Temperatures below 20 degrees F may kill flower buds and damage the roots and crowns of unprotected plants. Repeated freezing and thawing of the soil through the winter months can heave unprotected plants out of the soil and also cause considerable damage. The application of 2 to 4 inch mulch in late October to early November should give protection. Leaves are not good mulch, as they tend to mat together and do not provide adequate protection. Alternate freezing and thawing of the soil can heave plants out of the ground.
Damage is most severe in poorly drained soils.
Gardeners can increase the odds of their mums surviving the winter by protecting them in the fall. Mulches can be applied in early November. The mums should not be cut back in the fall. Recent research found that unpruned plants survived in winter better than those pruned in the fall. Several inches of mulching materials like straw, pine needles or evergreen branches can be used.