Light for carnivorous plants: sun, windowsills and LED grow lights

Why carnivorous plants need so much light

Many carnivorous plants grow fully exposed to the sun, which the ICPS LED page puts at about 100,000 lux at noon. Indoor growing does not need that much, but it needs far more than people use for themselves: the same page gives about 15,000 lux for bright shade and 1,000 lux for typical office lighting ^[1]. Pitcher plants have an extra handicap. The ICPS deck page notes that Sarracenia pitchers are not very good at catching light, which is why the plants want full sun ^[3].

Plants show it quickly when light is short. The ICPS flytrap checklist says a flytrap starts declining immediately without enough light, and that traps closing slowly can be a sign that the plant wants more light ^[4]. Color is another signal: Cephalotus grows best and is most colorful in strong light ^[5]. See weak color and floppy growth and traps not closing for the problem side.

How much light each group needs

The ICPS sorts carnivorous plants into three light classes and adds a note on whether each grows well under purple (deep blue and deep red) plant LEDs ^[1]. The last column adds what the genus guides say about growing them.

The ICPS adds that plants survive with less light than recommended but do not grow as well ^[1].

Outdoors: full sun, with exceptions

For flytraps, the ICPS recommends full sun where summers are humid or cool. Where summers are hot and dry, full morning sun followed by partial shade works, and 10% or 20% shade cloth is fine, while most shade cloth sold is 50% or more and too shady ^[2]. Sarracenia want as much sun as you can give, though in very hot, dry areas the pitchers can burn and some afternoon shade helps ^[6].

Darlingtonia is the odd one. Wild plants are often in partial or even dense shade, and the ICPS guide names keeping the roots cool in warm weather, not light, as the hard part of growing it ^[12]. Pygmy sundews will probably do better outdoors than under lights in a humid terrarium, according to their ICPS guide ^[13].

Windowsills: what they can and cannot do

The ICPS flytrap guide is direct about it: the plant needs more light than it can get on a typical house windowsill ^[2]. The checklist’s minimum of six hours of direct sun a day is, in its own words, hard to achieve in a house ^[4]. Pygmy sundews also need intense light, and a sunny window may not be enough ^[13].

The ICPS window sill page recommends windowsills for plants that do not need intense light. Mexican butterworts are its first choice, and many Nepenthes also do well. With a humid house and a window giving four hours of direct sun, flytraps, Cephalotus, Sarracenia rosea and many Drosera may do well, though the author suspects they will need supplemental light ^[11]. More on choosing and placing plants in growing indoors.

LED grow lights: the numbers

The ICPS LED page gives these targets for white LED light ^[1]:

The page gives 12 to 16 hours of light a day under its intense-light heading. Reflective material on three sides helps a lot by reducing edge effects ^[1]. Moderate-light plants can go at the edges of a grow area, with intense-light plants in the center directly under the lights ^[1]. The grow-light calculator scales these per-square-foot figures to your growing area.

Other ICPS sources give flytrap figures over different areas. The flytrap guide asks for 15,000 to 25,000 lux, about 25 W of actual white LED power per 25 cm (10 in) diameter circle, for about 14 hours a day, or about 12 W of purple plant light for the same circle ^[2]. The CPN article by the same author gives at least 15,000 lux for about 14 hours, better at 25,000, which takes 20 to 30 W of white LED per 30 cm diameter ^[14]. Converted to a square foot, those work out to roughly 45 W and roughly 25 to 40 W, against 30 W on the LED page. The LED page itself does not treat any figure as exact: it says light levels have to be found by trial and error because every setup differs, and that a lux meter helps with white light ^[1]. 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, with the lights about 10 to 15 cm above the plants ^[7].

Buying and setting up lights

• Read the actual wattage. Both the LED page and the flytrap guide warn against “watt-equivalent” claims ^{[1, 2]}.
• A plain white LED is enough. The checklist says you do not need special plant lights; a standard white LED spotlight works ^[4].
• Keep LEDs cool. Spotlights over 10 W should not sit inside an enclosure such as a desk-lamp shade, and panels need air around them; a 35 W panel can run 18 °C (30 °F) above room temperature ^[1].
• Expect dimming. LEDs can be down to 70% of their original output after 35,000 hours. One display terrarium on the LED page fell from about 15,000 to about 10,000 lux over six years ^[1].
• Match the fixture to the plant shape. Spotlights suit plants that hold their leaves out flat, but their narrow beam does not suit tall pitcher plants ^[1]. The Sarracenia guide says tall species are hard to grow under lights without mirrors or reflectors to light the sides ^[6].
• Use a timer. The LED page advises timers rather than plugging and unplugging cords ^[1].

White light or purple light

White LEDs suit plants you want to look at. Purple plant lights, made of deep blue and deep red LEDs, can cut electricity use by 50% to 70% for plants that are not on display, but the plants look dark or purple and it is hard to judge how bright the light is or how green new leaves are. The ICPS advises mixing in some white light so you can check the plants ^[1]. The CPN author uses 3000K LEDs with a color rendering index of 90 or more in the house ^[14].

Sundews are the sensitive group. The ICPS reports that many Drosera react badly to the wrong blue-to-red ratio, that temperate species such as D. intermedia can go dormant early and stay dormant for nine months if they dislike the color, and that many growers use panels with white and deep blue LEDs. Its advice when nothing else works is to use white light ^[1]. If a sundew stops making dew under new lights, see sundew not making dew.

Day length and seasonal cues

Some plants read the seasons from day length, and under constant light they misbehave. The ICPS LED page lists the ones that need a timer that shifts on and off times with the date: temperate Drosera, Mexican Pinguicula, Sarracenia more than a year old, and pygmy Drosera and Cephalotus if you want flowers or gemmae. Venus flytraps do not need seasons to grow well but flower together when they get seasonal cues ^[1]. Mexican butterworts with two leaf types kept under continuous light get stuck in their non-carnivorous winter form ^[15], and Cephalotus under a constant light period does not bloom ^[5]. The ICPS flytrap guide also warns against moving outdoor plants under lights for winter only, unless the timer follows local day length, because it puts them out of step with the seasons ^[2]. See dormancy for the rest of the winter picture.

Mistakes the sources warn about

• Counting on a windowsill to grow a flytrap well ^[2].
• Buying lights by “watt-equivalents” instead of actual watts ^[1].
• Shading plants with standard 50% shade cloth ^[2].
• Enclosing hot LED spotlights, which shortens their life ^[1].
• Keeping plants that need seasonal cues under a fixed day length ^[1].
• Switching sensitive sundews to pure blue/red light ^[1].