As our gardens fade in autumn, many of us buy potted chrysanthemums for a final display of blooms that may last through an early frost. Can they have a lasting place in our gardens?
It depends, according to Sharon Yiesla, plant knowledge specialist at The Morton Arboretum in Lisle. Even though chrysanthemums are biologically perennial, not all of the mums you buy in pots in September or August will be winter-hardy if planted in the ground.
“Some of them are hardy and will be tagged as hardy mums, but many aren’t selected or grown to be garden plants,” she said. “They’re intended to be enjoyed for a few weeks and then discarded.” If you are hoping to plant a mum in the garden, check the label carefully before you buy.
To make chrysanthemums into dense domes of bloom for sale in early fall, growers manipulate the plants. The mums usually are grown in greenhouses, where they are pinched back, heavily fertilized and often treated with growth-regulating hormones.
The wild ancestors of today’s chrysanthemums in eastern Asia were winter-hardy. In China, where some chrysanthemums were domesticated more than 3,000 years ago, they have long been revered as symbols of longevity because their blooms can survive so long into autumn. Today, hardiness is not always a priority for the growers who are choosing cultivated varieties of mums to sell for their short-term show.
Other perennials also bloom in fall and are more reliably hardy, such as toad lilies, Japanese anemones, and native plants including turtlehead, false sunflower, goldenrod and asters.
If you want to experiment to see if a potted mum will survive in the garden, check the label to make sure it is a hardy mum and it is rated for USDA climate Zone 5.
Remove the mum from the pot and plant it in the ground as soon as you can. “It needs as much time as possible to develop its root system before the ground freezes,” she said. “The earlier you buy it and plant it out, the better.”
Choose a full-sun site with well-drained soil that contains plenty of organic matter. Water the plant well and cover its roots with a layer of mulch.
A chrysanthemum plant that comes back in spring will not naturally grow into the tidy, domed shape it likely had when you bought it, but it will still be an attractive garden plant. To make it neater and more compact, pinch the stems back in early summer. Stop pinching by mid-July so flower buds have time to form.
Water and fertilize the plant regularly. Chrysanthemums are not drought-tolerant and once they dry out, they will not recover.
Your mum may flower any time in the late summer or early fall, depending on the cultivar.
There’s a way to get more predictable results: Buy and plant hardy garden mums in spring like you do other perennials. “That way you’ll know what you’re getting,” Yiesla said. “You can make sure you’re planting a hardy cultivar with the right bloom time in a size that will suit your garden.”
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or plantclinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.