Terrariums and humidity: which carnivorous plants need them

Less humidity than you might think

The ICPS window sill page makes a point that surprises many beginners: most carnivorous plants do not need as much humidity as their wild homes suggest ^[1]. It adds a caveat about numbers. Relative humidity is the percentage of the water the air could hold, and air holds less water at higher elevations, so the same reading means drier air in a mountain town than at sea level ^[1].

Humidity needs vary a lot by plant group. The table collects what the ICPS guides say.

If a sundew loses its dew or a Nepenthes stops making pitchers, low humidity is one of the causes to check; see sundew not making dew and Nepenthes not making pitchers.

The Venus flytrap disagreement

Sources differ on whether flytraps need humid air.

• ICPS. Ditch the plastic cup. The checklist says the grower added it to protect the plant in shipping and from neglect in shops, not because the plant likes it, and that terrariums are not needed by flytraps and can harm them. Plants from greenhouses and cups take a while to adjust to drier air, but they do adjust ^[7].
• CPN. The article “Grow Venus flytraps indoors” states that flytraps do not need high humidity. Its author keeps indoor plants in leaky terrariums that never fog, only so he can see them and refill trays less often, and reports no difference between plants grown with and without them ^[18].
• NC State Extension. The Plant Toolbox entry lists high humidity among the plant’s needs, along with wet, sandy soil and full sun to partial shade ^[19].
• New York Botanical Garden. It takes a middle line: the need for extra humidity depends on your home, misting is often unnecessary if watering is done carefully, and a terrarium with adjustable ventilation helps if dry air empties the water dish too fast, though heat can build up in summer and make the plant wilt ^[20].

The positions apply to different situations. The ICPS and CPN advice is written for flytraps in pots indoors. The Plant Toolbox entry is a short profile covering garden and container use alike and does not discuss terrariums ^[19]. None of the four recommends a sealed container or routine misting for a potted flytrap. More in the Venus flytrap profile.

When an enclosure earns its place

• Heliamphora. The main case for a terrarium indoors, below 70% room humidity ^[3]. See Heliamphora.
• Humid-loving sundews. Subtropical species and the three Queensland sisters ^{[4, 5]}; see the Drosera adelae profile.
• Seeds. The ICPS seed guide puts seed pots in plastic bags or a sealed terrarium, which keeps humidity high and keeps out fungus gnats ^[21]; see sowing seeds.
• Cuttings and divisions. Nepenthes cuttings stay sealed in a bag until they root and grow, then move to a terrarium or a partly opened bag to adjust to drier air ^[6]. Heliamphora divisions are bagged for about a month if the terrarium humidity is below 85% ^[3], and Cephalotus cuttings root under a humidity dome ^[11].
• Emergency repotting of a flytrap. If a new plant has to be repotted before it settles, the checklist suggests a plastic bag for a few weeks afterward ^[7].

The seed guide also notes that many species grow well directly under lights with no terrarium at all ^[21].

Setting up a terrarium that does not cook or rot plants

Keep the plants in pots. A photo caption in the ICPS seed guide states that planted terrariums are not recommended ^[21].

Let air move. The ICPS terrarium page explains why its author avoids old fish tanks: they are hard to reach into, tend to fog from poor air circulation, are hard to clean, and make temperature and humidity hard to control. His alternative is a Plexiglas cover that sits on a standard plant tray, with a removable front and room for vents; it has never fogged ^[22]. The window sill page and the CPN article both describe deliberately leaky terrariums for the same reason ^{[1, 18]}.

Watch the heat. Warm, humid and still air is the combination the sources warn about. The Heliamphora guide reports that plants infected with a common vascular fungus can die in under 10 days at a constant 28 °C (82 °F) and high humidity, and warns that bagged plants placed too close to lights or in sun can be killed by heat ^[3]. The subtropical sundew guide gives the same warning for seed bags under fluorescent lights ^[4]. The ICPS terrarium author leaves about 2 cm between his light fixture and the terrarium top ^[22].

Get the light in. Mirror tiles behind the shelf make a large difference to the light the plants receive ^[22]. For Heliamphora in an enclosure with reflective sides, the ICPS gives at least 20 W of LED, or 1,200 lumens, per square foot for 15 hours a day ^[3]. See light for other plants.

Clean with care. Never use an ammonia cleaner on Plexiglas, which fogs the plastic; the ICPS suggests a Plexiglas cleaner or a mix of reverse-osmosis water, a little isopropanol and a tiny amount of detergent ^[22].

Humidity without a terrarium

For plants that need only a little help, a water tray goes a long way. The NYBG notes that the moist pot and tray create some local humidity around a flytrap ^[20], and the ICPS author refills open trays more often in dry air but grows many flytraps without any cover ^[18]. A room or basement that stays above 70% humidity can even hold Heliamphora without an enclosure ^[3]. Tray methods are covered in water, and choosing plants for a house in growing indoors.

Mistakes the sources warn about

• Leaving a flytrap in its shipping cup ^[7].
• Sealed, fogged tanks with no air movement ^[22].
• Heat build-up from lights, sun or summer rooms in closed containers ^{[3, 20]}.
• Putting cool-climate plants in warm terrariums, such as tuberous sundews, D. schizandra and cold temperate sundews ^{[15, 5, 17]}.
• Missing mildew on Cephalotus. The ICPS describes plants that survived because they were treated as soon as mildew was noticed, and notes that whole collections have been lost to fungus attacks ^[11].