Cheerful, hardy and long-lasting, colorful mums (Dendranthema spp.) are a favorite with home gardeners and florists alike. Equally happy in the ground or in a container, mums tolerate a wide range of temperatures and are the perfect pick-me-up for a fading summer garden.
In general, mums are hardy in U.S. Department of Agriculture plant hardiness zones 5 through 9, although this can vary slightly by species. According to the USDA map, the lowest minimum temperatures mums can survive are right around 20 degrees below zero Fahrenheit. However, it's important to note that this is true only if the mums are hardy garden mums, and only if they are planted in the spring so that their roots are allowed to become established.
Because most mums bloom in late summer and early fall, many home gardeners plant mums in late summer and early fall as a way to spruce up their gardens. Mums planted this late will not likely survive cold winters because they have shallow roots. Container plants are a bit more protected from the cold, so they have a better chance of surviving.
Increase your mums' chances of survival by surrounding them with a 4-inch layer of mulch after the ground has frozen, and resist the urge to shear off the dead foliage. Yes, that first hard frost of winter will likely kill the plants to the ground, but that doesn't mean the roots are dead. Hardy garden perennials will return to life in the spring if their roots are established and protected.
Plant hardy garden mums in the spring if you want their roots to survive single-digit winter temperatures.
Most mums can survive hot weather if they are given plenty of water, but heat tolerance varies widely by species and cultivar. If you have temperatures that average in the 90s and 100s during the summer, it's best to plant mums where they will receive some afternoon shade. It's also a good idea to look for heat-tolerant cultivars. Blooms are triggered by fading summer days and cool nights, however, so if nighttime temperatures are too warm -- above average for your area for several nights in a row -- blooming might be delayed.
Florist and Fancy Mums
Potted mums sold as gifts -- like those often seen at a florist -- are exotic and not hardy enough to survive winter temperatures, even in USDA zone 9. These mums, which feature unusually shaped or very large flowers, are usually treated as annuals by home gardeners. Examples of these types of mums include spider mums and football mums.