Overwintering is the practice of protecting non-hardy or sensitive plants from the damaging effects of cold temperatures and freezing weather, allowing them to survive the dormant season. Gardeners choose to overwinter plants to preserve expensive, rare, or sentimental specimens. The process allows favorite plants to grow larger and mature over multiple seasons, leading to more substantial displays and earlier growth the following year.
Common Plants to Bring Inside
The plants most commonly moved indoors for winter protection fall into three main categories, each requiring a slightly different overwintering approach. Tender perennials behave as annuals in cold climates because their root systems cannot withstand freezing soil temperatures. Plants like zonal geraniums, fuchsias, and lantana are killed by hard frost, requiring an indoor environment to maintain them in an active or semi-dormant state. They are often kept in containers for an easy transition inside before temperatures drop below 50°F.
Tropical plants and sub-tropical shrubs require warm indoor conditions, often with high light, to prevent cold damage. Hibiscus, mandevilla vines, citrus trees, and bougainvillea suffer cellular damage when exposed to temperatures near freezing. While some tropicals may slow their growth indoors, they must be kept in a bright, warm location to maintain their foliage and health.
The third category includes tender bulbs, tubers, and rhizomes that store energy underground but cannot tolerate wet, cold soil during the winter. Dahlias, cannas, caladium, and tuberous begonias will rot in freezing ground, even if the foliage is killed by frost. These plants are typically dug up, separated from the soil, and stored in a cool, dark, dry environment to force complete dormancy, ensuring viability for spring replanting.
Preparing Plants for Indoor Storage
Bringing plants inside requires preparation to ensure a smooth transition and prevent pest infestations. The first step is a thorough pest inspection of all foliage, stems, and the soil surface for insects like aphids, spider mites, and mealybugs. These pests rapidly multiply indoors due to the lack of natural predators. Any signs of infestation should be treated with a strong jet of water, insecticidal soap, or horticultural oil. A second treatment is recommended one week later to eliminate newly hatched eggs.
Pruning reduces the plant’s size for easier placement indoors and removes damaged or diseased growth. Generally, plants should be cut back by no more than one-third of their total size, focusing on leggy stems and excess bulk. Ground-grown plants must be carefully dug up and potted into containers slightly larger than their root ball. This process should be completed a few weeks before the first hard frost, allowing the plant time to adjust before moving inside.
Plants intended for active growth (like tropicals) are moved indoors immediately after cleaning. Those intended for dormancy (like tubers) follow a different path, usually forced by allowing the foliage to die back naturally after the first light frost. Once the foliage is cut back, these storage organs are dug up and allowed to cure or dry slightly for a few days. They are then placed in a cool, dark location between 40°F and 50°F in material like peat moss or sawdust.
Strategies for Protecting Outdoor Plants
Plants that are semi-hardy or remain in the ground require protection in situ to survive the cold. A primary defense is applying a thick layer of organic mulch, such as straw, shredded leaves, or wood chips, around the base of the plant. This 2-to-4-inch blanket insulates the root zone from extreme temperature drops and prevents damaging freeze-thaw cycles that can heave plants out of the soil. Keep the mulch a few inches away from the main stem to prevent rot and pest issues.
For taller, more exposed plants like sensitive shrubs or small trees, physical barriers provide protection from harsh winter winds and sun scald. Wrapping the plant loosely with burlap or a breathable frost cloth reduces moisture loss and shields the foliage from drying winds. The covering should not be pulled taut against the leaves, as direct contact transfers cold and causes damage. Creating a simple tripod or cage structure before wrapping helps maintain an insulating air pocket.
Temporary structures like cold frames or hoop houses protect low-growing plants and container groupings. These structures trap radiant heat emitted from the soil during the day, maintaining a slightly warmer microclimate overnight. The coverings must be vented or partially removed on sunny days to prevent the internal temperature from rising too high, which can prematurely break dormancy.