Any plant that needs stratification would be a good indicator for your area. I'm a bit north of you in Ohio but depending on your microclimate, you and I can go between a 5 or 6.

Tulips and other bulb like hyacinth and daffodils some to mind. Deciduous trees and bushes maybe.

Everything you've listed sounds like hardy perennials that can take the cold inout area and infact need it to thrive.

Does that help?

There really isn't any such thing as an indicator plant for hardiness zone. The reason is that USDA hardiness zones measure only one thing and that is the average coldest temperature in a year.

Individual plants of the same type vary in their cold hardiness and each year is different. One year your area might set a cold record, but you have 3 feet of snow on the ground(an insulator) and most things survive. Next year you have a mild winter, but no snow and lots of things do not survive.

Just consider yourself to be a zone 5/6. I consider myself a zone 4/5 since it varies on the year. I guess I could consider myself a zone 4/5/6 given the real temps year by year.

I see where you are going with your question but all of the plants you listed are hardy to my zone or lower and reseed just fine. Something like a Canna would be something you could use. It will not over-winter in the ground in zone 5 but can be left (with mulch) in zone 6.

There are a zillion other plants like that but I need to have much more coffee before I can think what they are. ;o)