This is because it can “spread by seed, with those seeds able to germinate in tricky places under pretty inhospitable conditions” – including on rooftops, which the notorious Japanese knotweed cannot reach.
Unlike the dreaded Japanese knotweed, though, buddleia is often planted deliberately for its looks and its ability to attract wildlife.
The Royal Society for the Protection of Birds (RSPB say that buddleia “is great at attracting various insects to feed on its nectar. This is why it has been planted widely in wildlife-friendly gardens and other open spaces, after being introduced from China.”
In fact, the Royal Horticultural Society offers tips on how to grow the (admittedly beautiful) bush online.
What does it look like?
The Wildlife Trusts’ description of the plant makes you understand why it’s so popular in British gardens.
A buddleia bush has “large, drooping spikes of densely clustered, small, purple (or sometimes white) flowers. It has long, narrow leaves and the flowers have a honey-like fragrance,” they say.
They add that “Its familiar purple flowers bloom from June to October and attract all kinds of butterflies and moths looking for nectar sources.“
Look, we never called it ugly, did we?
So, wait – how does this benign-looking bush affect my home?
The same way Japanese knotweed does – by planting roots in the less stable parts of your home’s foundation, rendering it less secure.
“It produces lots of small, light seeds, which spread extremely easily. It can grow in many places, even in cracks in buildings several floors up,” say the RSPB.
“Once established in a building wall or roof, the plant sends down, fibrous roots and can cause significant damage to buildings as it grows,” says Jonathan Barton, director of PBA Solutions.
And buddleia can grow in extremely harsh conditions. The Wildlife Trusts note that its winged seeds “find it easy to colonise stony ground.”
Always promising to read the word “colonise” in relation to a seemingly normal flower-y bush, right?
Oh, and side note – remember when we mentioned how good it is for butterflies and moths earlier? Yeah, well that might not even be a net good.
The RSPB say that “Yes, Buddleia davidii can attract many butterflies, but if it is at the expense of rare invertebrates that would otherwise be living there, it is preferable to plant non-invasive flowers for the butterflies to nectar at.”
You can’t always trust a pretty face...
What can I do if I see, or have, the plant?
Luckily, unlike Japanese knotweed, you don’t have to remove buddleia from your garden completely as the roots aren’t the problem.
But the RSPB advises you “prune it severely as soon as the flowers have faded – that means it doesn’t have a chance to spread its seeds.”
Once you’ve trimmed off the flowers, “Remember to dispose of the prunings properly, not by dumping them over the garden fence. And when your Buddleia davidii dies, consider replacing it with non-invasive shrubs,” they say.