FARGO — What's the best part about the spritely colored potted mums sold in late summer at every national chain, hardware store and garden center? Yes, they beautify front steps and porches cheerfully, but they also keep Halloween decorations at bay for a few weeks, so jack-o'-lanterns and black cat decor don't appear in early September, which rushes the season a bit.
Potted mums, short for chrysanthemums, extend our flowering season, and many are still blooming when our first snowfall arrives, as most mums tolerate a little frost. Potted mums sold in late summer are usually considered non-permanent seasonal decorations to be left in their pots, or planted in the landscape for temporary instant color.
Potted chrysanthemums are heavy drinkers and might require daily watering, or at least daily monitoring. If mums are allowed to wilt when in full bud or bloom, flowering is quickly diminished.
Wouldn't it be nice if mums, with their brilliant fall color, could be planted in our perennial flower beds and grow year after year, instead of just in seasonal pots? Well, they can.
Chrysanthemums are truly perennials, but we must choose wisely, selecting varieties that bloom within our length of season and are winter-hardy for zones 3 and 4. Most potted mums sold for temporary seasonal decoration aren't varieties meant to be winter-hardy in our perennial gardens. Sometimes they'll survive if planted, often they don't.
Mums for perennial gardens
Mums have long been the queen of fall-blooming perennials. A North Dakota State University (then Agricultural College) Extension circular written in 1947 on the culture of perennial mums in northern gardens describes the need to carefully choose winter-hardy varieties that bloom within our season's length, and there weren't many from which to choose.
Today's gardeners are luckier because plant breeders have made great strides. The University of Minnesota has been a leader in North American mum breeding, a program started in the 1930s, before which there were no garden mums that flowered within Minnesota or North Dakota's growing season, according to the university. The program has since released 76 mum cultivars for planting in Hardiness Zones 3 and 4, although some have become unavailable heirlooms.
In 1977, Minnesota researchers discovered the genetic "cushion" habit of mums, having dome shapes with flowers completely covering the outside plant surface, instead of blooms only at the tops of upright plants, like previous types. The cushion habit quickly became the world's standard for mum growth, as the University of Minnesota released the "Minn" series of cushion mums including Minnautumn, Minnyellow, Minnruby and others that remain good perennial garden choices even today.
In 1990, university breeders inspecting fields of seedlings observed some with unprecedented size, vigor and increased winter hardiness, which became the Mammoth Mum series. Their habit is termed "shrub cushion" and the plants can eventually become a flowering dome-shape, 3 to 4 feet high and wide.
There are six steps to achieving the most perennial mum success:
• Choose varieties bred for our zones 3 and 4, such as the Minnesota cultivars.
• Plant in spring from starter plants purchased from garden centers. Mums survive best if given the entire growing season to establish before winter.
• Locate in full sun in a spot that typically receives plentiful winter snow cover. Avoid open, windswept areas.
• Fertilize generously in spring and early summer, but stop by July 4 to allow plants to toughen before fall.
• Leave tops intact over winter, which has repeatedly been shown vital for mum winter survival. Prune off tops in early spring as new growth emerges from soil.
• Cover with 12 inches of straw or wood product mulch in November after soil freezes. Wet leaves or grass clippings can become too soggy, smothering mums. Mum roots are shallow and easily damaged by alternate freezing and thawing of soil, but mulch applied after the soil freezes keeps roots comfortably and consistently frozen until mulch removal in early April.
For a complete list of adapted mum cultivars introduced by UMN, including the "Minn" and "Mammoth" series, visit https://mnhardy.umn.edu/varieties/flowers/chrysanthemums and click on each of the four growth habit categories near the bottom.