"Mums are ubiquitous in fall gardens. However for mums to be truly hardy, they are best planted in the spring. The mums for sale in fall nurseries have been coddled to set buds for September blooms and are putting an awful lot of energy into blooming, not growing roots. Planting these out in the garden in late summer / early fall doesn't guarantee sufficient time for the plants to become established. This is not a problem in warmer climates, where a bit of deadheading will satisfy most mums after bloom, but in areas with sub-zero winters, freezing and thawing of the soil will heave the plant out of the ground and kill the roots."