Venus Flytraps' Yearly Rest
Venus Flytraps need their yearly dormancy like humans need sleep. If they are deprived of their dormancy, their health will decline.
Like many plants that grow in a temperate environment (areas of the world that experience a cool to cold winter), Venus Flytraps have become accustomed to growing rather liberally during the growing season, sometimes using stored food more quickly than they are able to replace it if they don't catch many insects, and then growing very conservatively during their dormancy period, replenishing their food supply (mostly from photosynthesis), storing that food in the leaf bases (the "rhizome") underground, giving them a generous supply of food for their growth the following Spring when they emerge from dormancy.
Some plants have a dormancy during which almost all activity ceases, or the only growth that happens is below the ground in the root zone. However, Venus Flytraps continue to grow and to photosynthesize even during the winter, during their dormancy, so it's important to give them light during dormancy.
Venus Flytrap leaves generally grow more upright or erect during the warm weather and summer months, but grow shorter, with smaller traps that stay very close to the ground during cool to cold weather, such as throughout their winter dormancy and early Spring. Although there is a substantial amount of growth and activity in the rhizome and roots underground during dormancy, the rosette of leaves visible above ground at the soil surface is much smaller during dormancy, often 3-5 times or more greater in diameter or leaf length during the warm growing season.
Starting in late summer as the nights become cooler and the days shorter, Venus Flytraps' new leaves grow more slowly and progressively smaller, until they form a very compact rosette at ground level. This gives Venus Flytraps some protection from freezing air temperatures by being close to the thermal mass of the earth, which is usually quite a bit warmer when the air above it is cold, generally warming and cooling much more slowly than the air.
When the older leaves die as new leaves are formed, the blackened leaves and leaf areas can be trimmed from a Venus Flytrap. This helps the plant to look neat, and more importantly prevents the dead leaves from shading the new leaves from the sun they want and need.
There is however an exception to trimming the dead leaves in the fall, and that is when the Venus Flytraps are left outside all winter, in places that experience only light frosts and no hard freezes (hard frost can kill Venus Flytraps). When grown outdoors all year round, the dead leaves can help protect and keep the plant warmer during its winter dormancy, a little like a protective mulch functions on other garden plants, or a blanket over a cold person. Although Venus Flytraps can recover from having some leaves frozen and killed (the part of the leaf that is above ground), it requires using some of their stored food to grow new leaves, which means that there will be a little less food for the plant's initial Spring growth. It's better for the Venus Flytrap to protect its leaves from freezing during the winter.
Venus Flytrap dormancy can last for 12 to 16 weeks or more. If the weather is cool and highly variable in the Spring, Venus Flytraps might stay dormant for longer than the average period in order to avoid the possibility of frost. However, if grown in a greenhouse or other enclosed area that nevertheless gets plenty of light, Venus Flytraps can and often do emerge from dormancy perhaps one to four weeks earlier than they would in their natural environment, in response to the less variable, more stable temperatures in a protected or controlled environment. This earlier Spring awakening from dormancy doesn't harm Venus Flytraps if they are allowed to emerge from dormancy when they want to do so, and if the temperature in their dormancy environment is not raised too high too fast.
How Cool Should it be During Dormancy?
Venus Flytraps do not need to be very cool or cold during dormancy, and should be protected from frosts and freezing conditions if possible. In order to give Venus Flytraps a comfortable dormancy, most of the hours of the day (but not all) should be somewhat cool to fairly cold, for example in the 40-55 degree Fahrenheit range (4-13 degrees Celsius). But it's fine for the temperatures to rise during the day, especially in sunny weather, into the 70s Fahrenheit or even the 80s (21-30 degrees Celsius). So long as most of the hours of the day are on the cool to cold side, the rest of the hours can be warmer. Remember, no freezing. So long as a Venus Flytrap's leaves are alive and exposed to sunlight, they create food for the plant through photosynthesis and store that food for later use. If the leaf is frozen, it dies and another one must grow using some of the stored food to replace the dead leaf.
How Long Should Dormancy Last?
Although Venus Flytrap dormancy usually lasts about three-and-a-half to four-and-a-half months in nature, 12 to 14 weeks is more typical in the sheltered and at least partially controlled conditions that Venus Flytraps are typically grown under. The shortest length of time one can expect for Venus Flytrap dormancy is about 10 weeks. For that brief a dormancy to occur, the plants need to experience a Fall, Winter and Spring in which the plants are protected from freezing, the air temperature is a little warmer and more stable than what they usually experience in nature, with daily temperature variations that are not too extreme (not much too hot and then much too cold within a period of hours or several days). Those conditions are rarely found in nature, but can be achieved in cultivation when growing Venus Flytraps in a greenhouse or other sheltered place where they receive light and the temperature can be controlled with an artificial heater.
Regardless of the length of dormancy, Venus Flytraps should be allowed to enter into and emerge from dormancy when they want to do so. The shorter leaves and slower growth in the Fall is a sign that the plants are preparing to enter dormancy, and a sudden, rapid growth spurt in the Spring is a sign that they have decided to emerge from dormancy. A grower should generally not try to force a Venus Flytrap to become dormant when it is still actively growing, nor try to force it to emerge from dormancy by keeping the temperature of its surrounding air too high or too constantly warm. This type of manipulation of Venus Flytrap dormancy is likely to adversely affect the health of the plant. All a grower can safely accomplish with regard to giving Venus Flytraps a comfortable and not-too-long dormancy is to provide a little warmer and stable temperatures in the fall, winter and spring than they would be exposed to in nature, prevent the Venus Flytraps from freezing, and avoid wide temperature fluctuations over the period of one or several days. Under relatively stable and not-too-cold nor freezing conditions, Venus Flytraps will enter dormancy later than they would in nature, prolonging their growing season, and emerge from dormancy earlier than they normally would, giving them the opportunity for an early commencement of their Spring growth spurt.
How Much Water During Dormancy?
Venus Flytraps use much less water during their dormancy because they aren't growing much. In addition, water doesn't evaporate nearly as fast in cool to cold weather as it does during warm or hot weather, especially if there is also considerable wind. The plants should be watered thoroughly (in the morning ideally, to give the soil surface some time to dry before night time and colder temperatures that can promote fungal infection) and then allowed to dry out until just moist (much more dry than wet) before they are watered thoroughly again, and that regimen should be continued throughout dormancy and during the active growing season, allowing the plants and soil to dry considerably between thorough waterings.
During dormancy, Venus Flytraps can be susceptible to fungal and bacterial infection and rot, even to the point of death. Cool, wet conditions promote these kind of infections, so when the plants are cool in dormancy, care should be taken not to allow them to stay wet for too long, and to be more dry than wet most of the time, without ever drying out completely. It requires a little experience to learn and know how often to water one's plants, but with some observation and testing of the soil for moisture content (by simply poking a finger down into the soil to see how wet it still is under the surface) you'll learn a regimen that is suitable for your plants in your environment. As a rough comparison of water needs, during the active growing season, depending on container size, temperature, wind, etc., Venus Flytraps might need water every 2-5 days; during cool dormancy, those same plants in the same containers might need water only every 10-14 days or so.