Bringing tender outdoor plants inside for the winter is a practical strategy for preserving varieties that cannot tolerate freezing temperatures. This process, known as overwintering, allows gardeners to save favorite container plants, tropical specimens, and non-hardy perennials that would otherwise perish in the cold. It requires a structured approach to safely transition the plants from the high light and humidity of the outdoors to the drier, dimmer conditions of an indoor environment. Successfully overwintering provides a head start on the next growing season once the threat of frost has passed.
Selecting Overwintering Candidates
The decision of which plants to bring inside should focus on frost-sensitive perennials and tropicals grown in containers. Tender perennials, which are often treated as annuals in cooler climates, are excellent candidates, including varieties like geraniums (Pelargonium), fuchsias, begonias, and coleus. Tropical container plants such as hibiscus, citrus trees, and bougainvillea also require indoor protection, as their tissues are damaged when temperatures fall below 40°F. Specific herbs, most notably rosemary and bay laurel, can be brought inside to continue active growth or enter a semi-dormant state.
Pre-Move Preparation and Pest Eradication
The transition indoors should begin well before the first expected frost, ideally when nighttime temperatures consistently drop into the 45°F to 50°F range. Before moving, plants should be pruned, cutting back long, leggy growth by up to one-third to manage size and reduce the amount of foliage that must adjust to lower light levels. This step also helps to minimize the hiding places for pests that may be hitchhiking indoors.
Pest eradication is necessary, as outdoor insects can rapidly multiply in the stable indoor environment and infest existing houseplants. The entire plant, including the undersides of leaves and the pot exterior, should be thoroughly washed with a strong stream of water to dislodge insects and debris. Submerging the entire pot in a tub of water for several hours can force hidden pests, such as slugs or millipedes, to the surface of the soil.
A final chemical treatment with an insecticidal soap or horticultural oil, such as neem oil, is recommended to eliminate remaining pests and their eggs. After cleaning, remove the top inch of soil and replace it with fresh potting mix, as the surface layer can harbor insect larvae and fungal spores. Upon entering the home, plants must be isolated from all other indoor plants for a period of two to four weeks, allowing for daily monitoring and immediate treatment of any developing infestations.
Indoor Environment and Winter Maintenance
The indoor winter environment must be managed to prevent shock and encourage survival. Light is often the most limiting factor, as the intensity of light received near a south-facing window is significantly lower than full sun outdoors. Plants that were actively growing may require supplemental lighting from full-spectrum LED grow lights for 12 to 16 hours a day to maintain healthy foliage and prevent stretching.
Temperature management depends on the plant’s overwintering goal; most tropicals prefer a stable range of 65°F to 75°F during the day and 50°F to 60°F at night. However, some tender perennials, like geraniums, can be kept in a cooler, semi-dormant state, near 45°F to 50°F, if light is insufficient for active growth. Plants should be situated away from cold drafts from doors or windows and dry heat from vents or radiators, as these fluctuations cause leaf drop and stress.
The shift indoors necessitates a reduction in the watering schedule, as lower light and cooler temperatures slow the plant’s metabolism and water uptake. Water only when the top one to two inches of soil is completely dry to the touch, and suspend fertilization entirely until late winter. To combat the low humidity typical of heated indoor air, grouping plants together creates a localized microclimate, or placing pots on trays filled with water and pebbles can increase moisture around the foliage.
Re-Acclimation in Spring
The transition back to the outdoors in spring, known as hardening off, is a gradual process that strengthens the plant’s cellular structure against wind and intense sun. This process should begin once the threat of the last hard frost has passed and involves sequential exposure over a period of seven to ten days. Plants that have been indoors are accustomed to a stable environment and a low level of ultraviolet radiation.
Starting on an overcast day is ideal, placing the plants outside in a fully shaded, sheltered spot for just one or two hours. Direct sunlight at this stage will cause severe leaf burn, or sun scald, because the protective waxy cuticle on the leaves has not fully developed indoors. Each subsequent day, the amount of time spent outdoors is increased by an hour or two, and the plant is moved into slightly brighter conditions.
By the end of the re-acclimation period, the plants should be able to tolerate full-day exposure to the elements and are ready for their final placement in the garden. This slow process ensures the plant can synthesize the compounds necessary to protect itself from harsh outdoor conditions, preventing shock or plant death.