Hydrangeas are admired for their lush, colorful blooms that brighten gardens and patios. While hydrangeas planted in the ground benefit from the insulating warmth of the soil, those grown in containers are highly susceptible to cold damage. The entire root system is exposed to freezing air temperatures, which can lead to death. Successful overwintering requires careful preparation and placement to ensure the plant enters and remains dormant. This process protects the root ball, securing the plant’s health and guaranteeing a robust return of flowers the following season.
Pre-Winter Preparation
Preparing the hydrangea for its long winter rest begins in late autumn, typically when the leaves start to change color or just before the first hard frost. The goal is to encourage the plant to naturally transition into dormancy, which is required to set flower buds for the next year. Stop fertilizing the plant entirely in late summer, as new growth stimulated by fertilizer is tender and easily damaged by cold temperatures.
As the weather cools, gradually reduce the frequency of watering, allowing the soil to dry slightly more than usual between applications. This reduction signals to the plant that it is time to slow down its growth processes. Continue watering the roots until the plant is moved to its final storage location or the soil freezes solid, as dry roots are more susceptible to cold damage.
Pruning requirements differ significantly depending on the variety of hydrangea. Bigleaf hydrangeas (Hydrangea macrophylla) and oakleaf hydrangeas bloom on old wood, meaning their flower buds for the next season are already formed in the fall. Pruning these types before winter risks removing next year’s blooms, so only dead or diseased wood should be removed.
Smooth hydrangeas (Hydrangea arborescens) and panicle hydrangeas (Hydrangea paniculata) bloom on new wood, developing their flower buds in the spring. These varieties can tolerate being cut back in the fall or early winter without compromising the following season’s flowers. For all types, leaving the dry flower heads in place provides a small degree of natural protection for the developing buds beneath.
Selecting the Ideal Storage Location
Choosing the appropriate environment is the most important factor for successfully overwintering potted hydrangeas. The ideal location must provide a cool, stable temperature that keeps the plant dormant but prevents the root ball from freezing solid. The temperature should consistently remain between 35°F and 45°F (1°C to 7°C) throughout the winter months.
An unheated garage, shed, or a cool cellar is often the most practical choice for providing these conditions. These spaces naturally shield the pots from extreme temperature fluctuations and harsh winter winds. If using an unheated garage, position the pots away from the garage door, as that area is prone to sudden, severe temperature drops.
If a suitable outbuilding is unavailable, a cool, dark basement can work, provided the temperature is not warm enough to encourage premature sprouting. Too much warmth will cause the plant to break dormancy too early, resulting in weak, pale growth that will die when moved outdoors. Outdoor grouping is an option for gardeners in milder climates, but it requires heavy insulation and is generally the riskiest approach.
Protecting the Plant and Pot
Once the ideal location is chosen, the focus shifts to physically insulating the pot, as the root ball is the most vulnerable part of the plant. The pot should be wrapped with insulating materials to create a thermal barrier against the cold.
Materials such as bubble wrap, layers of old blankets, or horticultural fleece are effective for wrapping the pot’s exterior. This insulation should cover the container from the rim down to the base and be secured with twine or duct tape. Placing the wrapped pot on a thick piece of foam board, wood, or bricks prevents contact with the ground, which is often the coldest surface.
A highly effective method is the “pot-in-a-pot” technique, where the hydrangea’s container is placed inside a larger container. The space between the two pots is then filled completely with insulating material like straw, shredded leaves, or wood chips, creating a buffer zone around the roots. Avoid using non-breathable plastic sheeting, which can trap moisture and heat, leading to fungal diseases and rot.
Grouping several potted hydrangeas together can also provide collective insulation, as the combined mass of the pots helps to stabilize the temperature of the root balls. After insulation, the surface of the soil should be covered with a layer of straw or mulch to minimize moisture loss. This protection ensures the roots stay consistently cool and shielded from damaging fluctuations.
Post-Dormancy Transition
While in storage, your hydrangea requires very little attention. The plant will still utilize a small amount of moisture, so the soil should be checked monthly by inserting a finger a few inches deep. If the soil feels completely dry, administer a small amount of water to prevent the roots from drying out, but avoid saturating the soil, which could lead to root rot.
The most delicate phase of the winterizing process is the transition back outdoors in the spring. Wait until the danger of a hard freeze has passed, often several weeks after the last expected frost date, before beginning the move. A sudden shift from a dark, cool garage to a bright, warm patio can shock the plant and damage the newly emerging buds.
The process of re-acclimating the hydrangea, known as hardening off, should be done slowly over one to two weeks. Begin by moving the pots to a sheltered spot outdoors that receives only a few hours of morning sun. Over successive days, gradually increase the plant’s exposure to sunlight and wind.
Once the hydrangea is fully outside, resume a regular watering schedule, increasing the amount as the plant begins to leaf out. Fertilizing should only recommence once new green growth is clearly visible on the stems. This gradual transition ensures the hydrangea wakes from its dormancy healthy and ready to produce a full season of blooms.