The butterfly bush grows quickly, blooms abundantly and attracts peacock butterflies and red admirals like no other. But anyone who doesn’t intervene annually ends up with a tall bare shrub with flowers at two metres’ height. And anyone who leaves the spent flower spikes standing will find seedlings all over the garden.
Pruning: rigorous and in March
Buddleja davidii blooms on new wood. This means that every shoot that grows this summer will carry flower spikes in July and August. Anyone who prunes the shrub cautiously gets more branches but smaller flowers. Anyone who prunes back hard gets fewer but more vigorous shoots with larger spikes.
Prune in late March (as in the Dutch maritime climate, and suitable for most of Britain and Ireland), when the worst frost has passed but the buds have not yet broken. Saw all last year’s branches back to a low framework of 30 to 50 centimetres above the ground. Leave two to three buds on each side branch; these will produce the new flowering shoots. A mature butterfly bush consists of six to ten main branches that remain for several years, with the annual flowering branches on them.
Anyone who doesn’t dare prune this short ends up after a few years with a shrub two to three metres tall that only blooms at the tips. The bottom metre remains bare. It feels counterintuitive to remove so much wood, but the plant recovers within six weeks.
Location and soil
Place the butterfly bush in a spot with sun. In partial shade the shrub becomes taller and floppier, and falls over after rain. Full sun gives compact growth and the most flowers.
The soil can be almost anything: clay, sand, loam, calcareous or slightly acid. Buddleja also grows on waste ground, railway embankments and in the rubble of a demolished wall. What it cannot tolerate is prolonged wet soil in winter. On heavy clay that is waterlogged in winter, plants regularly die from root rot.
Heijnen-planten writes about Buddleja davidii ‘Black Knight’ that the shrub does well after making new shoots, and experience confirms this: after spring pruning the plant quickly gets going.
Water and feeding
An established butterfly bush rarely needs extra water. Only in a very dry summer, when the leaves start to droop, do you give it a thorough watering. Watering young plants in the first year during drought helps them root well.
Feeding is not necessary. The shrub grows abundantly even without feeding. Anyone who does feed does so after pruning in March with compost or a handful of organic fertiliser around the base. Too much nitrogen produces long floppy branches that blow over.
Winter hardiness
Buddleja davidii is winter-hardy throughout the Netherlands. Severe frost can kill the above-ground branches, but the rootstock shoots again in spring. Because you prune the shrub short annually anyway, it makes little difference if winter has killed part of the branches.
Only very young plants, planted in autumn, need protection in the first season with a layer of leaves or straw around the base. After that this is no longer necessary.
Why the butterfly bush spreads invasively
Each flower spike carries hundreds of seeds that ripen in September and October. The wind disperses them over tens of metres. They land on bare ground, in cracks between paving and along fences and germinate the following spring. Within three years there’s a flowering shrub.
In urban areas, on waste ground and along railway lines, Buddleja davidii forms dense thickets that displace native vegetation. The plant is therefore on the invasive exotic species list in Belgium.
In the garden you solve it simply: cut off the spent spikes before the seeds ripen. This also encourages a second, smaller flowering in September. Seedlings that do appear can be pulled out while they’re small, or chopped off with a hoe. The root remains shallow if you intervene in time.
Common mistakes
Pruning too late. Anyone who prunes in May or June removes flowering branches that have already set buds. Flowering then comes weeks later or fails completely. Stick to March and everything goes well.
Pruning too cautiously. A butterfly bush that is only topped grows half a metre taller each year. After five years the flowering is so high that you can no longer reach it to cut off spent spikes.
Forgetting to deadhead. If you leave the spent spikes standing, you get two problems: the plant makes seeds instead of a second flowering, and next year you’ll be weeding out seedlings everywhere.
Planting on wet ground. In a hollow where rainwater accumulates, the rootstock will rot in winter. Choose a spot where water drains away or plant on a raised area.
Cultivars and colours
Besides the wild species with lilac flowers, there are dozens of cultivars in white (‘White Profusion’), dark purple (‘Black Knight’), pink (‘Pink Delight’) and blue-purple (‘Empire Blue’). All are pruned in the same way: hard back in March.
The dwarf forms such as the ‘Buzz’ series remain more compact but follow the same principle: flowering on new wood, so prune short for the best flowering.
Practical tip
Every garden is different. Soil type, location and the specific cultivar can make a difference in growth rate and flowering time. For questions about toxicity of plants to children or pets, it’s better to consult the National Poisons Information Centre or a veterinarian than to rely on general articles.