Fall preparation for winter depends on the hydrangea. If you are talking about hardy hydrangeas in Minnesota, the Hydrangea paniculata, which includes the cultivars Grandiflora (also known as Pee Gee), Tardiva, Pink Diamond and Unique, they all bloom on new wood, which is wood produced the same year as the flower bud. Therefore, pruning in the spring is recommended. Hydrangea arborescens (Annabelle), which is commonly referred to as snowball hydrangea, blooms on new wood and should be pruned down to a foot or two off the ground in spring.

If you are growing Oak Leaf hydrangea, latin name Hydrangea quercifolia, climbing hydrangea/Hydrangea anomala, Endless Summer, or Nikko Blue/Hydrangea macrophylla, they are considered marginally hardy in Zone 4. So you should plant them in a sheltered location and give them winter protection. These produce flower buds on wood produced the previous year. Mulch the crown of the plant with marsh hay or straw once the ground has frozen. Winter mulch is meant to protect the plant from temperature fluctuations such as freezing, thawing, and refreezing. When a plant comes out of dormancy in the dead of winter, it can be damaged or killed.

Zones 5 and 6 will also need to provide minimal protection to save the flower buds on the Oakleaf hydrangea and the Bigleaf hydrangeas, which includes Nikko Blue and Lacecaps. The flower buds produced on the previous year's wood will live if the winter is mild, but at -10 degrees F or below, the buds will be killed. This means no flowers will be produced if the buds are killed during the winter's cold temperatures.