Yard and Garden: Cool Season Annuals

AMES, Iowa – Annuals are a great way to make your landscape and containers more diverse and colorful. As temperatures cool in late summer to fall, many summer annuals such as calendula, coleus and impatiens look “tired”. These annuals can be replaced with ones that perform well in cooler autumn temperatures.

In this week’s Yard and Garden article, Aaron Steil, a consumer horticultural expert at Extension and Outreach at Iowa State University, gives advice that can help Iowaians enjoy colorful annual plants by November. Below are some frequently asked questions about cool-season annuals that can be used in late fall and early spring.

What are cool seasonal annuals?

Cool-season annuals are annuals that prefer cold temperatures and grow best in spring or fall. Many tolerate mild frost, often surviving slight damage to flowers or leaves up to 28 degrees Fahrenheit or sometimes up to 25 F. Great accessories for containers and garden beds in the shoulder season to add color in the late fall or early spring season.

Which cool-season annuals perform well in Iowa?

In the fall, consider planting annuals such as pansy (Viola), kale and kale (Brassica oleracea), snapdragons (Antirrhinum majus), Inventory (Matthiola incana), whipped (Delphinium stakes), bachelor buttons (Centaura cyanus), marigold (Calendula officinalis), twinspur (Diascia), lobelia (Lobelia differed), nasturtium (Tropaelous), Niembergia (Niembergia), sweet alyssum (Lobularia maritima), daisies (Osteospermum), pink and sweet William (Dianthus), chard and beet (Beta vulgaris), dusty miller (Jacobaea maritima, also known as Senecio cineraria), petunia (Petunia), sugar peas (Lathyrus odoratus), Nemesia (Nemesia), California poppy (Eschscholzia californica) and the bells of Ireland (Molucella laevis).

There are a number of species traditionally grown as perennials that can be treated as annuals and grow well during the cool season, including their mothers (Chrysanthemum), coral bells (Heuchera) and the black-eyed Susan (Rudbeckia hirta).

When to plant cool seasonal annuals?

During the cool season in Iowa during the cool season, annuals can be planted from mid to late September. For early spring color, plant cool-season annuals from mid-March to early April, depending on weather conditions. Some species perform best when sown directly, but many species need to be started indoors and transplanted outdoors. If you are starting from an indoor seed, check the seed package to determine the length of time from seed to transplantation. For many years, it is six to eight weeks. Count back from the planned date for planting outdoors and then sow the seeds. For many species, this means sowing seeds from late July to early August for cool-season annuals planted in the fall and from early January to early February for those planted in early spring.

Certain cool-season annuals, such as mothers, are sometimes grown as perennials. Can I expect these plants to return year after year?

Plants like mothers, coral bells, and black-eyed Susan are sometimes grown as perennials, but thrive when treated as annuals in the cool season. These species do not overwinter very reliably when planted in the fall. Autumn planting does not give enough time for root implantation, so they overwinter badly. It is best to treat them as annuals if they are planted at the end of the season.

Some species traditionally treated as annuals, such as pansy, dianthus, and bison, often survive cold winter temperatures when planted in the fall and begin to grow and bloom again in early spring. These plants often do not tolerate warm summer temperatures, but in the heat they often turn brown and die. For this reason, they are not treated as perennials, despite being winter-hardy in most years.

What should I do with the annuals planted in the cool season in the fall when winter arrives?

Annual plants that are planted in the fall, cool-season, can be left in the soil during the winter months, some species can even overwinter under the snow and begin to grow again in early spring. In early spring, they can be replaced with new, cool-season annuals or allowed to bloom during the cool spring season, and when frost is over, they are allowed to bloom in much of Iowa in early or mid-May.

What should I do with annuals planted in the early spring when the summer arrives?

Since almost all cool-season annuals do not grow well at warm temperatures, especially above 80-85 F, they can be replaced with summer annuals after the risk of frost.

Photo to share: fasting grass.

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