The term "hardy annual" seems like an oxymoron. Annuals by definition sprout, flower and fade in one growing season. Perennials live at least three seasons, but cold tolerance varies. “Tender perennials" must be either used like annuals or overwintered in a protected environment. So are "hardy annuals" and "tender perennials" synonymous terms?
"Tender perennials" live forever in the warmer regions of our nation. My California relatives think coleus is perennial! Scented geraniums are a terrific example of a tender perennial. A plant collector's dream, they are only cold hardy to Zone 9 (average minimum temperatures 20 to 30 degrees). We are in Zone 7 (0 to 10 degrees average minimum temperature). We expect killing frost every winter. Best enjoy tender perennials as accent plants.
Chickweed is an excellent example of a "hardy annual.” Its seeds and seedlings are unfazed by frost. Many hardy annuals reseed and can be used to fill breaches in the developing landscape. Just protect the seeds with a bit of mulch. Cosmos, bachelor buttons, snapdragons and alyssum can be used in this way to create an informal floral border.
There are also "half hardy" annuals. These can be sown directly into the garden after soil temperature warms in the spring. The resulting plants bloom later than ones started indoors. The flowers arrive about the time early-blooming perennials are fading, thus bridging an awkward gap during the dog days of summer. Many of our favorites are in this category including petunias, zinnias, cleome and nicotiana.
Let's not neglect cool-season annuals. These cannot take the heat of our summers. Lettuces, cilantro, sweet peas and calendula are among them. It is best to start these indoors and set them out as early as possible. It can be frustrating when our spring warms too quickly or summer lingers too long into autumn because they bolt (go to seed) or wither rather than adding to the garden.
"Tender" annuals reflect their tropical origins. These are damaged by cold weather and cannot tolerate frost. They also require very warm soil temperatures to germinate. It is best to start them indoors for summer flowering. Impatiens, vinca and coleus are examples of tender annuals.
All annuals complete the cycle of life in one growing season. Those which are "hardy" produce frost-tolerant seeds and seedlings. "Half hardy" annuals need warm soil temperatures to germinate, and the seedlings are not frost tolerant. Tender perennials have limited frost tolerance and are used as annuals here.
Editor's note: Illustrations are by Ann Bartlett's artist daughter, Roxanne.
Master gardener Ann Bartlett never lets lack of experience with a plant stop her from trying it in the ornamental beds around her home.