Cool-Season Flowering Annuals for the Garden (2024)

As spring arrives, many gardeners are planning their planting scheme and mapping out road trips to their favorite garden centers. The anticipation of warmer days on the apartment deck or the backyard patio surrounded by blooms can tempt many gardeners to plant tender, warm-season flowering annuals when the threat of frost and freeze still loom in South Dakota. Planting many flowering annuals too early in the spring could lead to slowed growth, frost damage or worse—the dreaded need to replant. However, all is not lost!

Flowering annuals that appreciate and thrive in the cool weather of late-April and early-May can bring a gorgeous array of bloom colors, shapes and sizes to your garden for early season-enjoyment. This article will highlight several annuals for early-season planting, but first a quick exploration of the Midwest climate is necessary to ensure the right plant is being planted at the right time.

Understanding Frost-Free Dates, Soil, and Air Temperatures

Understanding how to determine the average frost-free date for your location is a key factor to knowing when it is appropriate to plant any garden plant. While some cool-season, flowering annuals can tolerate a frost or freeze, this is the exception rather than the rule. South Dakota State has compiled historical data to provide maps with corresponding ranges of dates for the average last freeze (32 degrees Fahrenheit) and the last hard freeze (28 degrees Fahrenheit). These dates are based on a 50% probability the last 32-or-28-degree temperature will occur on or before a certain day. Gardeners in the southeast corner of South Dakota can typically plan for a frost-free date after May 3, while gardeners in the Black Hills region may need to wait until as late as June 9 to feel confident that the threat of a freeze has passed. This range of historic frost-free dates is why Mother’s Day is often used as the kick-off to the primary gardening season for most of South Dakota.

It is important to remember that season-to-season variability does occur, and soil and air temperatures for your location can be monitored through the Mesonet at South Dakota State. On this website, you can click the location marker nearest to where you live and see up-to-date data on the soil and air temperature. You can also explore historical data for your region.

Soil temperature plays a critical role in seed germination, transplant establishment and plant growth rate. Some cool-season annuals can tolerate soil temperatures as low as 45 degrees Fahrenheit; however, most cool-season annuals will grow best in soil temperatures at 65 degrees Fahrenheit or warmer. If you are curious about the soil temperature in a specific location of your garden, you can insert a soil thermometer four inches into the soil to take measurements. The author has used a digital meat thermometer to check soil temperatures throughout the garden, but be sure to wash it off before using it for cooking again!

Air temperatures can fluctuate widely during the spring months, with nighttime lows approaching freezing and sometimes 70-plus degrees Fahrenheit during daytime hours a few days later. This large difference in warm, daytime temperatures versus cold, nighttime temperatures triggers flower development for many cool-season annuals. As temperature fluctuations decrease and summer months bring even warmer temperatures, many cool-season annuals will set fewer flowers or stop flowering completely until the cooler months of fall return.

General Care of Cool-Season Annuals

Start cool-season annual seeds indoors to get the earliest bloom time possible. Even though many cool-season annuals can tolerate cool soil and air temperatures, optimal germination occurs at warmer temperatures. You can set up a seed germination and growing space in your home or let your local garden center do the first steps of growing for you. Remember, cool-season annuals should be properly acclimated to outdoor growing conditions before planting. Be sure to harden-off transplants for optimum plant establishment.

As with any flowering annual, you’ll want to ensure that you follow the planting and care instructions on the seed packet or pot label. Plants will need more water at time of establishment, but be careful not to overwater cool-season annuals. Cooler temperatures mean that plants will not be transpiring as rapidly, and water use will not be as high compared to summer months. Additionally, timely spring rains can offset the need to provide supplemental water. Routine deadheading of flower blossoms will prevent plants from trying to set seed and extend bloom production. Some cool-season annuals will have minimal flower blooms during the heat of summer. Continue to support healthy foliage growth throughout the summer and, in many cases, you will see a second display of blooms in the fall.

Suggested Cool-Season Annuals for the Midwest

There are several cool-season annuals that are suitable to plant in South Dakota gardens early in the season; the list below is by no means exhaustive. Some of these annuals are old standards. A few may be new to you—take this opportunity to try something new in your garden during the spring months. This list is generally organized from the most to least cold-hardy plants.

Pansy

(Viola x wittrockiana)

Pansies can tolerate frost and will perform the best in the garden when temperatures are above 55 degrees Fahrenheit but do not exceed 75 degrees Fahrenheit.

Pansies are the poster child of cool-season annuals. They have become very popular as a potted plant for fall containers next to ornamental kale and garden mums; however, pansies are also early-spring bloomers when tucked into gardens in April and May. The delicate blooms come in solid or bi-color combinations, including: indigo, violet, yellow, burgundy, pink and white. Pansies may die back during the warm summer months, which is why some gardeners choose to wait until fall to incorporate this plant into the garden for extended enjoyment as weather starts to cool.

Monkey Flower

(Mimulus x hybridus)

Monkey flower can tolerate frost and will grow well in daytime temperatures of 50–60 degrees Fahrenheit and nighttime temperatures as low as 40 degrees Fahrenheit.

Monkey flower has a small, mounded growth habit and produces showy flowers in shades of yellow, orange and red. Flowers may also be bi-colored with showy contrasts of red and yellow in a freckled pattern. Monkey flower grows naturally in marsh regions, making it ideal to grow in damp, shady conditions. Do not plant this flower in full sun; it must be grown in partial-shade to-shade conditions. This lesser-known plant may be a bit harder to find at your local garden center, but the search is half the fun!

Alyssum

(Lobularia maritima)

Alyssum can tolerate temperatures as low as 28 degrees Fahrenheit, and the plants will do well in a temperature range of 50–60 degrees Fahrenheit. Heat-tolerant varieties are becoming widely available.

Alyssum is a low-growing (10-inches tall or less), sweet-scented annual that is used in garden borders or filling spaces in showy containers. Newer releases of alyssum have a trailing habit that makes them appropriate for stand-alone hanging baskets. The most-common flower color is white, though lavender, indigo and even pink blooming alyssum are also available. Alyssum should be grown in full-sun to partial-shade in soil that is well-drained. This plant is especially attractive to pollinators and other beneficial garden insects.

Snapdragon

(Antirrhinum majus)

Snapdragons can survive temperatures as low as 25 degrees Fahrenheit. Snapdragons grow best when daytime temperature are 65–70 degrees Fahrenheit and cooler, nighttime temperatures of 50–60 degrees Fahrenheit promote and prolong flowering.

Snapdragons have been bred for a variety of uses and heights in the gardens. Dwarfing varieties may be as short as six-to-eight inches and perfect for garden borders. Medium (14–18 inches) and tall varieties (24–26 inches) can serve as a statement piece in the garden. Taller plants may need staking. The flowers come in a wide variety of colors and consistent deadheading is key to prolonging the bloom time. Snapdragons have low drought tolerance and may not thrive during hot summer months. Grow snapdragons in the full sun with even soil moisture.

African Daisy

(Osteospermum hybrida)

Osteospermum (the common name, African Daisy, is not widely used) will thrive in daytime temperatures of 50–70 degrees Fahrenheit. Lower nighttime temperatures (50–60 degrees Fahrenheit) will promote increased flowering. Plants will tolerate a light frost.

Osteospermum became popular due to their large, daisy-like flower in colors including bright purples, oranges, yellows and pinks. The upright, spreading habit with an average height of 12–24 inches makes this a great early showpiece in the garden or for stand-alone spring containers. Osteospermum will perform well in the spring weeks, but blooms typically stall completely in the heat of summer. Removing spent blooms and nurturing the foliage can result in a second flower display in the fall. The plant does well in full-sun to partial-shade and does not tolerate being overwatered.

Petunia

(Petunia x hybrida)

Petunias tolerate a wide range of temperatures. Established plants can survive a light frost, while also tolerating temperatures above 90 degrees Fahrenheit in the heat of summer. The optimum temperature range for this versatile plant is 65–75 degrees Fahrenheit with slightly cooler nighttime temperatures.

Petunias are ubiquitous to many home gardeners—they are available with flowers in nearly every color, and breeders continue to release flowers with uniquely pigmented petals to keep gardeners on their toes from year to year. The oldest grown varieties, grandifloras, developed a bad rap due to their need for deadheading to keep flower performance going strong throughout the year. Many newer varieties are self-cleaning (no deadheading necessary), greatly reducing the time needed to keep the plants in tip-top shape. Petunia growth habits are now as varied as their color schemes—older varieties tend to grow in neat compact clumps, whereas new releases have been bred with excellent trailing habits that result in stunning baskets or carpeted gardens. Petunias thrive in full sun but will tolerate light shade. You can expect this plant to bloom throughout the entire summer when well cared for.

Dianthus

(Dianthus barbatus, D. chinensis, D. chinensis x barbatus, D. hybrida)

Dianthus are ideally grown when daytime temperatures are 55–65 degrees Fahrenheit and nighttime temperatures are 50–55 degrees Fahrenheit. Dianthus may tolerate a light frost.

While many common names exist for this plant (Sweet William, Carnation and Pinks), the genus name Dianthus is used widely. The species listed above are all typically grown as annuals. It is hard to find another plant that competes with the bright pink, purple and magenta blooms of Dianthus. The plant grows in short, tight clusters, and deadheading greatly increases the bloom period. It performs best when planted in full sun but will tolerate light shade. You may notice Dianthus reappearing in the garden year over year, as it does reseed freely.

Lobelia

(Lobelia erinus)

Lobelia thrive in cool, daytime temperatures of 55-60 degrees Fahrenheit and nighttime temperatures of 50–55 degrees Fahrenheit. Lobelia is sensitive to frost.

Lobelia are profuse bloomers, with blooms nearly covering the foliage completely in shades of pale-to-deep blues, white, pink or lilac. The low-growing, trailing habit results in low carpets of blooms in the garden or spectacular spring hanging baskets. Lobelia grows well in full-sun to partial-shade conditions with well-drained soils. Lobelia may die out completely in the warm, summer months; however, heat-tolerant varieties are becoming more widely available. Lobelia is poisonous if ingested in large quantities, so keep it away from small children and plant-eating pets.

Diascia

(Diascia hybrids)

Diascia grows best when days are 68–75 degrees Fahrenheit and nights are 62–65 degrees Fahrenheit. Growth will slow significantly at temperatures below 50 degrees Fahrenheit.

Diascia has a low, mounding growth habit, so it is typically found near the front of garden beds and borders, or spilling from containers and hanging baskets. The delicate flowers come in white and soft pastels, including white, lavender and coral. Diascia should be grown in full-sun to partial-shade and does not do well in wet soil conditions.