How Late Can You Plant Potted Mums?

Chrysanthemums, usually colloquially shortened to just "mums," comprise about 20 different plant species and scores of cultivars. Once botanically known as Chrysanthemum, these plants are grouped into a new genus called Dendranthema. Mums survive winters by becoming dormant and sprouting from their roots in spring. Not all mums are created equally--some quickly die if winter temperatures become too cold. In general, expect mums in the garden to grow well across USDA hardiness zones 4b through 9.

Mums growing in containers can be planted in the outdoor garden as soon as available at the nursery. Once fall deepens and mums are in full flower, they need to be planted as soon as possible in the ground to establish their roots before the ground freezes--especially if you want these plants to survive winter and return next year. Ideally, you should plant mums at least four weeks before frosts occur in your climate. This provides some time for the container-confined roots to grow into the garden soil and acclimate before the flowers and stems brown and the plant becomes dormant.

For vigorous, healthy mums, plant them in a fertile, moist but well-drained soil. They appreciate fertilizer applications up to three times a year, but too much fertility causes plants to grow tall with weak or scrawny stems. Choose a sunny location, receiving at least six hours of direct sun daily. If the root ball of a container-grown mum is compacted with a dense matrix of roots, jostle some roots free by scratching the root ball or making cuts 2 inches deep with a shovel blade or knife so replacement roots grow outward into the soil after planting.

According to herbaceous perennial plant expert Allan Armitage of the University of Georgia, the fall-flowering mums are complex genetic hybrids from plants native to China and Japan. Newer hybrids bred initially by A. Cumming in the 1930s are called Korean hybrid mums. Modern mums don't necessarily possess the same hardiness characteristics as their wild species parentage. Florist mums, as Megan Bame of Learn2Grow points out, have fibrous roots that don't survive cold winter soil as well as the thicker stolon-like roots of garden mum varieties. Thus, unless you know much about a mum's genetic lineage, it may or may not survive winter in your garden depending on your climate.

Armitage explains that chrysanthemums develop flowers based on exposure to certain lengths of nighttime darkness. In the Southern U.S., where winters are cool but relatively mild (USDA zones 7 and warmer), early- and late-flowering mum varieties grow well, although some florist mum types may only reliably come back each spring in USDA zones 8b and 9. In the Northern U.S., (USDA zones 4b through 6) hardier garden mum varieties are the best choice since they naturally bloom in the longer nights in autumn. Armitage mentions that florist mums in the Southern garden can bloom in spring with the long nights. Florist mums that survive the Northern winter grow over summer and likely do not develop and open their flower buds until fall when killing frosts and freezes already occur.