Winter dormancy: which carnivorous plants need it and how to provide it

What dormancy is, and what triggers it

Dormancy is a seasonal pause: a plant from a place with cold winters stops growing, protects its growing point and waits for spring. For the Venus flytrap, NC State Extension describes the main trigger as shortening day length in autumn, with cool temperatures of roughly 2 to 10 °C (35 to 50 °F) reinforcing it; in the wild the rest runs from about November into early spring, three to five months depending on the weather ^[6]. The ICPS LED page makes the same point for temperate carnivores in general: they read day length and the changing quality of sunlight to decide when to grow, bloom and rest ^[7].

Not every carnivorous plant rests in winter, and not every rest looks the same. Some keep a few low leaves, some pull back into a tight winter bud called a hibernaculum, Mexican butterworts switch to a different kind of leaf, and some sundews from Mediterranean climates rest in summer instead.

Which plants need a winter rest

Two other groups need a mention. Pygmy and tuberous sundews mostly come from places with wet winters and dry summers, so they grow in the cool season and go dormant in summer, as a stipule bud or an underground tuber ^{[16, 17]}. Cephalotus stems die back after flowering, and also under constant light, which can look alarming but is expected ^[18]. For Nepenthes the ICPS window sill page describes no dormancy, only a warning that cold drafts from windows can kill growth tips in winter ^[19].

The dormancy planner turns this table into a month-by-month plan for your hemisphere and setting.

Outdoors: protection by USDA zone

The ICPS guides give hardiness by USDA zone for the main outdoor genera.

Where sources disagree. NC State Extension’s Plant Toolbox calls the Venus flytrap winter hardy in zones 7 to 10 and growable in zones 5 and 6 with winter protection ^[20], a more optimistic reading than the ICPS guide’s “problematic” for zones 7 and 8 ^[4]. The difference is mostly about risk. The ICPS author, who lives in zone 7/8, stopped growing flytraps outside after a surprise late freeze killed them ^[4]; his CPN article explains that the plants had survived being frozen solid twice that winter but had already started growing when the late freeze came ^[21]. Extension’s dormancy article advises protecting outdoor plants from hard freezes below about −7 °C (20 °F) ^[6]. For bog construction and winter mulch, see bog garden.

Early and late freezes are the real danger

Fully dormant plants handle a surprising amount of cold. Potted Sarracenia can stay frozen solid for a few weeks, provided they are fully dormant and kept from freeze-drying, and snow is a good insulator worth piling onto the plants ^[1]. Flytraps can survive a week frozen solid under the same conditions, and the ICPS names freeze-drying as the biggest danger, so do not let them dry out ^[4]. Darlingtonia can be frozen solid if fully dormant with the rhizomes covered, and plants of high-elevation origin are more cold tolerant ^[8].

The problem is a freeze that arrives while plants are not yet dormant or have already woken. The CPN article puts it plainly: if the growing point will freeze, an outdoor flytrap must be dormant by then or be moved to shelter ^[21]. The ICPS rhizome rot page describes freeze-damaged Sarracenia rhizomes turning mushy, most often in S. psittacina, S. minor and S. leucophylla; a plant can be saved if some of the rhizome is still firm and white, by cutting away everything soft or brown and replanting ^[22]. If a surprise freeze threatens unmulched plants, the Sarracenia guide suggests covering them with row cover ^[1].

Overwintering under cover

Sarracenia in pots can spend winter in a cold greenhouse, cold frame, unheated garage, unheated sun porch or even a basement. The storage place must stay cool to cold, because plants kept too warm start growing early and struggle when moved outside in spring ^[1]. The same guide normally recommends removing only brown leaf parts, since green leaves feed the rhizome, but plants stored under cover benefit from a fall trim of long pitchers for air circulation, because old leaves can harbor pests and fungus ^[1]. That ties in with grey mold during dormancy.

For a flytrap overwintered in a garage window, the ICPS suggests extra light but warns against bringing it into the house under lights for winter unless the timer follows local day length; switching back and forth puts plants out of step with the seasons ^[4]. Extension’s article gives the same advice in other words: consistency matters most ^[6]. Hibernacula need care too: D. filiformis and D. tracyi can rot in winter if the soil surface is too wet, so their guide suggests extra sand on top ^[9].

For flytraps given a rest as houseplants, the New York Botanical Garden describes letting the light dwindle naturally, lowering the temperature gradually to about 45 to 50 °F (7 to 10 °C), stopping feeding and keeping the medium just barely moist, then resuming normal care around March ^[23]. Extension recommends at least 10 weeks at 35 to 50 °F (2 to 10 °C) ^[6].

The refrigerator question

This is the clearest disagreement among sources, and it applies only to Venus flytraps kept indoors.

• ICPS. The flytrap guide says plants only need dormancy if they will face freezing temperatures outside, and that putting an indoor plant in the refrigerator to make it dormant is a waste ^[4]. Its author’s CPN article says older books assumed every flytrap needs a winter rest because their authors grew in greenhouses; his plants indoors under 15 hours of LED light a day do not slow down in winter, and “No refrigerator is necessary” ^[21].
• New York Botanical Garden. Dormancy is not needed indoors but may benefit the plant; experts differ, and a rest is worth considering if you cannot give enough winter light ^[23].
• NC State Extension. It agrees with the ICPS for plants under strong grow lights but expects windowsill plants to go dormant on their own, and says forcing such plants to keep growing through winter usually gives weak plants ^[6].

None of these four sources recommends a refrigerator for a flytrap. In the ICPS guides the refrigerator appears for seeds, not plants: cold temperate sundew and Darlingtonia seeds get about four weeks of damp cold before sowing ^{[2, 8]}, covered in cold stratification.

More on the plant itself in the Venus flytrap profile.

Day length matters as much as cold

For several groups the light schedule is what keeps the cycle working. The ICPS LED page says temperate Drosera need a natural light cycle or they can go dormant permanently, that most Mexican Pinguicula need gradual changes in day length to live long and bloom, and that Sarracenia more than a year old need light cycles that match the season ^[7]. Mexican butterworts kept under continuous light get stuck in the non-carnivorous winter form ^[14]. Cold temperate sundews are tricky in greenhouses and terrariums for the same reason, and D. linearis drifts in and out of dormancy at the wrong times in mild climates and declines ^[2]. Flytrap seedlings show the cycle too: they grow for about four months and then stop, expecting winter, unless kept warm under intense light and fed heavily ^[4]. Timer settings are covered in light.

Mistakes the sources warn about

• Mistaking dormancy for death. Blackening flytrap leaves in November are usually the start of dormancy ^[6]; see traps turning black.
• Moving outdoor plants inside for winter. Both the ICPS and Extension advise against switching regimes ^{[4, 6]}.
• Storage that is too warm, which wakes Sarracenia early ^[1].
• Letting dormant plants dry out, which leads to freeze-drying ^[4].
• Cutting off green Sarracenia leaves in autumn, which can starve a plant without a large rhizome ^[1].
• Giving cold temperate plants no real winter, which the guides for D. rotundifolia, D. anglica and cold temperate Pinguicula say they need to survive long term ^{[2, 3]}.