Overwintering Carnivorous Plants
You have put a lot of time, energy and money into your carnivorous plant collection, and that investment can be protected during cold weather with a few basic safeguards. Many carnivorous plants can be grown outdoors well outside of their native range, but seasonal extremes can be a source of stress without the proper precautions.
The guidelines below are what we follow here at Carnivorous Plant Nursery in the Catoctin Mountains of Western Maryland (USDA hardiness zone 6a), where winter temperatures of -9°F (-23°C) are not unusual. Depending on your location, your plants may appreciate more or less protection, but the methodology is universal.
What is dormancy and why is it important?
First, it’s helpful to consider what bog plants experience in temperate latitudes during winter months. Not only are daylight hours reduced, but the lower sun angle (and possibility of snow cover) mean that there is simply not enough light for photosynthesis to sustain plants that require full sun during the growing season. Freezing temperatures will damage plant tissue, preventing new growth and winter can be a very dry season. It sounds contrary at first, but consider that ice does not make free water available to plants. Winter heaving as a result of cyclical freezing and thawing can also disturb roots and in extreme cases uproot plants entirely.
Just like trees drop their leaves in Fall, carnivorous plants require a winter dormancy period. This adaptation causes foliage to recede, concentrating sugars and carbohydrates into the perennial tissues of the plant. Depending on the genus, this can be a corm (Dionaea), rhizome (Sarracenia), hibernaculum (Drosera), turion (Aldrovanda and aquatic Utricularia). In every case, they exist at or beneath the surface of the soil (or sink into deeper water) where they’re protected from extreme temperatures.
Critically, this adaptation is not simply a stimulus response but an important part of the plants’ life cycle and necessary for long term health. If kept in artificial Summer conditions (indoors), temperate carnivorous plants will remain green in the short term but slowly decline and ultimately perish without the rest afforded by dormancy.
What does this mean for the plants in your collection? Dormant plants require no light and only enough water to prevent the soil from going dry.
Let’s run through the best precautions to take for the most common methods of growing carnivorous plants.
Potted Plants Outdoors
The minimum pot size we recommend if you plan to leave it stationary year-round is 16" (40 cm). Anything smaller will almost certainly result in plants suffering from wind and freezing temperatures. Potted plants are like aquariums. The bigger they are, the better they resist swings in temperature. By extension, the best pot is the earth! If you choose to keep them outdoors, one effective method is burying the pot(s) down to the soil surface level and mulch them in as described for bog gardens below.
The safest way to winterize potted plants is by moving them into a cold but protected location like a garage, shed, window well or under a deck or porch. The tray watering method is best for maintaining an adequate moisture level. Unlike during the growing season, allow the tray to go dry between waterings. The surface of the soil should stay moist but never saturated. Exactly how much water is required will depend on your specific conditions, but here’s a good rule of thumb; if pressed with a finger, the soil should yield under gentle pressure, but your finger shouldn’t pull away wet.
Indoor Plants & Temperate Plants in Tropical ClimatesIf you’ve already risen to the challenge of keeping temperate carnivorous plants indoors, providing Winter dormancy is relatively simple. This can be accomplished by simply moving your plants to a cold (but protected) location as described above. Best results will be achieved by maintaining dormancy for the natural duration (November - March), but the minimum is 45 days at 45°F (7°C) for proper rest.
If you live in a tropical climate, or it’s otherwise impractical to move your plants outdoors, this can also be achieved artificially with refrigeration as follows:
- De-pot your plants and bare-root them. (The soil can be saved and reused for several years as the acidity of peat dramatically slows fouling. If the texture has become muddy or slimy or smells foul, discard it.)
- Remove any foliage longer than 1” from the base.
- Wrap the rhizomes, corms or hibernacula in moist long-fiber Sphagnum moss and place them in a freezer bag. Poke about a dozen small breathing holes.
- Store these in the crisper drawer in your refrigerator until Spring. Typical refrigerator temperatures are fine (33-40°F / .1-5°C).
A light dusting of sulfur powder directly on the rhizomes prior to wrapping helps to prevent mold or fungus. Remember to check the moisture level occasionally. The long-fiber Sphagnum should remain moist, but never be saturated. If there’s standing water in the bag it’s too wet. Check on them periodically during the winter rest. If you see signs of growth it's certainly time to bring them out into growing season conditions.
A bog planter is a pot larger than 16” (40cm) diameter with drainage holes on the sides instead of the bottom. This is the minimum recommended size to leave in-situ overwinter if you’re in zone 6 or colder.
We recommend covering your bog planter with a single layer of burlap to facilitate cleanup in the Spring, then mulching with 6+" (15+cm) of pine needles. Top this with bird netting to hold the mulch in place and secure it all with lawn staples around the perimeter of the planter.
Our recommended method of growing temperate carnivorous plants is in-ground, whether it’s a proper bog garden or in-ground boxes. As described above, the earth is a great heatsink and will drastically smooth out rapid drops in temperature. In our collective growing experience, we’ve found that the actual low temperature isn’t as important as how fast it drops. Additionally, this helps to prevent unseasonable thawing during warm winter spells which can prematurely trigger growth, resulting in wasted energy. Combining in-ground growing with winter top-cover has proven successful for us, even with Florida and Gulf-Coast native species when temperatures drop below 0°F.
Though snow itself is an excellent insulator, it is not reliable as a top-cover in many locations. We use 2 different methods to winterize our beds, depending upon the size and shape of the area to be covered.
For smaller and/or irregularly shaped beds, our preferred method is mulching with 6+" (15+cm) of pine needles. Pine needles are superior to leaves because many leaves, like oak, will compact into dense mats that prohibit beneficial breathing (moisture and air exchange). Start with a sheet of burlap, and place the mulch on top. This makes spring removal easier and neater. Top the pine needles with bird netting, and anchor that down with lawn staples to keep it in place.
For our larger, rectangular growing beds we use horticultural frost protection fabric. This is white, porous, like felt, and allows light and moisture to penetrate. It’s easily layered, and we generally apply 3 layers by pre-folding which keeps temperatures approximately 10 to 20°F above ambient. It’s durable with proper handling and storage and can be reused for multiple seasons. Keep an eye out for mice, voles and other creatures which may also appreciate the cozy home you’ve created. They don’t generally damage the plants directly, but their burrowing can cause significant damage. Manual removal of nests followed by traps can both be effective remedies.