Wow! Great question, marcindy! It's one that I've been puzzling over all summer.

I have 2 Herbstsonne rudbeckias in my long, sunny garden. They're striking and get about 7 feet tall. One I've surrounded with Agastache Blue Fortune and I like the look of that, especially because the agastache covers the bottom part of the Herbstonne, which is mostly green leaves anyhow. I don't might the grayish blue tones.

I wouldn't use Russian Sage because, in my experience, it becomes rampant when planted in rich soil. My RS grows into and between everything and becomes difficult to pull out. Plus its bloom time is not as long as the Herb's.

I tried Verbena bonariensis but that didn't work either. So I'll be looking for everyone's suggestions to you!

Molie

While species Agastache have been reliable performers/reseeders in my zone, I've not found that any of the cultivars have returned after a season or two. I've grown any number of them from seed via winter sowing without seeing any success in their second year. Even the volunteer Verbena bonariensis that showed up in my garden a couple of years ago has disappeared.

Normally seed-grown winter sown perennials are hardier than nursery-grown plants but I haven't found Agastache particularly hardy compared to other perennials that are more zone-appropriate.

Wish I could be of more help. I love RS and have paired it with Gaura lindheimeri in my cottage garden bed but thanks to wet conditions it hasn't prospered. In my garden the RS either gets full sun + too much water or appropriate moisture + too little sun.

The zone thing may still be a problem but check out Tricyrtis hirta/toad lily. Mine are blooming now in both full shade and part sun--might work for you. Missouri Botanical Garden website rates it hardy to Z4. The flowers are speckled purple & white. Oh, and the bees love them.

I've seen many posts on this forum on how difficult it is to keep agastache going in cooler wetter climates. I have Black Adder and Blue Blazes and love them, but I live in the Southwest where agastaches are everywhere.

How about salvia? There are many beautiful deep purple/blues that would make a striking contrast with the heleniums and rudbeckia. I've grown East Friesland, Marcus, May Night. All are hardy to Z5.

Or lavender - Hidcote or Munstead are both striking deep blues. I grew Hidcote in Z5 and it came back reliably every year.

If a shrub would be acceptable, caryopteris Dark Knight is a vibrant deep blue.

None of these has as long a flowering season as RS, but all have deep color.

Cheryl