Mineral burn from tap water, salty soil and fertilizer

Why carnivorous plants are so sensitive

Carnivorous plants evolved in places where the soil holds almost no nutrients, and they react badly to minerals most houseplants never notice. The ICPS states plainly that they can be killed by regular potting mix, by fertilizer, and by water with moderate or high levels of dissolved solids ^[1].

“Dissolved solids” means the minerals carried in the water. NDSU Extension explains that total dissolved solids (TDS) are measured by evaporating a water sample and weighing what is left behind, and that handheld meters estimate the same figure from how well the water conducts electricity, expressed in milligrams per liter, which is the same as parts per million (ppm) ^[6]. Those minerals stay behind when water evaporates ^[6], and the ICPS notes that peat gradually accumulates salts from the water it is given ^[7].

What mineral burn looks like

The damage shows up differently in each group:

• Venus flytraps: new traps fail to develop, or turn black before they open, as the roots die in salty soil ^{[2, 5]}.
• Sarracenia: the leaves show salt burn ^[3]. Salt also burns root tips, which lets rot fungi climb into the rhizome; the ICPS shows plants killed by water at 350 ppm combined with drying out ^[8].
• All groups: salt build-up in the growing medium is one of the common problems the New York Botanical Garden lists for plants not watered with distilled water ^[9]. A white crust on the soil surface or pot rim fits that picture, since dissolved solids are what remain when water evaporates ^[6].

Mineral burn is easy to confuse with normal leaf turnover. A flytrap that keeps making healthy new leaves while old ones blacken is fine; see traps turning black and the troubleshooter.

Cause 1: tap, well, bottled or filtered water

How to check. Measure your water with a TDS meter, or look up your supplier’s water report. The ICPS limits vary slightly with the plant and the stage of growth:

The same pages give a rule of thumb for the source: water from surface supplies is usually acceptable, while well water is doubtful ^[10] or, for Heliamphora, generally too salty ^[4]. Check your meter before trusting it. NDSU Extension recommends calibrating conductivity meters regularly, including whenever the batteries are changed or the meter has been dropped ^[6]. The water checker turns a reading into a verdict.

Fixes that do not work. Ordinary filtering does not remove dissolved salts. Cartridges that contain chemicals to remove salts do work, but the ICPS warns they must be replaced often or they release the collected salt back into the water ^[2]. Softening does not make water less salty either ^[4]. The ICPS checklist calls bottled drinking water deadly to flytraps, along with most well water ^[11], and the New York Botanical Garden warns that bottled or filtered water will probably carry too much dissolved salt ^[9].

What to do. Use distilled, reverse osmosis or clean rain water whenever your tap water is over the limit ^[2]. Details on each option are in water.

Cause 2: fertilizer in the soil or the water

How to check. Has the plant been fertilized, or potted in a bagged mix? The ICPS describes fertilizing the soil of flytraps or sundews as living dangerously ^[12]. Fertilizer adds dissolved solids quickly: by the ICPS’s figures, half a teaspoon of a water-soluble orchid fertilizer per US gallon (about 1 ml per 2 liters) raises the TDS by 350 ppm ^[4]. In Sarracenia, burning the root tips with fertilizer opens the way for rot ^[8].

Fertilizer often arrives unannounced. The ICPS warns that peat sold by companies specializing in fertilizer may contain fertilizer or lime without saying so clearly on the bag ^[13], and regular potting mix has usually been adjusted to a neutral pH and had fertilizer added ^[5].

Exceptions. Soil fertilization helps juvenile Sarracenia, and the ICPS says watering them with a solution that raises the TDS by 50 ppm should not hurt ^[14]. Lime is not a problem for every carnivore either: most Mexican butterworts prefer an alkaline mix, and the ICPS adds dolomitic lime to theirs ^[15]. See the butterwort genus page.

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

Cause 3: salty soil ingredients

How to check. Where did your peat, sand or perlite come from? Each can carry salt:

• Peat: some brands contain salt at levels toxic to carnivorous plants, and washing reduces it ^[13]. The CPN article on indoor flytraps calls the most common peat brand much too salty and rinses all peat before use ^[5].
• Sand: salt can enter during processing, when water is sprayed on to keep the dust down, so the ICPS suggests soaking sand in purified water ^[10]. Play sand, plaster sand and builders sand should not be used as they come ^[13].
• Perlite: often salty for the same reason. The ICPS washes it, keeps only the pieces that float, and uses it only for plants outdoors that get rained on ^[13]. For indoor flytraps, the CPN author avoids perlite and pumice altogether ^[5].
• Coir: for Nepenthes, the ICPS looks for packaging that says the coir was exposed to rain for at least three monsoon seasons, so the salts have been leached out ^[16].

What to do. Buy plain sphagnum peat with nothing added, washed silica sand, and rinse ingredients before mixing. The soil guide and soil mix calculator cover the recipes.

Cause 4: old soil that has built up salts

How to check. How long has the plant been in the same soil? The ICPS Cephalotus guide notes that peat breaks down and accumulates salts from the water over time ^[7]. The CPN article repots flytraps every year or two for this reason ^[5], and the New York Botanical Garden advises repotting slower plants at least every two years to refresh the medium ^[9].

What to do. Repot, or flush. Cephalotus in its sandy mix needs water flushed through the pot now and then to remove accumulated salts ^[7], and growers stuck with salty well water should leach Sarracenia often by watering from above and repot more often ^[8].

Rescuing a plant with mineral burn

1. Switch to distilled, reverse osmosis or rain water straight away ^[2].
2. Flush the pot by running clean water through it; Wisconsin Extension notes that this leaches excess salts from houseplant soil ^[17].
3. Repot into fresh, unfertilized mix. For a flytrap that will not eat, do this as soon as possible; if it is still feeding, feed it well and repot after a few weeks. Do not reuse the old soil indoors ^[5].
4. For Sarracenia, inspect the rhizome while it is out of the pot and cut away any brown or soft tissue, as described in rhizome and root rot ^[8].

Use repotting for the potting steps.

Preventing mineral burn

• Test your water and keep it under the ICPS limit for your plants ^{[2, 3]}.
• Never fertilize the soil of adult plants, and check every bag for added fertilizer or lime ^[13].
• Rinse peat, sand and perlite before use ^[13].
• Repot every year or two, or flush pots regularly where the mix allows it ^{[5, 7]}.