The Mixed Border is a bed thirty feet wide and twenty-three deep. Big. If plants are a foot at the front, they'll need to be ten, twelve, fifteen feet at the back. Some perennials are that big and more—check out my Helianthus verticillatus.
But across the back of the bed it's just as much about shrubs and trees. Here's a variegated ash—what a beauty—that is, at least for me, so slow-growing that you can only see it if you're standing at the back of the bed.
That's the giant Siberian filipendula in back of it, i.e., completely blocking the view of the ash from the front of the bed. This has since gotten moved to the Pink Borders—the fluffy clusters of flowers aren't really the pure white you see here; there's a hint, a contamination, of pink in then was too obvious to ignore. I've just planted in its place a, fittingly, Siberian perennial aralia that's supposed to get even bigger than the usual eight or ten feet. So the ash will still need to double in size to be part of the front-the-front show.
Meanwhile, the ash's performance is no less thrilling for being, so to speak, private. Click on the pictures—and then again—to see its full intensity as well as the wider context.
I always plant Russian Giant cannas nearby; on a good Summer they get ten feet tall too. Their immense purple banana-like foliage is a vivid contrast with the comparatively tiny white-and-green leaves of the ash.
If anyone nearby is in bloom as well—like the PG hydrangea at the immediate back, with unusually large flowers, called 'The Swan'—that's swell too. But the show is powerful even if there's not a flower in sight.
Get variegated ash from Greer Gardens, who says that the tree is "A fast grower, reaching 50’ in height and about 25’ in height." How lucky for them! I'll prune mine, someday, to keep it shorter than, oh, 15 feet. At this rate I've got another five years or so before I get out the stepladder and loppers.

Any time I visit Avant Gardens, my heart leaps with anticipation: I can see their sensational dogwoods again. Sensational for at least three reasons: They are still rare, and so are a thrill to discover. They have indelibly memorable foliage. And (often) a striking overall habit. That they also bloom—these are dogwoods, for heaven's sake—is by comparison just icing on the cake.
Given how showy a dogwood in full flower can be, these must be powerfully ornamental trees to upstage even that exciting a show. Indeed! Here's the easiest: a variegated Korean dogwood hybrid called Wolf Eyes.  What a show! The foliage almost out-competes the flowers. (I'll see if I can return to shoot this individual in bloom.) The Korean dogwood's usual wide greeting-the-sky habit, heavily fluffed with white-edge leaves. It's an easy tree to establish if you can provide a mostly-sun exposure, decent soil, and acceptable drainage in the Winter. Then you can just stand back and marvel.
If you want to work harder and worry more, consider one of both of these two, which are worth the agita in that they are, if possible, even more thrilling. "Giant" dogwood is an accurate name for Cornus contraversa: Old trees can be forty feet tall and thirty wide—twice the size of American dogwood. Even the straight green-leaved species of Giant dogwood should be used more. It's striking tiered branching habit adds a remarkable layered look to a tree that's impressive already just for sheer size.
The variegated cultivar makes a great tree a stunning one, with bright mostly-white leaves in large tiers.
This one at Avant Gardens is just an adolescent, still thinking through that tiered thing. Older trees are showstopping, especially when backed by darker foliage that shows what is, eventually, a planar gap between foliage layers that's a couple of feet thick.
But you can't buy variegated Giant dogwood at Home Depot, and even if you could, you probably will have trouble establishing it. The tree is hardy to Zone 5, but when young (which is the only size you can afford to buy) it's mightily afraid of less-than-perfect Winter drainage, as well as drought and/or hot Western sun in the Summer. So plant it where it gets morning sun only, where it's definitely higher than the surrounding garden, and in generously rich soil. Water deeply once a week in hot weather, and cross your fingers. If it gets through the first Winter, you're (usually) safe.
(Another option is to buy the largest one you can find, and hang the cost. But woe to you if you still don't give it afternoon shade, great drainage, and good soil.) No wonder the variegated Giant dogwood is a head-spinner. It's alive! (And notice that at Avant Gardens its bed is, indeed, higher than the surroundings. And that the tree has some shade from an enormous oak to the back.)
A bit easier is the variegated form of the alternate-leaved dogwood, Cornus alternifolia 'Variegata', which at Avant Gardens is dramatically backed with purple beeches. It has a similar tiered branching, so another common name is pagoda dogwood. (Giant dogwood is also called Pagoda dogwood. So shop for the latin name, not the common one.)
This is a half or even a third the size of the variegated Giant. Twelve feet would be fabulous. In my experience, it insists on the same culture as for the variegated Giant, although because alternifolia is much much hardier, it seems to be more tolerant of less than perfect siting when young.
Either tree, then, is a triumph, an automatic star of your landscape. And yes, both of them are a bit of Mt. Everest to establish, and you may or may not be up for the suspense, let alone the expense of trying again in a different spot, and then yet again in yet another spot.
I've taken the easy way out for the moment, with an alternifolia cultivar, gold-variegated, called Golden Shadows. I'll post on that later. So far so goot, but wow is it pokey.

An unprepossessing opening picture, true.
It was a glorious bright-sunny day, so the interior seems dark dark dark. These are the pair of South-facing windows in our dining room; they're unusually large for an 18th C. house. With such low sills my guess is that they're a 19th C. addition. Whatever: They are a pleasure welcoming the Winter sun as well as the Summer view. I planted all kinds of plants to make that view worthwhile. Here's the show out the lower-left sash.
Boy do I love these plants. We'll look at most of them, and soon. Right now, the gold-leaved Japanese maple.
Yup, that's the color, and all season long: Glowing gold but not over the line into "Jeez, honey, where are my sunglasses?" yellow.

It's a Japanese maple too, mind you, so has that tribe's multi-trunked classy look to the branches and trunks. So yes, it's good even in Winter, when (alas, sniff, sigh) the leaves have fallen. But if we're lucky (I haven't been so far, but I live in faith), the Fall weather is such that just the tips of the leaves turn a cherry Fall red, leaving a round interior of each leaf still gold—hence the "Full Moon" of the common name. The tree is very slow growing, so buy the biggest you can afford. After a couple of decades, it might be only 18 feet tall. It's happy in amazing amounts of shade too; conversely, it handles full sun without scorching only if the soil is rich and it doesn't have to beg for water. I never water mine even though it's on the South side of the house and gets full West sun. So I guess my soil passes the test on both counts.
More on the companion plants later—and soon.