Hydrangeas have been rediscovered by gardeners who appreciate the low-maintenance and long-lasting bloom of the old-fashioned snow ball or Annabelle Hydrangea arborescens. Newer, hardier hybrids of old mophead, lacecap and oakleaf hydrangea varieties have made it possible for more northern gardeners to enjoy these grand old shrubs. Hydrangeas’ winter needs vary according to their variety, hardiness and garden environment.
Hydrangeas are perennial deciduous shrubs that grow from 3.5 to 10 feet tall depending on variety. Flowers grow in globular or cone-shaped clusters measuring from a few inches to more than a foot in diameter. H. arborescens (also called smooth hydrangea) and H. quercifolia (oak leaf) are native to the U.S.
Most hydrangea varieties thrive in USDA Plant Hardiness Zones 5 to 8. H. arborescens and H. paniculata (panicle), familiar as grandiflora or Pee Gee hydrangeas, are hardy to zone 3 or 4 and some newer hybrids in other groups are equally hardy. Some climbing hydrangeas (H. anomala sp. Petiolaris) are only hardy to zone 6. No amount of winter preparation will save a hydrangea that is not hardy in a climate area, but even the hardiest bushes grown in northern zones benefit from winter protection.
Established hydrangeas may be pruned during winter to remove dead wood or branches that cross or crowd the center of the shrub. Winter pruning should be avoided on oak leaf and H. macrophylla, which includes mophead and bigleaf hydrangeas, until they are mature; these hydrangeas bloom on the previous year’s growth and winter pruning will eliminate next summer’s blooms. Panicle and arborescens bloom on new wood and can be pruned anytime during winter. The old-fashioned arborescens variety “Annabelle” can be cut down to the ground during winter and will spring back with a 5-foot-tall shrub the next summer.
Any hydrangea will benefit from a 1 or 2 inch winter mulch of leaf mold, compost or wood chips around its crown. Gardeners in zones 4 and 5 should cover crowns with 4 to 5 inches of organic mulch. If rodents are a problem, surround the plant with a fence of hardware cloth, pushed into the ground. Hydrangeas planted in their northernmost range may benefit from a wrap in burlap. Mulch and wraps should be added only after dormancy sets in to avoid the die-back of tender growth that results from hard freezes.
Keep watering hydrangeas until the first freeze in November or December. Place mulch under and wraps around shrubs after the first freeze but before the ground freezes. If pruning is necessary on old-growth bloomers, complete it by early winter. Complete all pruning tasks on new-wood bloomers before buds develop in early spring. Wraps should be removed and mulches replaced in early spring for hardy varieties and when the ground warms for more tender plants.