Sprinkle diatomaceous earth powder at the base of your plant. The D-earth particles are sharp and will cut the soft body of the snail or slug to a fatal degree. It will also do the same thing to the exoskeleton of insects at their joints.
Also, you can go out at night with an led headlamp and hand pick them off your wisteria as best as you can. We have more of a problem with slugs in the USA. As a matter of fact the first time I had ever seen land snails was in a field in Torremolinos Spain, hundreds of them and to this day not a one in my country.

Besides the usual coffee grounds and egg shells, if you don't want to go out at night hunting snails, place a flower pot saucer upside down and slightly elevated. They go there to hide for the day and are easy to collect.
You are lucky, Loosestrife. It is warm enough here that they don't disappear in the winter, so snail & slug control is a 365 day job.