Hydrangea fall care differs depending on the type of hydrangea growing in your garden. These flowers vary greatly in their cold tolerance and their growing environments, with some varieties growing as far north as U.S. Department of Agriculture zone 3, and others as far south as zone 9. These hardy shrubs provide blossoms all summer long, but improper fall care could harm your plant or reduce the amount of flowers the following year.
Hydrangeas that bloom on old wood may need frost protection in the fall to help prepare them for winter chills. Phyllis Heuerman from the Frederick County Master Gardener Program advises protecting hydrangea macrophylla in zones 6 or colder by creating a chicken wire cage to surround the hydrangea shrub. Plant the chicken wire cage a couple inches in the soil. Fill the cage with autumn leaves, which provide natural insulation from cold. Leave the cage up until frost danger passes in the spring.
Discontinue fertilizing hydrangea plants in the autumn, since you don't want to promote new growth that can be damaged by winter chill. Resume fertilizing in the spring, when frost danger passes for your area.
Prune hydrangea paniculata in the autumn. This type of hydrangea blooms on new wood so you can safely prune it in the autumn without affecting the amount of blooms you'll have the following year. Gardeners can prune hydrangea paniculata within a few inches of the ground in the fall to avoid looking at the dead foliage all winter long. Prune hydrangeas that bloom on old wood--which includes oakleaf and macrophylla hydrangea--by August. These hydrangeas develop their buds for the next year during fall and will not bloom much if you prune them at this time.
Mulch the soil near your hydrangea plant to protect the roots from winter damage. Apply a 2- to 4-inch layer of mulch or pine straw underneath your hydrangea plant. Leave the mulch in place year round or remove it in the spring when your hydrangea does not need protection.
Continue to water your hydrangea plants in the fall months since they still need nutrients. Auburn University advises giving your hydrangea 1 inch of water per week unless the plant received adequate rainfall instead. To determine how much water this is, set a hose on low and run it into a bucket, timing how long it takes to get 1 inch of water in the bucket.