By Aaron Steil, Consumer Horticulture ISU Extension Specialist
(SBA/ISU Extension and Outreach) - As summer flowers fade and many garden plants wind down for the season, asters and chrysanthemums step into the spotlight. Their colorful blooms bring vibrant shades of purple, pink, blue, white, yellow, orange and red to the autumn landscape.
"Many gardeners plant asters and mums for their beautiful fall color, but they also play an important role in supporting pollinators," said Aaron Steil, consumer horticulture specialist with Iowa State University Extension and Outreach. "At a time of year when fewer plants are blooming, these flowers help sustain many insects as they prepare for winter or complete their migrations."
A valuable fall food source
Late-season flowers are especially important for pollinators. Bees are actively gathering pollen and nectar reserves before winter, while butterflies such as monarchs need energy-rich food sources during their fall migration south. Asters are particularly valuable because they often bloom from late summer until frost and attract a wide range of pollinating insects. Some asters provide benefits beyond nectar production, serving as host plants for the larvae of butterflies and moths, like the pearl crescent butterfly.
"Spend a few minutes watching a blooming aster on a warm September afternoon, and you'll quickly see just how attractive these plants are to pollinators," Steil said. "The flowers can be covered with native bees, honey bees, butterflies and other beneficial insects."
Not all mums are created equal
Chrysanthemums are among the most recognizable fall flowers, but gardeners should understand that not all mums perform the same in Iowa landscapes. Florist mums, often sold in grocery stores and home centers for seasonal decoration, are generally not winter-hardy in Iowa and rarely perform well as long-term garden plants. Garden mums are more cold-tolerant and bloom early enough to be reliable landscape perennials.
"Garden mums and florist mums may look similar at first glance, but their garden performance can be very different," said Steil. "If you want a plant that has a better chance of returning year after year, choose hardy garden mums."
Why gardeners should plant more asters
Asters are some of the most dependable perennials for Iowa gardens. Most are long-lived, winter-hardy and bloom for weeks in late summer and fall. Their daisy-like flowers are available in a wide range of colors and plant sizes, making them useful in nearly any landscape setting.
"Asters are among the toughest and most rewarding fall perennials gardeners can grow," said Steil. "Once established, they provide years of reliable color and outstanding pollinator activity."
Care tips to keep fall bloomers looking their best
Garden mums grow best in full sun and well-drained soils. They benefit from consistent moisture during dry periods and often develop a fuller, bushier habit when pinched back several times in spring and early summer. For the best winter survival, leave stems standing through winter and apply a protective mulch after the ground freezes.
Asters grow best in full sun and well-drained soil as well. Some species can get quite tall and may flop and lose lower leaves by season’s end. Stake asters to prevent flopping; many benefit from being cut back slightly in late spring or early summer to create more compact plants with additional flowers. Planting taller selections in the back allows them to shine without crowding out other perennials, and those same perennials help cover their bare lower stems. Good air circulation also helps reduce problems with powdery mildew.
Top picks for Iowa gardens
For Iowa gardeners, asters offer options for every garden size. Taller selections include New England asters (Symphyotrichum novae-angliae), such as 'Alma Potschke,' 'Harrington's Pink,' 'Wedding Lace’ and 'September Ruby,' which are native and provide excellent pollinator value. Other good choices include smooth aster (S. laeve) and Tatarian aster (Aster tataricus), a robust species that can reach five to seven feet tall and creates a display of lavender-blue flowers in late fall.
For smaller spaces, consider asters that stay under 24 inches tall. 'Purple Dome' forms a dense mound and is among the most reliable asters for Iowa gardens. Aromatic asters (S. oblongifolium) such as 'October Skies' and 'Raydon's Favorite' are outstanding native pollinator plants. Compact New York asters (S. novi-belgii), including 'Professor Kippenburg,’ 'Wood's Purple,' 'Lady-in-Blue' and 'Snow Cushion,' fit well in smaller gardens. Gardeners can also consider Italian aster (Aster amellus), a European species valued for its drought tolerance and long bloom season. Cultivars such as 'King George,' with violet-blue flowers, and 'Veilchenkönigin' ('Violet Queen'), with rich purple blooms, perform well in sunny, well-drained sites.
When selecting chrysanthemums, gardeners should first decide whether they want seasonal color or a long-lived perennial. Fall is the perfect time to purchase mums for containers and garden displays, but these plants are best treated as annuals regardless of cultivar. Choose any flower color or form that appeals to you and enjoy the display through autumn. If a perennial planting is the goal, however, wait until spring to plant hardy garden mums. Spring planting gives roots time to establish before winter and greatly improves survival. Many hardy garden mum cultivars, including selections developed by the University of Minnesota, perform well in Iowa.
