Rhizome and root rot in carnivorous plants: causes and rescue

Normal decay versus rot

Some decay underground is part of normal growth. A Venus flytrap grows forward from one end of its rhizome while the oldest part rots away behind it ^[3]. Old Sarracenia clumps carry plenty of dead material too: the ICPS notes that the oldest part of a big rhizome is often dead, with old leaf bases around the outside, and that under low-nutrient conditions this material is fairly inert and harmless ^[2].

Rot is different. It reaches living tissue: the growing point wilts or turns brown, the rhizome or bulb becomes soft, and the plant may collapse even though it is standing in water. Extension plant pathologists describe the same pattern in other plants with root rot, which look drought-stressed but do not recover when watered ^[4]. If you suspect rot, unpot the plant and look; the troubleshooter can help you rule out other causes first.

Soggy soil and the wrong pot

How to check. How far is the base of the plant from the water in the tray, and does the soil smell? Carnivorous plants need wet soil, but most do not want their crowns sitting in it. The CPN article on indoor flytraps explains that Venus flytraps are not bog plants that tolerate saturated soil for long; the old leaf bases that make up the bulb rot in soggy soil, and pure peat tends to turn smelly. The aim is a mix that stays wet without becoming airless ^[1]. The ICPS flytrap guide keeps the water level more than 5 cm (2 in) below the soil surface, uses taller pots for wetter mixes, and avoids pure peat or pure sphagnum in short pots ^[5].

Other groups have the same weak point. D. filiformis and D. tracyi can rot in winter if the soil surface stays too wet ^[6]. For Cephalotus, the ICPS keeps every part of the plant at least 5 cm above any water level the pot may reach ^[7]. For Heliamphora, it wants a mix that holds plenty of water but keeps air spaces, and that never smells of rotting organic matter ^[8].

The fungi and water molds that cause root rot in general thrive in exactly these conditions. Wisconsin Extension notes that root rot pathogens favor wet soils with poor aeration ^[9], and UC IPM explains that Phytophthora water molds survive for years in moist soil as resting spores and release spores that swim through water-filled soil to infect roots ^[4].

What to do. Do not respond by letting the pot dry out; Sarracenia, for example, should never dry out and are kept in about 4 to 6 cm of water ^[10]. Fix the geometry instead. Use a taller pot or a sandier mix, and top flytrap pots with pure sand so that only the roots and the base of the bulb sit in the peat and sand. The CPN author uses about 2 cm of sand, which keeps the older part of the bulb from rotting and lets air reach the deeper soil ^[1]. For winter-wet sundews, mix extra sand into the top 1 cm of soil ^[6]. The soil guide has pot and mix combinations.

Roots burned by salt or fertilizer

How to check. What water do you use, and has the plant been fertilized? The ICPS rhizome rot page describes how this goes wrong in Sarracenia. Rhizoctonia and Fusarium are weak pathogens, but add a little too much fertilizer and they feed on the dead leaf bases, build up, and then attack live tissue. They also enter through root tips burned by fertilizer or by water that is too salty, then climb the roots into fresh tissue. Seedlings, which have little dead material, are harder to kill this way, but too much fertilizer or salt will kill a Sarracenia of any age ^[2]. The Sarracenia growing guide says the same: fertilizer in contact with old leaf bases can encourage rhizome rot ^[10].

What to do. Stop fertilizing the soil of adult plants, switch to low-mineral water, and leach salty soil; see mineral burn and water.

Frost on plants that were not fully dormant

How to check. Did an early or late freeze arrive while the plant was still growing? The ICPS explains that freeze damage in cold-sensitive species and clones turns the rhizome mushy, most often in S. psittacina, S. minor and S. leucophylla ^[2].

What to do. If any part of the rhizome is firm and white, remove everything soft or brown and replant it. To avoid a repeat, grow hardier species or clones, or protect plants better when freezes are likely ^[2]. Browning leaves on such plants are covered in Sarracenia pitchers turning brown, and winter protection in dormancy.

Hot roots

How to check. Did the plant collapse during a hot spell? Two genera are especially sensitive to warm soil. The ICPS says that when cobra lilies (Darlingtonia) collapse and die, it usually happens in hot weather, starting with the growth points at the edge of the pot, and that the soil must stay below 27 °C (80 °F) for long-term growth ^[11]. In Heliamphora, experiments reported in the ICPS guide showed that plants infected with a fungus that grows in the vascular system can die in less than 10 days at a constant 28 °C (82 °F) and high humidity ^[8].

What to do. Keep roots cool. The ICPS suggests larger pots for a more stable environment, and in-ground pots or bogs where summer soil stays cool enough ^[11]. See the cobra lily profile and the Heliamphora page.

Fast-moving infections

How to check. Did a healthy-looking plant go downhill within days? The ICPS describes Cylindrocladium as a pathogen that can wipe out a collection: a pitcher dehydrates, and within two to three days the whole plant is dead. It spreads in water and has no cure ^[2]. The Heliamphora fungus spreads through soil, rain water and contact with other plants, and reusing soil, using contaminated ingredients and watering from above can all carry it ^[8]. Root rot water molds in general also travel in splashing and running water, contaminated soil and infected plant parts ^[4].

What to do. Isolate the plant immediately ^[2], keep its tray water away from other plants, and do not reuse its soil ^[8].

Rescuing a rotting plant

1. Isolate it from the rest of the collection ^[2].
2. Unpot and clean it. For Sarracenia, pull old leaves off at the base like artichoke bracts rather than clipping them, then cut dead rhizome ends back until no brown material remains, even a small brown spot surrounded by good tissue. The plant gets smaller, but the ICPS considers that better than losing it ^[2].
3. Check the growing point. Cutting out an infected growth point can save a Sarracenia only if done very early, because the pathogen usually travels in the phloem and a growth point that looks healthy may already be infected ^[2].
4. Save what you can. For a collapsing Cephalotus, the ICPS cuts off all the good stems for propagation, removes any rotting or blackened underground parts, and repots what is left in new soil ^[7]. A flytrap’s old rotting rhizome can be broken off, and healthy leaf bases can be rooted as leaf pullings ^[3].
5. Repot in fresh mix. Do not reuse the old soil for indoor plants ^[1].
6. Fix the cause from the sections above before the plant goes back with the others.

On fungicides, the sources are cautious. Some infections have no cure ^[2], most fungicides do not adequately control root-rot water molds ^[4], and the ICPS warns that fungicides, even cinnamon, are deadly to Pinguicula ^[12]. If you do use a product on another genus, follow its label exactly.

Preventing rot

• Match pot height and mix to the water level, and top flytrap and sundew pots with sand ^{[5, 1]}.
• Do not fertilize the soil of adult Sarracenia, and use low-mineral water ^[2].
• Clean all dead material off rhizomes when repotting ^[2].
• Time divisions so cuts can heal: in USDA zones 9 and 10 or a greenhouse, divide in late fall as plants go dormant; in zones 7 and 8, divide outdoor plants in late winter after the extended freezes ^[2].
• Quarantine new plants for a week or two before they join the collection ^[2].
• Keep Darlingtonia and Heliamphora roots cool ^{[11, 8]}.

The repotting guide covers the routine that prevents most of these problems.