Delphinium
Salvia uliginosa

I have Jacob Kline Mondarda/Bee Balm in the back of my border. It blooms all summer long. Raspberry Wine is another bee balm that is fairly tall.

Oops - monarda not mondarda

Thanks for your response, yes I will try these, had bee balm some time ago and it got mildewy, will try again. Also, would like to try delpheniums.

Hollyhocks
Eremurus

Both of these plants are great in the back border, when their foliage wanes, the flowers are stealing the show. Below is pic of one of my single flower hollyhocks at summer's end, and at least 12 feet tall. My eremurus or foxtail lilies reach about 7 feet tall.

Here is a link that might be useful: Very tall hollyhocks

a couple more:
Campanula lactiflora
Verbascums
Filipendula rubra
Eupatorium 'Gateway'
Telekia speciosa
Thalictrum rochbruneanum

some sunflowers are perennial, or use the annual ones.

Plants with important basal leaves, like Crambe don't show or look well at the back.

I've grown the Gillenia for years, the fall color effect is like that of a deciduous shrub. Herbaceous peonies also tend to color noticeably in fall.

Wow, thanks for all of the great input, can't wait to take this list to my local garden centre and see what I think will work in this bed.
Like the rest of us in the Pacific Northwest, wish things would warm up and rain would subside, enough is enough. At least it makes the weeds easy to pull.....

Valeriana officinalis. kinda fun giant plant. in part shade, it will kick over 6 feet here. it has a very very strong smell that you might love/hate. tosses up white blooms, thick foliage.

Here is a link that might be useful: nice picture here

Plants with important basal leaves, like Crambe don't show or look well at the back.

I guess that depends on your viewpoint :-) The leaves are of crambe are large and coarse looking and are always plagued by insect predation - hiding them behind other plants may be preferred, much like one tucks hollyhocks behind other, fuller plants to disguise the rust issues. It is the flowering of the plant that offers the major interest - I could care less about seeing the ratty looking foliage.

Hollyhocks, delphiniums etc. do not produce large basal leaves in a mound or rosette that is present when the plant is in full effect - and an important part of their impact. Putting plants with big leaves that sit on or come to the ground (crambes, hostas, daylilies...) behind other kinds without this feature produces an elephant in the room feeling, where the eye is drawn through the lesser or taller plant forms to the bolder or lower leaves behind. Such plants have too much "weight" visually to be at the back.

In front is the grand glaucous foliage of seakale

--Gertrude Jekyll

It's the same as the relating of shrub foliage textures in borders, a dumpy big-leaved rhododendron or hydrangea at the back of a planting sucks or pinches the visual flow to the back at that point. Such specimens are boulders in the flow of an informal bed.

on the other hand, a little bit of dissonance in the bed is good from time to time. occasional peeks to the back, occasional reverses in height order, etc etc.

cooking, music, gardening, its all the same. you gotta do something wrong every once in a while.

I forgot to mention Orienpet Lilies, mine get about 6-7 feet tall, and are great performers and have perfume.

Here is a link that might be useful: Photos: Orientpet Lilies -- Va Va Va Voom!

foxglove. asiatic lily along with tallclover's suggestion. many happy returns from lillies!

My two favorites are Veronicastrum and Persicaria polymorpha.

The Veronicastrum is tall and spikey, long lasting mid to late summer bloom. lavender, white or pink (Apollo).
Persicaria is a great plant but eventually will grow quite wide and tall, more shrublike,but nice throughout the summer and fall and not invasive. Definitely back of the border plant. I have 3 spaced in a 100 foot border and they really anchor the border.

Also like eupatorium Gateway -5-6 feet, also have more than one in my border.

All good suggestions in the forum. Good luck
Also like eupatoriam.