- Macrophylla or big leaf (including mophead and lacecap hydrangeas, and blue or pink-flowered varieties like Nikko Blue)
- Panicle/paniculata (including white-flowered varieties that change to green and rose tones)
- Smoothleaf hydrangeas (like Limelight)
- Oakleaf hydrangeas
- Climbing hydrangeas.
These all have varying degrees of winter hardiness, so it's crucial to know their growing zone limits and their potential vulnerability in winter weather.
Paniculata and smoothleaf hydrangeas are generally very cold hardy, as are oakleaf hydrangeas and climbing hydrangeas, so these varieties don't usually need additional winter protection.
Macrophylla hydrangeas, however, are a different story. An occasional problem with macrophylla hydrangeas is their failure to flower in summer. There's a common misconception that this problem can be addressed by simply using fertilizer. But, actually, the more likely reason for this is a failure of the flower buds, which begin forming in late summer, to survive cold winter temperatures. Blue-flowering macrophylla hydrangeas, for example, are sometimes only winter hardy to Zone 6, meaning a cold winter can potentially kill the buds.
Consider your Hydrangea Location
The first consideration for winter hydrangea care is figure out if the shrub is planted in an appropriate location. If your macrophylla hydrangea has a hardiness zone classification where the lowest zone is even slightly higher than the one you live in, your hydrangea will very likely fail to form spring buds. Even if the zone is the same, if your shrub is too exposed to winter weather, this might also damage the buds.
Many mophead and other macrophylla hydrangeas are generally hardy in USDA Zones 6 to 9. Recent cultivars such as the "Endless Summer" varieties are meant to be hardy in Zones 5 to 8 (although the "Endless Summer Bloomstruck" variety is hardy to Zone 4). The "Cityline" hydrangeas are mostly hardy to Zone 5.
To be on the safe side, if you live in a cold zone with a typical Northeast winter, your hydrangea should be hardy to at least Zone 5. Planting your hydrangea near a structure that holds some heat (like a brick building or foundation) and in a spot that gets bright sunlight and shelter from the wind in winter will help.
If your macrophylla hydrangea only blooms occasionally in summer, or sometimes skips a year of blooming, moving it to a more protected site may increase the chances it will bloom. Mulching the base will help to some extent as well, but the main area to be protected is the budding branches.
Methods of Winter Protection
There are a few steps you can take to protect your mophead and lacecap hydrangeas in winter. One fairly common method is to create a simple structure that will help insulate your plant. This structure needs to allow air to circulate.
Placing garden stakes in a circle around the shrub and wrapping with burlap, chicken wire or an open-weave natural fiber fabric are all effective. Inside this makeshift structure, you can layer some lightweight insulation material such as pine straw or oak leaves. This will create a protective "zone" of warmer air and wind protection.
Keep this structure in place all winter and remove as temperatures begin to warm in spring. As wind or snow cause the insulation material to settle, thereby exposing the tips of budding branches, you'll want to add more material to replace it. Gather enough pine straw (long dried pine needles) and large oak leaves in autumn and keep in a paper lawn bag, sheltered from rain, to use later. Some gardeners find cutting a piece of styrofoam or cardboard to place over the top of the structure will prevent further damage to buds from winter weather.
Another method for protecting hydrangeas is to wrap them loosely in foam (such as egg-crate foam) or insulation material (such as the foil insulation used to mail climate-controlled packages).
Wrap the entire shrub, taking care not to break any branches, and secure gently but firmly with twine, clips and/or duct tape. Some creative gardeners make these wrapped shrubs look like big wrapped gifts, with a ribbon tied up in bows, coinciding with the winter holidays.
There are commercially-made structures you can purchase from your garden store for hydrangea protection as well, but a creative gardener can often make do with found materials and objects already in the garden shed.
Winter Water and Food for Your Hydrangea
Hydrangeas, as the name suggests, need hydration to thrive. Keeping them well watered before the ground freezes in winter helps the roots and shrub stay stronger for the season. Drying winter winds can also sap moisture from shrubs.
Feeding your hydrangea's surrounding soil with nutrients will also help it stay healthy during the winter. Soil that has used up too many nutrients can become "thin" and will make plants more vulnerable to winter damage.
A top-dressing of compost or composted manure makes a perfect meal for your hydrangea to digest over the long winter season. A mix of used coffee grounds and wood ash is also fine. These additions add acidity and alkalinity to the soil, respectively, and adding both together helps balance the soil pH levels, as well as feeds the soil with organic nutrients.
An added layer of lightweight natural mulch (such as pine bark mulch, oak leaves, hay, or pine straw) will help keep moisture levels consistent. Ideally, an early blanket of snow helps with this too, but these days we can't always rely on "normal" weather patterns.
Add six to eight inches of mulch over the compost layer, but only after the ground freezes. Mulching before a hard frost might attract rodents to the base of the plant, or fool the plant into thinking it's warmer than it is. Waiting until after hard frost will mean the plant has gone further into its dormant winter state, and the added protection will be like a winter blanket.
In spring, gently clear away the mulch when the danger of hard frost has passed.