BogTrapsA field guide to growing carnivorous plants

Care: most carnivorous plants

Water for carnivorous plants: TDS limits, rainwater and the tray method

Published October 15, 2026Awaiting editor review17 sources

Short answer

Use rain, distilled or reverse-osmosis water, or tap water only if its total dissolved solids (TDS) are low: the ICPS Venus flytrap guide puts the line at 90 ppm and its seed guide at 80 ppm [1, 2]. Ordinary filters do not remove dissolved salts [1]. Most bog plants stand in a tray of water that is never allowed to dry out, about 1 to 2 cm deep for a flytrap and 4 to 6 cm for a mature Sarracenia [3, 4].

Why dissolved minerals are the problem

Carnivorous plants come from nutrient-poor ground, and the ICPS seed guide warns that ordinary potting mix, fertilizer and water with moderate or high dissolved solids can kill them [2]. The concern is what is dissolved in the water, measured as total dissolved solids (TDS) in milligrams per liter, which is the same as parts per million (ppm) [5].

The damage builds up over time. The ICPS Cephalotus guide describes peat that slowly accumulates salts from the water until it causes problems [6], and the flytrap guide describes soil that becomes so salty that the traps on new leaves fail to develop [1]. In Sarracenia, the ICPS rhizome rot page explains that salty water burns the root tips, which gives weak fungal pathogens a way into the plant; its photos show a dead growth point on plants hit by water of TDS 350 and pH 8 combined with drying out [7]. The symptoms are covered in mineral burn from tap water and rhizome and root rot.

The TDS limits, page by page

ICPS pages give slightly different limits because each one was written for a different situation. Read them side by side rather than picking one at random.

ICPS page Situation it addresses Limit as the page states it
Growing Dionaea muscipula Tap water for carnivorous plants in general Usable below 90 ppm TDS; above that, distilled, reverse-osmosis or clean rain water [1]
Seed Germination Guide Seeds and seedlings Less than 80 ppm dissolved solids [2]
Display Sarracenia on your deck Large Sarracenia pots outdoors Under 100 ppm TDS, or plenty of summer rain that you can collect [8]
Sarracenia Rhizome Rot An example of damage, not a limit Plants damaged by water of TDS 350 and pH 8 together with drying out [7]

The strictest figure is for seedlings, which sit in the same small pot for a long time. The most relaxed is for big outdoor containers, and the Sarracenia guide notes that plants outdoors get plenty of water to flush the soil [4]. For scale, the author of the CPN article on indoor flytraps reports city tap water of about 50 ppm and expects most readers will need reverse-osmosis or rain water instead [9]. The seed guide agrees that in most places water under 80 ppm means buying or making reverse-osmosis, distilled or deionized water [2]. The water checker applies these same three limits to a reading you enter.

What removes salts and what does not

  • Filters. The ICPS flytrap guide states that filtering water will not remove dissolved salts. Cartridges that contain chemicals to strip salts do work, but they must be replaced often or they release the salt they have collected back into the water [1].
  • Softeners. The Heliamphora guide adds that softening does not make water less salty either [10].
  • Bottled drinking water. The flytrap checklist calls it deadly to Venus flytraps, the deck page specifies reverse-osmosis water and not bottled drinking water, and the CPN article warns that both bottled and filtered water can kill plants [3, 8, 9].
  • Well water. The checklist says most tap water from wells is also fatal to flytraps, and the Heliamphora guide calls well water generally too salty [3, 10].
  • What works. Distilled, reverse-osmosis and clean rain water [1], plus deionized water [2].

Rain is free but the volume adds up. The deck page estimates that each Sarracenia in a large pot uses about a liter of water a day in summer [8]. The CPN article suggests reverse-osmosis water as the fallback when you cannot collect enough rain [9].

Measuring TDS at home

The flytrap guide lists four ways to find out: ask your city, ask the water company, use a well test, or measure with a TDS meter [1]. Pocket meters work by measuring electrical conductivity (EC). Dissolved minerals let water conduct electricity, so the more minerals, the higher the reading; distilled water conducts very poorly [5].

The meter turns EC into an estimated TDS by multiplying by a conversion factor. NDSU Extension’s table for natural waters gives a factor of 0.50 at 100 µS/cm, 0.60 at 500 µS/cm and 0.65 at 1,000 µS/cm, and it keeps rising with conductivity [5]. Units vary between meters, but 1 mS/cm equals 1,000 µS/cm, and mg/L equals ppm [5]. At the low end of that table, with a factor near 0.5, a reading of about 180 µS/cm works out to roughly 90 ppm.

Meters drift. NDSU advises checking an EC meter against a calibration solution before sampling, and again after changing the batteries or dropping the meter, and collecting the sample in a clean container rinsed several times with the water being tested [5]. A reading that sits close to one of the ICPS limits is worth repeating with a calibrated meter.

The tray method

Most bog-type carnivorous plants are grown in pots standing in a tray or dish of pure water that is topped up before it runs dry. The flytrap checklist asks for water about 1 to 2 cm deep and warns never to let the dish dry out [3]. For Sarracenia, the ICPS guide recommends 4 to 6 cm of water, never letting the plants dry out, and an automatic watering system so a trip away is not a risk [4]. The ICPS also publishes a design for a float-valve system that refills trays to a set level [11].

Tray depth is only half of it. For a Venus flytrap, the guide says the water level should always be more than 5 cm (2 in) below the soil surface, so wetter soil calls for a taller pot and short pots need a sandier mix [1]. The CPN article explains why: flytraps are not bog plants that can live semi-aquatically, and the old leaf bases that make up the bulb rot in soggy soil. Its author never top-waters indoor flytraps except when repotting, and lets some trays dry before refilling them to about 3 cm [9]. The pot and mix side of this is covered in soil.

How the method changes by genus

Plant Watering method in the ICPS guides
Venus flytrap Dish 1 to 2 cm deep, never dry; water level more than 5 cm below the soil surface [3, 1]
Sarracenia in pots About 4 to 6 cm of water at all times [4]
Sarracenia in an in-ground bog Water level 7 to 18 cm below the surface, added on the side opposite the drain [4]; see bog garden
Cold temperate Drosera Pots always standing in water while growing, with the pot flooded regularly in hot weather [12]
Cephalotus About 1 cm of water under a 20:80 peat:sand mix, flushed through now and then; keep plant parts at least 5 cm above any likely water level [6]
Darlingtonia Methods differ: the guide’s author keeps tall pots in a few cm of water and top-waters every morning, when the water is coolest, while another grower it cites uses wide pots not standing in water and waters once or twice a day. Either way the soil must stay below 27 °C (80 °F) [13]
Heliamphora A few mm of water, or watering every day or two and draining the excess; pure water only [10]
Mexican Pinguicula Up to 2 cm while the leaves are carnivorous, and trays can dry for a few days; in the succulent winter phase, let the tray dry for about a week between waterings [14]
Nepenthes in the house A heavier sphagnum mix that may sit in water for a day or two while the moss absorbs it [15]
Terrestrial Utricularia A shallow tray like Drosera, or a container without drainage; in the wild the water is near or slightly above the surface [16]

Seedlings follow the same split. Wetland species sit in a little water with the surface sprayed so it never dries, while fog-forest and jungle species are kept very moist but not standing in water [2].

Flushing salts out

Even good water leaves something behind. The Cephalotus guide recommends flushing water through the pot occasionally [6]. With salty well water, the rhizome rot page advises leaching the salts out by overhead watering and repotting more often [7]. Mexican butterworts should be repotted early if a salt crust forms on the soil [14]. The CPN article advises against reusing old flytrap soil indoors; it can go in the garden or to outdoor carnivores that get rained on [9].

Mistakes the sources warn about

  • A dry tray. Both the flytrap checklist and the Sarracenia guide treat a dried-out tray as the thing never to allow [3, 4].
  • Trusting a filter, softener or bottle. None of them lowers dissolved salts [1, 10].
  • Waterlogging plants that are not bog plants. Mexican Pinguicula should not be grown like bog plants, and a flytrap crown kept too close to the water rots [14, 9]. The Mexican butterwort profile has more.
  • Fertilizer in the water. Sarracenia pump fertilizer water from their pitchers into the soil [4], and fertilizer leaking from Drosera regia pots into a shared tray can harm other sundews and flytraps [17]. See feeding for safer ways to give nutrients.
  • Warm water for Darlingtonia. Many growers put ice cubes made of pure water on the soil each day in hot weather to keep the roots cool [13].

Questions

Can I use tap water for a Venus flytrap?

Only if its total dissolved solids are below 90 ppm, the limit in the ICPS flytrap guide. Ask your water supplier or measure with a TDS meter; if the figure is higher, use distilled, reverse-osmosis or clean rain water [1].

Does a filter jug make tap water safe for carnivorous plants?

No. The ICPS flytrap guide says filtering does not remove dissolved salts. Cartridges made specifically to remove salts do work, but they must be replaced often or they release the salt they have collected [1].

Is bottled drinking water or spring water safe?

The ICPS checklist calls bottled drinking water deadly to Venus flytraps, and the ICPS deck page specifies reverse-osmosis water, not bottled drinking water, for Sarracenia [3, 8].

How do I turn a conductivity reading in µS/cm into ppm?

Multiply it by a conversion factor. For natural waters NDSU Extension gives about 0.50 at 100 µS/cm, 0.60 at 500 µS/cm and 0.65 at 1,000 µS/cm [5].

Plants this applies to

Related guides

Sources

  1. Growing Dionaea muscipula. International Carnivorous Plant Society. Tap water usable below 90 ppm TDS; filters do not remove dissolved salts; water level more than 5 cm below the soil surface; salty soil and failing traps. Accessed October 12, 2026.
  2. Seed Germination Guide. International Carnivorous Plant Society. Water for seeds under 80 ppm dissolved solids; reverse-osmosis, distilled or deionized water; watering seedling pots. Accessed October 12, 2026.
  3. Checklist for Growing Dionaea muscipula. International Carnivorous Plant Society. Water dish 1 to 2 cm deep and never dry; bottled drinking water and most well water are fatal to flytraps. Accessed October 12, 2026.
  4. Growing Sarracenia. International Carnivorous Plant Society. Pots in 4 to 6 cm of water, never dry; bog water level 7 to 18 cm below the surface; rain flushing outdoor soil; fertilizer water in pitchers. Accessed October 12, 2026.
  5. Using Electrical Conductivity and Total Dissolved Solids Meters to Field Test Water Quality (WQ1923). North Dakota State University Extension. How EC and TDS meters work, ppm and mg/L, EC-to-TDS conversion factors for natural waters, calibration and sampling. Accessed October 12, 2026.
  6. Growing Cephalotus follicularis. International Carnivorous Plant Society. About 1 cm of water under a sandy mix; peat accumulates salts; occasional flushing; plant parts above the water level. Accessed October 12, 2026.
  7. Sarracenia Rhizome Rot. International Carnivorous Plant Society. Salty water burns root tips and lets pathogens in; plants damaged by water of TDS 350 and pH 8 plus drying out; leaching well-water salts. Accessed October 12, 2026.
  8. Display Sarracenia on your deck. International Carnivorous Plant Society. Tap water under 100 ppm TDS or collected summer rain for large outdoor pots; about a liter of water per plant per day in summer. Accessed October 12, 2026.
  9. Grow Venus flytraps indoors. Carnivorous Plant Newsletter 48(4): 178-182. Indoor tray depths, no top watering, bulb rot in soggy soil, filtered and bottled water, not reusing old soil. Accessed October 12, 2026.
  10. Growing Heliamphora. International Carnivorous Plant Society. Pure water only; well water generally too salty; filtering or softening does not reduce salt; watering in terrariums. Accessed October 12, 2026.
  11. Mechanical Tray Water Control. International Carnivorous Plant Society. Float-valve system that refills water trays automatically. Accessed October 12, 2026.
  12. Growing cold temperate Drosera. International Carnivorous Plant Society. Pots always in water while growing; flooding in hot weather. Accessed October 12, 2026.
  13. Growing Darlingtonia californica. International Carnivorous Plant Society. Soil below 27 °C; regular top watering in the early morning; ice cubes of pure water in hot weather. Accessed October 12, 2026.
  14. Growing Mexican Pinguicula. International Carnivorous Plant Society. Not bog plants; tray up to 2 cm in summer, drying out between waterings in winter; repot when a salt crust forms. Accessed October 12, 2026.
  15. Window Sill Garden. International Carnivorous Plant Society. Nepenthes in the house in a heavier sphagnum mix, allowed to sit in water for a day or two. Accessed October 12, 2026.
  16. Growing Genlisea and terrestrial Utricularia. International Carnivorous Plant Society. Water near or above the soil surface; trays or undrained containers; pure water required. Accessed October 12, 2026.
  17. Growing Drosera regia. International Carnivorous Plant Society. Fertilizer leaking from D. regia pots into a shared tray can harm other Drosera and Dionaea. Accessed October 12, 2026.