I'm zone 5 and here are some heights on things mentioned above for me. A tall type of cosmos sulphurensis ( something like that) 2 ft bright lights 3-4 ft. amaranthus 4-5 ft. Some problems staying upright in high winds. Staking at times. canna like tropicanna , bengal tiger =4 ft for the plant and another foot and a half when in flower, which is not during the summer but late summer. I don't water these extra here unless potted. Expect a bit shorter plant in a pot. ( and more hassle with watering) Elephant ear may get taller for you but need lots of water. My older ones get 5 ft for upright leaves ( taller) like odora New ideas: Broom corn- gets really tall- maybe 10 ft. It's an easy annual. Looks like a grass most of the season. needs no extra water Verbena bonairensis- same height as bright lights but less ugly with browning seed heads on the cosmos. drought tolerant. More of a see thru plant than a wall. I think of more perennials than annuals for this- sorry. K

We had a similar situation in our backyard. We planted arborvitae, which did very well along our side fence, in pretty dense shade. However, in the past few years they have been decimated by deer. If you don't have a deer population passing through, arborvitae should work well. We used Canadian Hemlocks along our back property Line, which also has a significant amount of shade and competes with other trees. They grew quickly, but after their third year, they were attacked by wooly adelgid (an aphid-like disease), and in spite of our efforts to save them, they are looking pretty bad, and I need replacing. I'm told that Hemlocks are not susceptible to disease, but my experience was not good.

Nandina domestica? The flowers aren't brilliant but the foliage can be. (I'm thinking of the tall variety, not the low shrubby ones.) One of the elegant grasses, perhaps? As a foil for the Echinacea. Ribes odoratum or sanguinea? Penstemon?

There is one thing you most definitely need to know! 'Cherry Bells', Campanula punctata, is very pretty--and VERY invasive. Be really careful where you plant it and/or hover over it and be ready to POUNCE and get rid of it if you find it going everywhere in a couple years. Based on my experience, I'd also be very surprised if it got more than 16" tall--and I think even that is pushing it. (Your Astilbe, if they bloom for you, should be taller than that.) I haven't grown Adenophora lilifolia, but I believe that one can be invasive too, and I'd also be very surprised if it got much more than 24". But individual growing conditions affect things like that, so you never know. The Digitalis mertonensis should be very pretty. Will be interested to see if it gets to three feet for you. Not invasive at least! ;-) You found some very pretty things--but be careful they don't eat you. When you said you were looking for some "tall" things, I assumed you wanted AT LEAST 3', so the things I suggested are that--mostly 3-4'. You probably won't find any of the things I suggested anywhere but a "real" garden center--definitely not places like big box stores. Too bad you can't come to one of our swaps! Kinda long drive, I think! But my "purple fluff" Thalictrum has started to self seed a little bit, so I've given a few of the plants away at our last couple swaps. You might need to look for some of them mail order if you decide you want any of those things. Or, if Billings has a good garden center somewhere, it might be worth a drive if it's not close to you. A yellow columbine I have that's fragrant gets "about" 3' tall if you can find it. It's Aquilegia chrysantha. 'Denver Gold' is one named variety, or you can find it as an unnamed, straight species plant too. The drawback--it self seeds insanely! But it's one I consider "worth it" to put up with pulling the seedlings out. I second the bleeding heart! The Old Fashioned ones are the tallest, Dicentra spectabilis, but they probably won't get up to 3' either--but they ARE very pretty when they bloom! Mine is blooming right now! (But one thing I just thought of, the bleeding hearts usually go dormant over summer when it gets hot.) Skybird