The transition requires multi-step preparation and acclimation to ensure a healthy winter for the entire plant collection.
Understanding Cold Tolerance and Damage
Plant damage from low temperatures falls into two main categories: chilling injury and freezing injury. Chilling injury occurs in tropical and subtropical species when temperatures drop below 50°F to 55°F. This damage is caused by cell membranes hardening, which inhibits metabolic processes like photosynthesis and nutrient translocation. Symptoms include water-soaked spots, wilting, and reduced growth, which can appear even after the plant is moved back to warmer conditions.
Freezing injury results from the formation of ice crystals within the plant tissues when temperatures fall below 32°F. Extracellular ice crystal growth pulls water out of the cells, causing severe dehydration and cell collapse. Intracellular freezing, where ice forms inside the cell’s protoplasm, is immediately lethal because it mechanically disrupts the cell’s structure. Environmental factors like high humidity and wind can intensify the perceived cold, making plants more vulnerable to damage even if the thermometer reading is slightly higher than the critical threshold.
Temperature Guidelines for Common Plant Groups
The most common container plants grown outdoors are sensitive to mild cold snaps, making the nighttime temperature a crucial indicator for moving them inside. The general guideline for most houseplants is to begin the move when overnight temperatures consistently approach 50°F. Waiting too long increases the risk of cold shock and exposure to chilling injury.
Tender tropical plants, such as hibiscus, impatiens, and certain palms, are the most sensitive and should be moved indoors first. These plants originate from environments where temperatures remain warm year-round, making their cells highly susceptible to damage once the temperature drops below 50°F. The absolute latest cutoff for these species is before the temperature dips below 45°F to avoid irreversible harm.
Semi-hardy plants, including citrus trees, geraniums, and general houseplants like ficus or peace lilies, have a slightly higher cold tolerance. They can remain outside until nighttime temperatures are consistently near 40°F to 45°F, but it is safer to bring them in earlier, around the 50°F mark, to prevent stress. Succulents and cacti generally prefer to stay above 40°F to 50°F. Since these fleshy plants store large amounts of water, they are vulnerable to freezing injury below 35°F, as the expanding water destroys the cell structure.
Preparing Plants for Indoor Transition
Bringing plants indoors requires preparation to prevent the introduction of pests and minimize shock from the environmental change. A few weeks before the anticipated move, plants should be gradually acclimated to the lower light levels they will experience inside. This involves moving them from full sun to a shaded outdoor spot for one to two weeks, which helps them adjust to the reduced light intensity and prevents severe leaf drop.
Thorough pest management is essential, as outdoor pests like spider mites, aphids, and fungus gnats can quickly infest an entire indoor collection. Every part of the plant, including the undersides of leaves and stem crevices, must be inspected for hitchhikers. A strong jet of water can physically dislodge many pests, and a subsequent application of insecticidal soap or Neem oil will eliminate the remaining insects.
Cleaning and pruning prepares plants for their winter stay by removing dead or diseased foliage that might harbor pests or fungi. The pot should be cleaned, and the top layer of soil can be replaced, as many pests lay eggs near the surface. For plants that can tolerate it, soaking the entire pot in water for about 15 minutes can force out soil-dwelling pests and ensure a pest-free transition.
Adjusting Care Needs Indoors
Once plants are situated indoors, their care routine must be significantly altered to reflect the reduced light and activity of the winter environment. Indoor light levels are drastically lower than outdoor sun, even near a bright window. Plants often need to be placed in the brightest possible location, such as a south-facing window. Supplemental grow lights may be necessary for tropical species that prefer 12 to 16 hours of light per day to prevent them from becoming leggy or dropping leaves.
The most common cause of indoor plant demise during winter is overwatering, as cooler temperatures and lower light cause most plants to enter a semi-dormant state. Since growth slows, the plant’s demand for water decreases, and the soil takes much longer to dry out. Watering frequency should be reduced, allowing the top inch or two of soil to dry out completely to prevent root rot.
Heated indoor air is typically much drier than the summer outdoor environment, which is problematic for tropical plants requiring high humidity. The dry air from heating systems can cause leaf tips to brown or curl, so methods for increasing local humidity become important. Grouping plants closely together creates a microclimate of higher humidity, and placing pots on a tray filled with pebbles and water allows for passive humidification as the water evaporates.