Carnivorous plant soil: peat, sand, perlite and sphagnum mixes by genus

What carnivorous plant soil has to do

Carnivorous plants come from wet, nutrient-poor ground, and the ICPS seed guide warns that regular planter mix and fertilizer can kill them ^[7]. The soil therefore has two jobs: hold plenty of water while staying low in nutrients and salts, and still let air reach the roots. The CPN article on indoor flytraps describes the balance: pure peat tends to turn stinky, pure sand would not wick up enough water, and the goal is a mix that keeps roots wet without becoming anaerobic ^[8]. In the default mix, the ICPS says, the sand is there to open up the soil and keep it from getting too soggy ^[1].

The right mix also depends on the pot and how you water. For Venus flytraps, the ICPS says the wetter the soil, the taller the pot should be; short pots need a sandier mix, and the water level should stay more than 5 cm below the soil surface ^[9]. The Cephalotus guide goes further: “Do not blindly follow someone else’s soil mix advice, including the advice here.” Its point is to check whether the soil stays wet but not soggy and adjust ^[3]. The watering side is covered in water.

The ingredients, one by one

Sphagnum peat

The ICPS CP mix page recommends Canadian sphagnum peat in North America and German sphagnum peat in the EU, with no fertilizer added. It warns that planting media sold by fertilizer companies may contain fertilizer or lime that is not obvious on the bag, and says to avoid sedge peat mined in the US and any forest humus ^[1]. Quality varies from lot to lot, so stems and other debris may need picking out ^[1].

Many growers wash peat. Washing removes some of the nutrients and spores that feed moss, cyanobacteria and liverworts, and it lowers salt; some brands carry salt at levels toxic to carnivores. The trade-off is that many carnivorous plants appreciate the small amount of nutrients in unwashed peat ^[1]. The CPN author washes his peat and finds the most common brand is usually much too salty ^[8].

Sand

The ICPS asks for sharp silica or quartz sand with grains of about 1.5 to 2 mm, sold as washed and graded sand. It recommends #14 sand-blasting sand, with #12 and #16 acceptable. Play sand, plaster sand and builders sand should not be used straight, and pool filter sand is too fine; fine particles set the mix like concrete. The page’s verdict is that no sand is better than the wrong sand ^[1]. The subtropical sundew guide gives a slightly finer target of about 1.2 mm (12 to 16 mesh) ^[10], and the CPN author prefers 14 and 16 mesh ^[8].

Perlite

Perlite is a good alternative to sand with drawbacks. In some areas it is hard to find perlite that is not salty, because the water sprayed on it to control dust during manufacture may be salty. The ICPS keeps perlite damp because its dust harms the lungs, washes it in a bucket and uses only the pieces that float, and uses it only for plants outside that get rained on ^[1]. The Sarracenia guide likewise allows perlite in place of sand only outdoors, where plenty of water flushes the soil ^[11]. For Venus flytraps, the ICPS flytrap guide says to avoid perlite, pumice and other potentially salty components ^[9].

Long-fiber and live sphagnum moss

Long-fiber sphagnum moss is dried Sphagnum. The ICPS says Chilean and New Zealand sphagnum decompose slowly and so last well as a potting medium, highly recommended in Nepenthes, Heliamphora and Darlingtonia mixes, while dried Wisconsin sphagnum is not much better than a peat substitute ^[12]. Origin also matters for sustainability: New Zealand moss is grown sustainably and is expensive, Chilean moss is not grown sustainably, and Wisconsin moss definitely is not and may contain dried carnivorous plants ^[12]. The page suggests growing your own in a wide, short pot in the water tray, out of hot sun ^[12]. Two cautions: sheet moss sold for hanging baskets is not Sphagnum whatever the package says ^[5], and the ICPS recommends gloves when handling North American sphagnum and other organic potting materials because of sporotrichosis, a disease caused by a soil fungus ^[12].

A sand top layer

Several ICPS guides finish pots with pure sand. On flytraps, about 1 cm of sand makes older leaves less likely to rot completely ^[9]; the CPN author uses about 2 cm and adds that it keeps moss off, lets air into the soil and does not attract fungus gnats ^[8]. Cephalotus and Heliamphora growers use a 1 cm sand layer against moss, and Mexican butterwort pots get up to 5 mm against fungus gnats ^{[3, 6, 4]}.

What “equal parts” really means

Ratios are by eye, not by scale. The ICPS author says that a one-to-one mix means it looks one-to-one when mixed, which can take three scoops of fluffed peat to one of sand ^[1]. The CPN article reads “50:50” as “not overly sandy” and “not overly peaty”, with more sand in shallow pots and more peat in deep ones ^[8]. Plants from very wet, peaty places get a peatier mix and plants from sand or gravel a sandier one, and an exact copy of wild soil is not required ^[1]. Keep the ingredients damp while measuring and mixing ^[1]. The soil mix calculator turns the ratios below into volumes for your pots as a starting point.

Mixes by plant group

Why some groups break the pattern

• Cephalotus. The 50:50 mix may work better at first, but over time the peat breaks down and collects salts from the water, which is why the guide moves to about 20% organic matter ^[3]. See the Cephalotus profile.
• Mexican butterworts. They are not bog plants. Most prefer alkaline soil, hence the lime; a house needs a heavier, more water-absorbent mix than a humid greenhouse ^[4].
• Heliamphora and Nepenthes. Heliamphora need a mix that holds a lot of water yet keeps air spaces, made of ingredients that last, because they often go two or more years without repotting ^[6]. Nepenthes cuttings root in a loose, well-drained medium with little or no nutrients, and coir should be labeled as leached by monsoon rains so the salts are gone ^[5].
• Drosera regia. It needs wet soil but seems less tolerant of anaerobic conditions than most Drosera and Sarracenia ^[21].
• Pygmy sundews. The extra sand slows the moss that tends to overgrow them ^[22].

Mistakes the sources warn about

• Potting mix and the wrong sand. The checklist rules out potting mix, play sand and builders sand ^[2]. The CPN article adds that potting mix is too coarse, pH-adjusted and fertilized ^[8].
• Fertilized or limed peat. Read the bag; avoid products from fertilizer companies ^[1]. Too much fertilizer in the soil kills even easy plants such as bladderworts ^[24]; see feeding.
• Perlite, pumice and vermiculite for flytraps. The CPN author avoids all three: perlite and pumice tend to be salty and vermiculite quickly breaks down to mush ^[8].
• Fungicide in butterwort soil. The ICPS says fungicides of any kind, including cinnamon, are deadly to Pinguicula even in the soil ^[4].
• Reusing old soil. The CPN article says not to reuse flytrap soil indoors ^[8], and the Heliamphora guide notes reused soil can spread a fungus that infects the plants ^[6].
• Letting live sphagnum take over. It can overgrow small flytraps, sundews and Heliamphora ^{[9, 18, 6]}.

When soil wears out

Soil does not last forever. The ICPS flytrap guide says the soil eventually gets too salty and new traps fail to develop, so a well-kept plant needs repotting about every year ^[9]. Subtropical sundews can be kept for years with repotting every 3 to 5 years ^[10], and Mexican butterworts need fresh soil after about five years, or sooner if a salt crust forms ^[4]. Timing by genus is in repotting, and salt damage in mineral burn from tap water.