Buddleia bloom

Buddleia is a very free seeding plant. I have one just the other side of my garden wall. Its on the side of an unadopted lane and owned by no-one, totally uncared for but flowers year after year forming a lovely screen in summer. My neighbour on the other side has an offshoot from this plant which has seeded naturally but is now a feature in his garden and treated as any other plant in that location. This is an early flowerer which is long past its prime but the seed heads are being enjoyed by the birds.
In the back garden my neighbour has a different variety which is still in full flower at this time of year. This blip is a bloom from an offshoot of this plant which has seeded itself in old pot in my garden. I am undecided as to whether to keep it or just let it do its thing this year then reclaim the pot.