Sundew not making dew: light, humidity, water and dormancy

What the dew is, and when dry leaves are normal

The glistening drops on a sundew’s tentacles are its trap: a sticky secretion that holds insects. The secretion is built mostly from carbon, which the plant gets from photosynthesis, so its production depends on light ^[2]. Field and laboratory studies on the round-leaved sundew agree that plants adjust how much they invest in sticky leaves to their conditions, making less when they are shaded or when nutrients are easy to get from the soil ^{[2, 5, 6]}. A sundew without dew is a plant telling you something about its surroundings.

Not every dry sundew is in trouble. Several groups have a seasonal rest when leaf production stops:

• Cold temperate species form a tight winter bud and stay dormant for months ^[4].
• Every form of the forked sundew, D. binata, goes dormant at some point, even under lights ^[7].
• Warm temperate species cycle between growth and semi-dormancy in greenhouses and terrariums ^[8].
• Tuberous sundews die back to underground tubers by late spring and sit out the summer ^[9].

If your plant is in active growth and its leaves are still dry, work through the causes below, or use the troubleshooter.

Too little light

How to check. This is the first thing to rule out. The ICPS lighting table lists most sundews as needing intense light, about 15,000 to 25,000 lux of white light for 12 to 16 hours a day. The exceptions are D. adelae and D. prolifera, which prefer moderate light of about 8,000 to 15,000 lux, and D. schizandra, which wants low light of about 3,000 lux ^[1]. Most subtropical sundews want more light than a windowsill provides ^[3], and for pygmy sundews even a sunny window may fall short ^[10].

The research supports this. In a shading experiment on D. rotundifolia, shaded plants produced less sticky leaves ^[2]. In a field study at a Finnish bog, tentacle density was about 50% higher on sundews growing in hollows than on those on the more shaded hummocks ^[6].

What to do. Move the plant to more sun or add white LED lighting; the light guide covers fixtures and distances. Do not push the shade-loving species too hard: the ICPS warns that intense light could be too much for D. adelae and D. prolifera ^[1], and says D. schizandra wants no direct sun at all ^[11]. See the D. adelae profile for that group.

Dry indoor air

How to check. Which species is it, and where does it sit? The ICPS says most carnivorous plants need less humidity than their wild habitats suggest, but notes that windows are where houses are most extreme: in heated homes, radiators and heating vents are often right beside or below the glass, so plants get cold drafts and blasts of warm, dry air in turn ^[12]. Subtropical sundews prefer higher humidity than most houses offer ^[3]. Among the Queensland rainforest species, D. adelae does best in very humid conditions and makes a poor house plant unless the house is very humid, D. prolifera needs high humidity, and D. schizandra demands it ^[11].

What to do. Move the plant away from heaters and drafts, or grow it in a terrarium. The ICPS author uses leaky covers that never fog up rather than sealed tanks ^[12]. Some growers succeed with subtropical sundews on open shelves under lights, as long as the spot is out of drafts ^[3]. Not every sundew wants this: tuberous species do not need high humidity and tend to overheat in terrariums ^[9]. See terrariums and humidity.

Mineral-rich water, fertilizer or salty soil

How to check. Measure the water you use. The ICPS rule, set out in its flytrap guide, is that tap water is acceptable only below 90 ppm total dissolved solids, and that ordinary filters do not remove dissolved salts ^[13]. Then check the soil: the subtropical sundew guide asks growers to read peat and sphagnum bags to be sure nothing has been added ^[3].

Extra nutrients do more than risk burning roots. In the shading experiment, adding inorganic nutrients reduced leaf stickiness just as shade did ^[2], and in a comparison of two British bogs, sundews at the site with more available nitrogen invested less in sticky leaves ^[5]. The ICPS describes fertilizing the soil of sundews as living dangerously ^[14], and its subtropical guide says to put no fertilizer of any kind in the soil, since even when it does not kill the plants it changes the soil chemistry ^[3].

What to do. Switch to distilled, reverse osmosis or clean rain water if your supply is above the limit ^[13]. If the soil has had fertilizer or hard water, repot into fresh, unfertilized peat and sand or sphagnum ^[3]. The water guide, the water checker and mineral burn cover this in depth.

A dried-out pot, a recent repotting, or handling

How to check. Did the pot dry out, has the plant just been repotted or moved, or have the leaves been touched? Growing notes from Kirstenbosch National Botanical Garden for the South African D. aliciae say that drying out makes sundews droop and produce less mucilage, and that dew production may stop after transplanting or after a leaf has been handled ^[15].

What to do. Water and keep the plant moist; the same notes say this revives a dried-out plant, and that new leaves will grow and produce dew again after transplanting or handling ^[15]. Avoid touching the leaves to test them, and stand the pot in a tray of water so it cannot dry out ^[8].

A temperate sundew going dormant

How to check. Know which group your plant belongs to and check the season. D. rotundifolia and D. anglica need a definite winter and can be dormant about six months of the year, northern forms of D. intermedia up to nine, and D. linearis about nine. D. rotundifolia spends a third to half of the year as a winter resting bud, or hibernaculum, and North American and European D. intermedia form them too ^[4]. D. filiformis also forms winter buds ^[16]. A forked sundew in dormancy stops making new leaves and dies back to the ground, and plants under lights can do this unpredictably ^[7]. Tuberous species die back as temperatures rise, and heat above about 25 °C (78 °F) can trigger it early ^[9].

What to do. Let the rest happen. Cold temperate sundews can struggle in greenhouses and terrariums because they expect seasonal cues; the ICPS advises keeping resting buds cool and just damp ^[4]. For the Florida form of D. filiformis grown in terrariums, the ICPS notes that well-fed plants are less likely to go dormant ^[16]. Plan winters with the dormancy guide and the dormancy planner; the D. rotundifolia and D. binata profiles give species details.

A plant worn out by flowering or hunger

How to check. Has the plant just flowered, or gone a long time without food? The ICPS says warm temperate sundews can live for years if you stop them blooming, but may decline badly or die after flowering ^[8]. Even annual subtropical species such as D. burmannii can last for years if flower stalks are removed as soon as they appear and the plants are well fed ^[3]. D. adelae declines if it is not fed well ^[11].

What to do. Cut off flower stalks when you first notice them unless you want seed ^[8]. Feed only leaves that still have dew, using live insects or rehydrated food, and use less if it goes moldy ^[3]. Be careful with rich food on the Queensland species: dried bloodworms can burn holes in the leaves of D. prolifera and D. schizandra, which do best on small live flies ^[11]. If a subtropical plant keeps declining despite feeding, the ICPS suggests starting again from root, stem or leaf cuttings ^[3]. See feeding.

Keeping the dew coming

• Give strong light for the species group, measured if possible with a lux meter ^[1].
• Use water under 90 ppm TDS and unfertilized media ^{[13, 3]}.
• Keep humidity-loving species away from heater drafts, or under a leaky cover ^[12].
• Expect and allow winter or summer dormancy in the groups that need it ^{[4, 9]}.
• Remove flower stalks from short-lived species unless you want seed ^[8].

The sundew genus page explains the cultivation groups, and the Cape sundew profile covers one subtropical species in detail.