Depending on where you are and what the surface of your perennial bed is like, you may get some pretty good expansion of your current plantings next year. So keep that in mind when you interplant some new plants. I'd love to see a pic of the area, then we can help you better with ideas. Are you interested in some grasses? Do you have some things blooming early?

Maybe some taller narrow perennial sunflowers/coreopsis, Liatris are pretty and narrow.

I'm in Connecticut, zone six here's what I have in my phone I'll try and post some more tonight. And just envision a spindly looking Russian sage next to the agastache

Verbena bonariensis comes to mind. These tend to reseed, how heavily depends on your zone.

Culver's root (I started a thread about it a few days ago) would work.

Knautia, scabiosa, daisies for something a bit more substantial. Poppies for something a bit different.

There are a lot of plants that grow flower stalks from a basal mound.

Painted daisies is another tall one, also Sidalcea 'Candy Girl'.

A second vote for Veronicastrium/culver's root. Knifophia/red hot poker, Penstemon, Agastache, Delphinium.

Here's a better picture. I planted shastas between the grasses in the back and was going to round out the front right corner with a few more coral bells in front of the agastache is centuraea Montana blue and dianthus

Would you consider one of the tall lilies like Chinese Trumpet Lilies or Orienpet Lilies? That would be a thin stalk with a lot of bang for your buck on top. I usually don't stake mine, but even if you need to, they're well worth the effort for the show they put on.

Thanks for all the suggestions everyone! I'll do some research to see what I like best

Well, I thought of some more...lobelia silphitica and cardinalis (if your bed run toward fairly high moisture).

The lobelias are really nice August and September for fall color. Mine are beginning to bud now.

Coreopsis tripteris is nice..but very tall and will spread (not horribly...but it will)...5 to 8 ft tall and just now starting to come into bloom (for the next six weeks or so).

Delphinium exaltatum (also just coming into bloom in my garden)

That's what I can think of (mostly because I see it in front of my own eyes).

Oh...not too narrow, but consider a fennel also...perennial, colorful, fragrant (if you pick foliage), edible (surplus), and host to swallowtail butterfly larvae.

Keep thinking...though this isn't a very long lasting plant (in my experience), there's always the bitter sneezeweeds, helenium, which are very colorful in mid to late summer and fit the tall narrow profile. Color ranges through patterns of yellow and reds.

Rudbekia 'Herbsstonne' would give you some tall 'stalky' color along the fence. You can divide it after a few years too, so you get a lot of bang for your buck.

That's one I forgot...and a good choice Virginia!

I also thought of Herbsstone.... the widely spaced big glacuous leaves.would give you exactly what you are looking for.

Liatris, Ox eye daisy, Joe Pye weed, Vernonia (ironweed). All these are natives which will attract butterflies, nice bonus.

Seconding Liatris!

Bellflowers produce a nice tall stalk... Just be sure you don't get the creeping variety or they will take over.

I described herbstone incorrectly.

What is the composite plant that gets about 6-7 feet tall, has sparsely spaced glaucous leaves?

For some inspiration you might look at the Hayefield website, whose owner's name just dropped out of my memory. Her whole setup is wild and fieldlike and she has a lifetime of information about letting plants seed in. .....just crept back in.....it is Nancy Ondra.....and she loves the plant whose name I can't remember either. Bad night for the synapses.

Marie, my Herbsstone gets over 7 feet tall! I think you described it accurately.

Perhaps you are thinking of Rudbeckia maxima. That has glaucous leaves.

Carindutch

exactly!

I only had one lonely one.

several,like in photo Are really effective

those leaves are it's best feature

Wish I could find it again

marie

it would be fun to figure out its placement with its substantial low large leaveze and second fiddle flowers

The placement of the one in photo is very good....