Yard and garden: Cool season annuals
AMES, Iowa — Annuals are a great way to add variety and color to the landscape and containers. As temperatures cool in late summer into fall, many of the summer annuals like marigolds, coleus and impatiens are looking “tired.” These annuals can be replaced by those that do well in the cooler temperatures of fall.
In this week’s Yard and Garden article, Aaron Steil, consumer horticulture specialist with Iowa State University Extension and Outreach, offers some advice that can help Iowans enjoy colorful annuals into November. Below are some frequently asked questions about cool-season annuals that can be used in late fall and early spring.
What are cool-season annuals?
Cool-season annuals are annual plants that prefer cool temperatures, growing best in spring or fall. Many are tolerant of a light frost, often surviving down to 28 degrees Fahrenheit or sometimes even 25 degrees with little damage to flowers or leaves. They are great additions to containers and garden beds in the shoulder seasons to add color late into fall or early in the spring season.
What cool-season annuals do well in Iowa?
For the fall, consider planting annuals like pansy (Viola), ornamental cabbage and kale (Brassica oleracea), snapdragons (Antirrhinum majus), stock (Matthiola incana), larkspur (Delphinium consolida), bachelor’s buttons (Centaura cyanus), pot marigold (Calendula officinalis), twinspur (Diascia), lobelia (Lobelia erinus), nasturtium (Tropaeloum majus), Nierembergia (Nierembergia), sweet alyssum (Lobularia maritima), cape daisy (Osteospermum), pinks and sweet William (Dianthus), swiss chard and beets (Beta vulgaris), dusty miller (Jacobaea maritima, aka Senecio cineraria), petunia (Petunia), sweet pea (Lathyrus odoratus), Nemesia (Nemesia), California poppy (Eschscholzia californica) and bells of Ireland (Molucella laevis).
There are several species traditionally grown as perennials that can also be treated as annuals and grow well in the cool season including, mums (Chrysanthemum), coral bells (Heuchera) and black-eyed Susan (Rudbeckia hirta).
When should I plant cool-season annuals?
For the fall season in Iowa, cool-season annuals can be planted mid-to late September. For early spring color, plant cool-season annuals mid-March through early April, depending on the weather conditions.
Some species do best when directly sown, but many need to be started indoors and transplanted outdoors. If starting from seed indoors, consult the seed packet to determine the length of time from seed to transplant.
For many annuals it’s six to eight weeks. Count back from your projected planting date outdoors and sow seed at that time. For many species, this means sowing seed in late July through early August for cool-season annuals planted in fall and mid-January to early February for those planted in early spring.
Certain cool-season annuals, like mums, are also sometimes grown as perennials. Can I expect these plants to come back year after year?
Plants such as mums, coral bells and black-eyed Susan are sometimes grown as perennials, but also grow well when treated as a cool-season annual. These species will not reliably over winter when planted in the fall. Fall planting does not allow sufficient time for root establishment, therefore they don’t overwinter well. They are best treated as annuals when planted late in the season.
Some species traditionally treated as annuals, like pansy, dianthus and bachelor’s button will frequently survive the cold winter temperatures when planted in fall and begin growing and blooming again in early spring. These plants often do not tolerate the warm temperatures of summer, however, often browning and dying in the heat. Because of this, they are not treated as perennials despite being winter hardy most years.
What do I do with fall planted cool-season annuals when winter arrives?
Cool-season annuals planted in fall can be left in the ground through the winter months, some species may even overwinter under the snow and begin growing again in early spring. They can be replaced with new cool-season annuals in early spring or left to bloom through the cool spring season and replaced with summer annuals after the danger of frost passes, in early to mid-May for much of Iowa.
What do I do with early spring planted cool-season annuals when summer arrives?
Since nearly all cool-season annuals do not grow well in warm temperatures, especially those that exceed 80 to 85 degrees, they can be replaced with summer annuals once the danger of frost passes.