Weak color and floppy growth in carnivorous plants: light and other causes

What normal looks like

Color and leaf shape vary for reasons that have nothing to do with health, so start by ruling those out.

• Genetics. Flora of North America notes that Venus flytrap petioles vary from long to short under both genetic and environmental influence, and that strong sunlight sometimes brings out red in the traps, but not always ^[4]. Cultivars differ widely in trap color ^[5]. The ICPS describes ‘Justina Davis’ as making no red pigment at all, so it stays green in any light, and ‘Korean Melody Shark’ as having long, non-leafy petioles that take up a lot of space ^[3].
• Season. In spring, as flower stalks emerge, many flytraps put up upright leaves with long, narrow petioles, while some cultivars do not. Wild-type plants hold their leaves upright seasonally, unlike the short-petioled ‘B52’ ^[3].
• Year-to-year variation in Sarracenia. The ICPS notes that some S. flava lids open green and slowly turn red, and that some hybrids are confused with red-tubed forms depending on how dark the tubes get in a given year ^[6].
• Flat summer leaves. Several Sarracenia make flat, non-pitcher leaves called phyllodia in midsummer ^[2]; in S. oreophila they usually outnumber the pitchers ^[7].
• Butterworts on a windowsill. The ICPS notes that Mexican butterworts on a windowsill are greener than plants in a greenhouse or under lights but grow and bloom very well ^[8].

Signs that light is the problem

Low light shows up as a pattern across the plant rather than one odd leaf. Flora of North America calls the Venus flytrap very sensitive to shading ^[4], and the ICPS checklist warns that a flytrap short of light starts declining immediately and that slow-closing traps can mean it wants more light ^[9]. In Sarracenia, pitchers from heavily shaded or dry sites may be smaller, flat like phyllodia, or weak and lying down, which is why botanists identify species from plants grown in full sun ^[2]. Cephalotus is most colorful in strong light ^[10], and shaded sundews make less sticky leaves ^[11]; see sundew not making dew and traps not closing.

The troubleshooter lists low light and naturally green cultivars as the two causes to check first.

Cause 1: a window or room that is too dim

How to check. Compare your light with the ICPS figures:

All classes are for lights on 12 to 16 hours a day. For reference, the ICPS puts full sun at about 100,000 lux at noon, bright shade at about 15,000 lux and typical office lighting at about 1,000 lux ^[1]. A lux meter helps when you use white light, but the ICPS warns that meters marketed for plant lighting may give numbers of little value, and that some trial and error is unavoidable ^[1].

Outdoors the rule is hours of sun. The ICPS checklist asks for at least six hours of direct sun a day for flytraps ^[9], and its deck-growing page sets the same six-hour bar for Sarracenia, noting that pitchers are not very good at catching light ^[12].

What to do. Move sun-loving plants outdoors where your climate allows, or add white LED light. The ICPS flytrap guide recommends about 25 W of actual LED power per 25 cm (10 in) diameter of growing area for about 14 hours a day ^[3], and the lighting page gives about 2,000 lumens or 30 W of white LED per square foot for 25,000 lux ^[1]. In hot, dry climates, the ICPS suggests full morning sun followed by part shade for flytraps, or 10 to 20% shade cloth, warning that most shade cloth is 50% or more and too shady ^[3]; for Sarracenia it suggests some afternoon shade ^[6]. See light and the grow-light calculator.

Cause 2: lights that are weaker than they look

How to check. Read the label on your bulbs. The ICPS asks for actual watts, not “watt equivalents” ^[3], and the CPN article on indoor flytraps warns about manufacturers who advertise watt equivalents instead of real power ^[13]. Old lights fade too: the ICPS says LEDs can be down to about 70% of their original brightness after 35,000 hours, sooner if they overheat ^[1].

What to do. Check the lamp height and the setup. The ICPS places lamps high enough to light the whole growing area but no higher, and finds that reflective material on three sides helps a great deal by reducing weak spots at the edges ^[1]. Tall Sarracenia are hard to light from above alone; the ICPS suggests mirrors or other reflective material to reach the sides of the pitchers ^[6]. Under purple plant lights, color is hard to judge, and the ICPS notes plants will not look natural without enough white light ^[3].

Cause 3: a plant that is green by nature

How to check. Find out what cultivar or species you have. If it is ‘Justina Davis’, or another form described as green, no amount of light will make it red ^[3]. Some cultivars, such as ‘Red Dragon’, turn fully red only in full sun ^[5]. Long petioles may simply be the plant’s genetics or the season ^{[4, 3]}.

What to do. Judge the plant by its growth, trap speed and new leaves rather than color alone. The Venus flytrap profile describes the main cultivars.

Cause 4: too much fertilizer

How to check. Has the plant been fertilized in the soil? The ICPS notes that soil fertilization can leave Cephalotus dark green and producing more non-carnivorous leaves ^[10], and in sundews added nutrients reduced leaf stickiness much as shade did ^[11].

What to do. Stop soil fertilizing and feed through the traps instead; see mineral burn and feeding.

Getting color and firm growth back

• Measure your light and match it to the ICPS class for each plant ^[1].
• Give flytraps and Sarracenia at least six hours of direct sun outdoors ^{[9, 12]}.
• Use real wattage, keep lamps at the right height, add reflectors, and replace aging LEDs ^{[1, 3]}.
• Do not fertilize the soil ^[10].
• Judge progress by new leaves rather than old ones; individual flytrap leaves last only a few months ^[9].

Spring’s tall, upright flytrap leaves usually come with a flower stalk; see Venus flytrap flowering. For year-round indoor setups, see growing indoors.