Perennials with Presence by Cherie Luke, Master Gardener
I’ve heard it said that trees and shrubs provide the backbone of a great garden. Every garden needs a background to help showcase all of your beautiful perennials, annuals, and bulbs. But trees and shrubs take time to grow, unless you have the money to purchase plants that are already quite large. If you don’t, then it helps to use large perennials that can fill in quickly. Perennials I refer to as “perennials with presence”, and here are three of my favorites.
Crambe cordifolia - Giant Kale - is a 6’ tall perennial with 2’ wide leaves that can spread 5’ across. It bears tiny white flowers from spring into summer.
Another favorite is Crambe maritima - Sea Kale. It reaches about 2 1/2’ tall and 2’-3’ wide. It bears large blue-green leaves and dense racemes of tiny white flowers in summer. 
And lastly, Persicaria polymorpha - giant fleeceflower. This perennial grows to about 6’ tall and 3’-5’ wide. It bears large white astilbe like blossoms that will bloom throughout the summer.
These herbaceous plants are all growing well here at 7600’ zone 4, and I have seen them all for sale at the Denver Botanic Gardens spring plant sale as well as seeing them in the Gardens themselves. The first 2 mentioned also happen to be edible.