Tropical plants can survive winter in colder climates, but their survival depends entirely on human intervention to protect them from temperatures they are not genetically equipped to handle. A “tropical plant” refers to any species that evolved in year-round warm climates and is highly susceptible to chilling injury. Most tropical foliage and flowering plants begin to suffer stress, which can lead to cellular damage and death, when temperatures consistently drop below 50°F (10°C). Since these plants lack physiological mechanisms to withstand cold, their continued health requires a strategy appropriate for the plant type and the local climate.
Categorizing Cold Sensitivity
Understanding a plant’s inherent cold tolerance provides the foundation for any winter survival plan. The USDA Plant Hardiness Zone Map is the most widely used system for predicting a plant’s outdoor viability, dividing North America into zones based on the average annual minimum winter temperature. Each plant species is assigned a hardiness zone, indicating the coldest zone in which it can survive the winter outdoors. Tropical plants are generally rated for Zones 9 through 11, meaning they cannot tolerate the freezing temperatures common in most other zones.
This classification distinguishes between truly tender tropicals and semi-hardy varieties. Truly tender plants, like hibiscus and crotons, are immediately damaged by temperatures in the 40s Fahrenheit (4°C to 9°C) and cannot survive a hard frost. Semi-hardy tropicals, such as certain palms or hardy bananas, can withstand brief dips below freezing, often surviving down to 20°F (-6°C) if their roots are insulated. Knowing the specific cold threshold determines whether a plant requires full indoor protection or can be safely overwintered outside.
Overwintering Plants Outdoors
Keeping tropical plants in the ground through winter requires labor-intensive protection to create a microclimate. For established, in-ground specimens like large palms or banana plants, the primary goal is to protect the root crown and the growing point, known as the meristem. A thick layer of organic mulch, such as straw or shredded leaves, should be applied around the base of the plant, extending out to the drip line to insulate the soil and maintain a stable root temperature.
Before the coldest weather arrives, the exposed portions of the plant may need wrapping. Trunks of palms can be wrapped in burlap or heavy cloth, and tender foliage can be tied up and covered with a frost blanket. For maximum protection, a temporary shelter can be constructed around the plant, such as a wooden frame draped with plastic sheeting, to shield it from harsh winds and freezing rain. Watering should be reduced in the late fall to prevent the soil from becoming saturated, as wet soil freezes more deeply and can lead to root rot.
Moving Container Plants Indoors
Bringing containerized tropical plants inside is the most common method for ensuring their survival, but it requires careful preparation to prevent pest infestations. Before moving the plants, a thorough inspection for insects like aphids, mealybugs, and spider mites is necessary. Any found pests should be treated using horticultural oil or insecticidal soap. Pruning the plant by up to one-third of its size can help reduce its overall volume and manage the shock of moving to lower light conditions.
The indoor environment should be selected to balance the plant’s needs with the home’s conditions. A location near a south-facing window offers the best light exposure, which is important for plants like citrus or flowering hibiscus. Temperatures between 60°F and 70°F (15°C and 21°C) are generally suitable, but the low humidity of heated indoor air presents a challenge. Grouping plants closely together or placing them on trays filled with pebbles and water can help create a localized zone of higher ambient humidity.
Adjusting Care Indoors
During the winter, most tropical plants enter a period of reduced growth, often called semi-dormancy, and their care routine must be adjusted. Watering should be significantly reduced, allowing the top inch or two of soil to dry out completely before adding moisture. Overwatering is a leading cause of death for overwintering plants. Fertilizing should be suspended until new, vigorous growth resumes in the spring, which prevents weak, spindly growth poorly adapted to indoor conditions.
Spring Transition
Reintroducing tropical plants to the outdoors once the danger of frost has passed must be done gradually to prevent sunscald and wind damage. This process, known as “hardening off,” prepares the plant’s tissues for the harsher outdoor elements. For seven to fourteen days, the plants should be placed outside in a sheltered, heavily shaded location for just a few hours each day.
The duration of outdoor exposure should be increased incrementally, moving the plants into brighter light and more wind each subsequent day. Rapid exposure to full sun can cause leaves to burn and turn white or brown, a form of shock that limits the plant’s ability to photosynthesize. Once the plants are fully acclimated, normal watering and a regular fertilization schedule can be resumed to encourage robust new growth.