Though technically perennials, mums are often grown as annuals, owing to shallow root systems inclined to heave right out of the ground during winter's freeze-thaw cycles. If you live where winter can be frigid (USDA Plant Hardiness Zones 3 through 6), go with early-blooming varieties—they are more likely to come back in subsequent years. Those of you who live south of Zone 6, on the other hand, can push the envelope, opting for varieties that need more time to reach their full potential. When winter looms, apply a mulch of crisscrossed evergreen boughs to help plants make it through the cold weather ahead.
Several years ago, horticulturists at the University of Minnesota developed a new chrysanthemum hybrid described by perennial breeder Neil Anderson, Ph.D., as a "hardy shrub mum." Dubbed the My Favorite Series, the robust plants were touted as reliably perennial as far north as Zone 3b. They have been hard to find in the past couple of years but returned to the market this year under a new name, the Mammoth Series. Seven colors are currently being sold at nurseries, with more varieties scheduled for release in 2007. True to their new designation, these oversize mums can measure 2 to 3 feet tall and 4 to 5 feet across by their third season, with a massive canopy of flowers—up to 5,000 per plant. Monarch butterflies adore these shrubby wonders.
Finally, to further increase the odds that your chrysanthemums will survive winter, consider planting them in spring as opposed to late summer, to give root systems ample time to become established.