Native to the sandy, acidic bogs of the eastern Carolinas, Venus flytrap (Dionaea muscipula) is a perennial herb that tolerates summer heat as well as chilly winter temperatures. For this plant to remain healthy and survive year after year, including as a houseplant, it must experience a cool winter dormancy period. Even though Venus flytraps are often sold and marketed as a humidity-loving tropical rainforest houseplant for terrariums, a wet and warm winter leads to their demise.
Venus flytraps naturally transition into their annual dormancy as day length shortens in fall. According to the American Botanical Society, that roughly coincides with Thanksgiving to Valentine's Day (November through February). In the natural habitat, this corresponds to chilly temperatures that cause leaves to become slightly reddened, and moist soil.
If growing a flytrap indoors in a terrarium or small glass bowl, the dormancy period requires temperatures around 30 to 40 degrees F at night and no more than 50 to 60 degrees F during the day. Floridata notes that the home environment is too warm for plants in winter. Short bouts with temperatures below freezing are OK, but can lead to leaf abortion. The Venus flytrap needs bright light or direct winter sun during this dormancy.
Once in dormancy, little or no trapping of insects for nutrition occurs. Because the plant is not actively producing flowers or growing new leaves and traps, its needs are low. Maintaining a barely moist soil and high ambient humidity prevents leaves or traps from prematurely browning over winter. According to Floridata, over-watering in winter is a bad cultural practice. Only use rainwater (or melted snow that has warmed) to water the plants, as it is free from salts and chlorine.
Venus flytraps can live as long as 20 years if they are grown properly, including experiencing annual dormancy each winter. The dormancy conserves energy when day length, temperature and insect food supply isn't ideal for regular growth. The dormancy also sustains a bio-rhythm that leads to natural production of the dainty flowers in late spring.
If growing Venus flytrap outdoors (U.S. Department of Agriculture Hardiness Zones 6b through 9a), cover plants with a piece of towel or sheet when temperatures at night are expected to drop below 25 degrees F, and remove it during the day. Indoor-grown plants need cooler winter temps, so placing them in an unheated room, basement window or garage/shed often provides the right temperatures. Make sure the plants get bright light during the day.