Astilbe is a perennial with beautiful, showy flowers atop glossy, fern-like foliage. Here’s how to grow and care for astilbes in your garden. Astilbes’ flower clusters vary in size from 6 inches to 2 feet and their height ranges from 6 inches to 5 feet, depending on the variety. If you have a shady area, try astilbes. They are a great way to add color and texture to a place where other flowers won’t thrive.