Feeding carnivorous plants, and why fertilizer is risky

Does your plant need feeding at all?

A carnivorous plant that grows outdoors usually feeds itself. The New York Botanical Garden expects a flytrap kept outside in summer to catch its own prey ^[7], and the ICPS rhizome rot page says adult Sarracenia should catch plenty of insects without help ^[2]. Plants indoors or in terrariums are different: they will catch little or nothing, which is why the ICPS bloodworm page exists ^[8].

For an indoor Venus flytrap, feeding is not optional. The ICPS flytrap guide says the plant must eat regularly, that you should feed it when its growth slows, and that it declines if feeding stops. The more it is fed, the larger it grows and the more often it needs repotting ^[1]. The author’s CPN article names strong light and regular feeding as the two keys to keeping flytraps indoors long term ^[9], and the checklist says to feed a plant as soon as possible if it stops producing new leaves ^[10].

Food does not replace light. The ICPS Sarracenia fertilization page states that no amount of fertilizer makes up for too little light or the wrong temperatures ^[3]; see light. Seedlings are a special case: until they are big enough to take a tiny piece of bloodworm, they are effectively starving and can only be given time ^[11].

What to feed

• Live insects. The flytrap checklist suggests catching flies in a jar and chilling them in the refrigerator for a few hours so they cannot fly ^[10].
• Dried bloodworms. These are freeze-dried midge larvae sold as fish food. Read the ingredient list and skip brands with anything besides bloodworms, since some additives damage plants. A 5 to 10 g container lasts a few years ^[8]. The ICPS page also notes that packages warn some people may have an allergic reaction to them ^[8].
• What not to use. The checklist warns that worms can eat their way out of a trap ^[10]. The New York Botanical Garden says not to feed meat, cautions that caterpillars can escape the same way, and notes that house ants can carry toxic substances ^[7].

Some sundews need gentler food. Drosera prolifera and D. schizandra burn easily from bloodworms and foliar fertilizer, and the ICPS recommends small live flies for them ^[12].

Feeding a Venus flytrap with bloodworms

The ICPS step-by-step page describes the method ^[8]:

1. Put a pinch of dried bloodworms on a dish or pot lid, not in the plant’s pot, add a drop or two of water, and blot off the excess.
2. Take a piece about one third the size of the trap. A bigger piece will be eaten, but the trap dies after digesting it.
3. Place it in the trap with a toothpick and touch the trigger hairs a few times so the trap closes.
4. Press both sides of the trap very gently with forceps, wait 30 minutes to an hour, and press again. Closing is only the first phase; without the second, sealing phase, there is no digestion.
5. If the trap reopens after about 24 hours with the food undigested, add a drop of water to the food, trigger the trap again and squeeze it gently.

A healthy trap can reopen and feed three or four times, so feeding the same trap repeatedly is fine ^[8]. For seedlings, the page suggests feeding one or two traps on each plant once a week ^[8], and the NYBG gives about every one to two weeks for adult houseplants ^[7]. When a seedling’s trap reopens, remove the remains of the previous meal before feeding again ^[1]. Traps that blacken after a meal are covered in traps turning black, and the plant itself in the Venus flytrap profile.

Feeding other genera

For other adult Sarracenia, the two ICPS pages by the same author pull in slightly different directions. The fertilization page says adults that are not catching insects are better off with some dilute fertilizer in the pitchers than in the soil ^[3]. The growing guide warns that fertilizer water poured into long tube pitchers can backfire, because the plant pumps it into the soil, and recommends full sun and access to prey instead ^[15]. The growing guide singles out the S. purpurea complex, including the purple pitcher plant, as the plants that like water in their pitchers and benefit from a little fertilizer there ^[15].

Why fertilizer in the soil is risky

The ICPS feeding index sums it up: fertilizing the soil of flytraps or sundews is living dangerously ^[17]. The flytrap guide puts it more bluntly, saying not to fertilize “unless you don’t care if you kill them” ^[1]. The reasons, as the sources give them:

• The plants are not built for it. Flytraps live in very low-nutrient soils and are not used to dealing with fertilizer ^[10]. For subtropical sundews, soil fertilizer that does not kill outright still changes the soil chemistry in ways the plants dislike ^[13].
• It feeds rot. In Sarracenia, old rhizomes carry dead tissue that is harmless in poor soil. Fertilizer lets weak pathogens such as Rhizoctonia and Fusarium grow on that dead material and then attack live tissue, and over-fertilizing burns root tips that let pathogens in ^[2]. See rhizome and root rot.
• It feeds moss and algae. Fertilizer encourages moss in Sarracenia pots, so they need transplanting more often ^[3], and moss, algae and cyanobacteria can quickly overgrow Cephalotus seedlings given dilute soil fertilizer ^[6].
• It spreads through trays. Fertilizer leaking from Drosera regia pots into a shared water tray can harm other sundews and flytraps ^[4].
• Even easy plants die from it. Terrestrial bladderworts can be killed by too much fertilizer in the soil ^[18].

The exceptions

• Juvenile Sarracenia. Soil fertilizer makes a large difference to their growth rate; under ideal conditions a mature plant can come from seed in as little as three years. In the ICPS trial, four slow-release high-nitrogen pellets per small pot worked, six did no harm, and outdoor plants are best fertilized just as spring growth starts ^[3].
• Drosera regia. Unlike other Drosera, it appreciates soil fertilization; the ICPS author uses four to six high-nitrogen pellets per pot each spring, and plants that are neither fertilized nor well fed decline ^[4].
• Heliamphora. It needs fertilization to grow well. Commercial growers add about 1 g of fertilizer per 40 L of water, raising its TDS by about 25 ppm; the guide treats 70 ppm as generally safe in the soil if the pots are top-watered well between feedings, and calls too much soil fertilizer one of the easiest ways to kill the plant ^[5].
• Cephalotus. It can take pellets or very dilute urea-free orchid fertilizer if the pot is flushed regularly, though the plants may turn darker green and make more non-carnivorous leaves ^[6].
• Mexican butterworts in greenhouses. They appreciate light soil fertilization in pure mineral mixes, but with fertilizer in the tray water it is very easy to over-fertilize and burn them ^[19].

Fertilizer also raises the dissolved solids in your water, which matters for the limits in water.

Mistakes the sources warn about

• Feeding pieces larger than about a third of the trap ^{[8, 7]}.
• Giving dead food without massaging the trap, so it reopens undigested ^[8].
• Feeding sundews that have no dew, or feeding dry food ^[13].
• Pouring fertilizer water into long Sarracenia pitchers ^[15].
• Overfilling Heliamphora pitchers until in-pitcher feeding becomes soil fertilization ^[5].
• Feeding young Cephalotus too much or too soon, which the ICPS says has killed plants ^[6].