteland and old walls, anywhere the soil is poor but well drained. In an ordinary garden, even fairly poor soil is more than enough, there is no need to over-enrich the planting hole.

Temperature and humidity

This is a fully hardy shrub, from USDA zone 5 to zone 9, tolerating frosts well below minus fifteen degrees once established. No protection is needed at all, even shrubs planted late in the season come through winter with no trouble whatsoever. In the coldest zones the top growth freezes off entirely and the plant comes again from the base, without the following summer’s flowering suffering for it.

The one point to watch is very young plants grown from cuttings or seed, planted too late in the season, they can be caught by frost in their first year if they have not had time to properly harden off. Past a year in the ground, the question never comes up again.

Pruning and upkeep

Pruning is the one thing that changes everything with buddleia, and it is also the thing most gardeners get wrong. Buddleia flowers only on the current year’s wood, the spikes form on shoots that grow out that same spring, not on last year’s old wood. So it needs a hard prune in late winter, once the hardest frosts have gone and before the buds break, cutting all the stems back to thirty or forty centimetres from the ground. That drastic cut, which looks brutal, is exactly what produces the longest, most generous spikes the following summer. A buddleia that is never pruned soon turns into a tangle of old wood with small, sparse flowers, all held high out of sight.

On top of that, deadhead the spent flower spikes right through summer, as soon as they turn brown. This encourages a fresh wave of flowering and, just as importantly, stops the plant setting seed, which matters a great deal anywhere the species is considered invasive.

Pests and diseases

Buddleia is one of the healthiest shrubs you can grow, it needs almost no watching for disease. Serious fungal or bacterial problems are practically unheard of on this species.

The one thing that does crop up now and then is red spider mite, especially in very hot, dry summers, they love the warmth and dry air around the foliage. Leaves speckled with tiny yellow dots and looking dull should put you on alert, a regular spray of water over the foliage in dry spells is usually enough to keep them away.

Propagation

Taking cuttings is by far the simplest and most reliable way to propagate a buddleia. Take semi-ripe cuttings fifteen to twenty centimetres long in late summer, from this year’s wood that has already firmed up a little, strip the lower leaves and plant them straight into open ground or a light potting mix. The strike rate is excellent, often close to one hundred per cent.

Sowing works well too, the plant produces an impressive quantity of tiny seeds that germinate easily, and that is exactly the mechanism behind its spontaneous spread in the wild. For that reason, it is better to propagate the varieties you love by cuttings rather than let the seed scatter at random.

Month-by-month care calendar

Green Hub tips

• Prune hard in late winter, once the hardest frosts have gone and before the buds break, cutting the stems back to thirty or forty centimetres from the ground, this drastic cut is what gives the longest spikes in summer.
• Deadhead the spent spikes right through summer, this prolongs flowering and above all stops the plant setting seed, which matters given how invasive it can be.
• Plant in full sun in well-drained soil, even poor soil, and water almost not at all once the shrub has been in the ground for a year.
• If you are planting a new buddleia, choose a sterile variety from a nursery, you keep the butterfly show without feeding the spread into wasteland and riverbanks.