Cool Temperate weed barrier plant?

[Speedy]

You mentioned it in your opening post but have you given Vetiver a go?
it does well for me here.

[Pakanohida]

Yeah, I might give Lomandra a go too, and I'm keen to see what vetch can do here. I've never grown it before.

I might also use some canna lilies to the north as part of a small wind break - it dies down over the winter here too, but I have heaps of it.

The ground is generally too heavy for bamboos around here.

I'm thinking perhaps cannas at the back, followed by lomandra and then a front row of comfrey. for a nice little weed/windbreak.

I used to grow clumping bamboo in dense clay soil of northern california with no water ever given to them. They are very versital.