hello. I'm new to gardening and have a question. I'm looking for a purple colored background perennial for some of my flower beds. The intent is for them to cover the view of the retaining wall, and also to provide a nice background for some colorful annuals. The plant should be about 3-4' tall and should not expand through rhizomes or vigorous self seeding since we don't want it to intrude on the annuals. I also want a perennial that will live for many years.

it seems as if Lavender would be a good choice, expect that it is apparently "woody" and I don't like the idea of woody plants since I feel that they get too established and it's too hard to dig them up if I change my mind. I just got rid of a whole bunch of woody weeds and it was quite a battle and I still wasn't able to dig them all up. So I'm trying to avoid creating a future problem like that..

I don't know if blazingstar or hoary vervain or salvia or Russian sage would be better choices or if there are other great options. The area faces West and gets good afternoon sun The 3' retaining wall will provide shade in the morning.

Looking forward to your suggestions. I live in zone 6.

I'm partial to Nepeta 'Walker's Low', it's hardy, can be clipped to shape after flowering and will flower again to a limited extent. It makes a nice foil for other plants. It may not be quite as tall as you would like but it's not an aggressive spreader or self-seeder and may top out at about 3ft.

There are also some purple leaved hardy hibiscus plants that would work too. They will reach your desired height and die back to the ground every year. Here are some varieties with the purple leaves. The blooms will be red or pink which may not fit what you're looking for but thought I'd throw it out there.
Summer Storm
Midnight Marvel
Kopper King

Salvias also make great plants along with veronica and if you can get them to grow some lavender colored lupines.

I'm not a buddleia (butterfly bush) fan, but I think there's some smaller ones on the market that might work for that area.