It's no risk to the foundation unless the foundation is not structurally sound (ie., already cracked). But I have seen wisteria take down an entire deck so those stairs will be a piece of cake for it to demolish. It is not so much a matter of allowing the plant to grow large - although it will and extremely rapidly if in a good setting - it is the torquing pressure of the twisting stem it uses to vine/climb with. You need a very sturdy support to resist this pressure - like a 6"x6" upright set in concrete. I've seen wisteria take out a steel clothesline pole (set in concrete) and a wooden fence as well. This is a very strong and very heavy plant.

I hope your wisteria is a grafted form and not seed grown. Seed grown wisteria can take MANY years before blooming - as much as 10-20 years.

Interesting, thank you. The wisteria has been growing wild here for years so not from seeds. (Not from me at least.) I transplanted some I found elsewhere around here.

How do people have it on their houses? We had wisteria growing on a trellis attached to the house in the house I grew up in and it didn't cause any structural problems. (Alas, it only bloomed when we moved in in 1958 and never since then... And we lived there for 30 years...)

The stairs may well have the posts set in concrete. And those posts look pretty solid to me. But maybe I shouldn't do this...

Very bad idea to grow it on those wooden stairs! In addition to the issues gardengal raises, once the stems wrap around the wooden railings and open stair risers and the stems get thicker as they age a few years, you'd never be able to unwrap them to paint or do other maintenance on the stairs. Also, the foliage will get very dense and can act like a sail in the wind, adding stress to the stair structure. A few years ago our young Japanese wisteria, grown as a small tree, blew over on a windy day, taking down its (very inadequate) wooden support with it! Luckily, we knew its original support was a temporary situation so there was no damage done, but it did highlight how easy it would be for a wisteria to cause damage in a situation like the one in your picture. Do not grow it there! I have both a Japanese and a Chinese wisteria 'tree' and I love them - BUT I'm also aware of the strength and vigorous they have so don't treat them carelessly/lightly!

OK. I'm getting the message. I do wonder though, how do people grow it on trellises on their houses. How does this place, the Deer Head Inn, an old building, survive with all that wisteria growing all around it for so long? (Deer Head Inn pic) How did our house survive? (That's wisteria on either side of the front door. It later moved onto the trellis across the top of the door. Never caused any problems. We probably pruned it often...)

It takes a lot of attention to pruning to grow it like that - especially on a wooden structure that needs painting regularly! A tenant growing it on a rental property is a different situation. The landlady would be concerned about what happens if the next tenant has no interest in maintaining the plant, and what happens when those stairs need painting, or the wood rots and needs replacing....?

Good points, though she's told me she doesn't want to be renting it after I leave. I am happy to prune it...