The coneflowers and black eyed susans can't reseed because you mulch heavily. I consider them hardy perennials; I'd expect the original plants to return this season but it's still early.

How about peony?

There are smaller varieties of Baptisia you could use- I have yellow, purple, and blue.

A gold or blue conifer - something with an interesting form.

Spring blooming- right now I have Kerria blooming, the double flowering are nice looking if you can tolerate the shade of 'yellow.'

I have Cercis blooming- one of the dwarf or weeping varieties would work.

Cornus florida are blooming, C. kousa and C. alternifolia are coming soon- I adore my Kousa and Pagoda dogwoods- look into all of their various varieties.

Also blooming right now are woodland phlox 'Sherwood Purple,' all my creeping Phlox, Bleeding Hearts, Veronica 'Georgia Blue,' some early Irises, ajuga, one variety of my catmints, and a couple of early salvia.

If I notice any others in my wanderings today I'll come back and mention them.

Don't forget about hostas- I have several enormous varieties in full blazing sun- as long as you water they do great. A huge variegated hosta like 'Sagae' or the bright golden/green of 'Sum and Substance' can be a real showstopper.

Don't forget about textures- you've got a lot of spikey upright things in the irises, so I'd look for some larger, flatter leaves as contrast. Geums are nice that way, as would be some hostas.

I took a few closer pics today, with me facing South. Please ignore the opening under my deck, my hot tub sprung a leak a few weeks ago so I've been working under there as time permitted :-/

But as you can kinda see, in the middle of the irises is a big empty patch. The canna lilies are in there, but they're only 2-3" tall right now. This section will be empty for awhile!

@indianagardengirl, I think peonies are beautiful! They only bloom for a week or so, though, don't they? If so, I'd definitely need something else for color, too, as I think they'd come up and go away at about the same time as the irises.

@cearbhaill (zone 6b Eastern Kentucky), I've never even heard of most of those! LOL I'm going to spend some time Googling them, thanks :-)

Penstemon varieties might be nice.

Azaleas or Rhododendrons? They would be a poor choice where I live but may be happy where you are? They are attractive bushes even when not in flower.

Yes, peonies flower for about a week to 10 days, weather dependent. And what a glorious week, sigh. Aren't irises only about 2 weeks? Anyway, they bloom mid to late May for me, and have handsome foliage the rest of the year, and seemed like they'd be the right height and texture for what you want.

I'm sure there's tons of perennials or shrubs in your zone that I don't have any experience with, so hopefully you'll find the right choice for you!

possibly monarda fistulosa

Wild Bergamot

Check out Blue Fortune or other Agastache for planting near the deck. Also, Whitewater Acanthus would add some interesting juxtaposition of textures & colors to your garden. Sunshine Blue Caryopteris would add contrast to your plantings & add late summer & fall color.

Plants that bloom around the same time as Irises include MIdnight Wine Weigela, dwarf Nikko Deutzia & Peonies, to name a few.