Chris wrote:

I've heard they are terribly invasive ... and so perhaps caution might
be a good tip! Can anyone confirm?

My perenial sweet peas are perfectly well behaved, growing in now, will
disappear at about the same time normal sweet peas do. I've had no
problems with invasion or anything else.

Can't comment on size of root or root run, I've not tried to dig mine up.

Grown in heavy soil with clay underneath in North Wales.

Sarah