Agastache, Rudbeckia, Gaillardia, and Delphinium: Short Lived Perennials
Q: Some of my favorite perennials flower all summer. I especially love Agastache, large-flowered Rudbeckia, Gaillardia, and Delphinium. Unfortunately, they only live two or three years for me. I thought perennials could live forever. Am I doing something wrong? They seem very happy one year then the next spring they don’t come back.
A: Horticulture hasn’t given these shorter-lived plants a good category name. They are always lumped in with “perennials”. In reality, they are somewhere between annuals and perennials. They are not biennials either! Regardless, we love them for their long-flowering impact in the landscape. But there are some things that gardeners can do to maximize their longevity.
Not cutting them back harshly in the fall often helps. These plants have one thing in common: a fine, relatively shallow root system more reminiscent of an annual than a true perennial. True perennials generally have beefier roots where carbohydrates are stored to fuel next year’s growth. The plants you refer to don’t have much capacity to store carbohydrates in their fine roots. Their best option in their quest toward “perennialness” is to count on reserves in the stems of the plant. If we cut the plants down to the ground in the fall (like we do for true perennials like peony and daylily) we are effectively removing all their stored energy. With this in consideration, leave the above-ground plant parts until new basal growth appears in April. I often remove just the seed heads in the fall though or else they pop up all over the garden in the spring.
Lastly, the stored carbohydrates in these plants can be maximized by fertilizing in the late summer as the flowers begin to fade and the plant is putting much of its energy into seed production. Fertilizing all perennials in late summer or fall will produce healthier plants the next year. In my opinion, if you only fertilized your perennials once a year, it should be in the fall.