Buddleja "tree" in a pot

The cultivated varieties of buddleia aren't quite as domineering as the wild form. I've yet to see any buddliea I've ever grown seed itself anywhere. I often leave some seedheads for the birds, so there should be [theoretically] a good chance of some taking, but there never is. If I want more, I just take cuttings - those are extremely easy. I often use the cut back branches as supports for other plants, and they often root, so I have to keep an eye on them!
It's also quite different if you actively sow seed [of most plants] compared with something self seeding naturally, and it's why so many plants produce such huge quantities of seed. Just like frogspawn - they don't all make it.

Actively growing a cultivated type in a pot is always a bit hit and miss @Mrs. B , and will come down to the factors mentioned by @misterQ. A bigger pot and some heftier soil might help though

 

@fairygirl growing Buddleja from seed see here:Buddleja davidii seeds
@The Buddleja Garden produces the site I have linked to above. The website is very useful for anyone interested in buddlejas I have found very helpful over the years. Especially the annual diaries.

 

Yes, I've seen that site before @NigelJ . I'm not really bothered about trying to grow from seed though, I was just trying to explain the differences between actively growing, and letting nature take it's course, which are often quite different processes

I left quite alot of seedheads on last summer/autummn, so perhaps with the wind here, someone further down the road has a hundred of my buddleias growing in their garden!

 

@fairygirl mine originally just grew from a random seed, and every one I've ever left heads on has thrown saplings up all round the garden, lol!
At my last place, me and a friend decided to landscape the hideous sea of gravel & brambles, and we pulled about 150 small buddleias out. (The day we picked to have a big bonfire was a 30⁰ day..
and the day we picked to lay the turf, there was a monsoon!!) I digress...
I'll see what happens but it's not as if mine is a prized specimen that can't get planted in a field with the goat willow.

 

This is why location is a factor @Mrs. B. The seed would struggle to be viable and then germinate and grow over winter conditions here.
People often moan about the amount of Verbena bonariensis they get seeding around. They're a rarity here for self seeding. If I get any, it's when they seed underneath another plant or similar. The adult plants don't always survive, let alone seedlings. I've only got two plants still alive, and that's after a very mild winter here.