PlantRoot rot is an oxygen problem before it is a disease: cut away every soft root, repot into fresh mix in a pot sized to what survives, and water only after the mix dries to the right depth.
What is actually happening underground
Roots need oxygen to respire, just as leaves need light. When the spaces in the mix stay full of water, oxygen runs out within hours to days, and the fine root tips, the most active part of the root system, die first. That dead tissue is the entry point for water molds and fungi that live in almost every potting mix and garden: Pythium and Phytophthora (oomycetes, which spread through saturated mix by swimming spores) and Fusarium. Once established, they move from the dead tips into healthy roots and, eventually, into the stem base.
The triggers are almost always the same: watering too often for the light level, a pot much larger than the root ball, a dense or broken-down mix, a blocked drainage hole, or a cachepot holding a puddle. Cold makes it worse, because cool roots use less water and the mix stays wet longer.
Confirming it: above and below the soil
Above ground, the plant wilts or yellows even though the mix is wet, lower leaves drop, and growth stalls. On snake plant and ZZ the first sign is often a soft, translucent or darkened stem base, and the whole leaf may topple. Fungus gnats and a sour or swampy smell from the mix are frequent companions.
The only way to be sure is to look. Slide the plant out of the pot and inspect the roots:
- Healthy roots are firm and springy, and white, cream, tan or orange depending on the species and mix.
- Rotted roots are brown to black, soft or mushy, and often smell.
- Pull gently on a suspect root: if the outer layer slides off and leaves a thin, stringy core, that root is dead.
- Check the stem base and any rhizomes too: a firm rhizome can regrow roots, a soft one cannot.
Rescue surgery, step by step
Do not wait for spring if the plant is collapsing. Wash the old mix off the roots under lukewarm running water so you can see everything. With scissors wiped in 70% isopropyl alcohol, cut away every soft, brown root back to firm tissue, cleaning the blades between cuts. Discard the old mix; do not reuse it on another plant.
Repot into fresh, open mix, such as a potting mix with a third of perlite or pumice added, in a pot only slightly larger than the remaining root system; after heavy pruning that is often a smaller pot than before. Scrub a reused pot with hot soapy water first. Water once lightly to settle the mix, place the plant in bright indirect light, and do not water again until the right drying depth for the species is reached. Hold off fertilizer until new growth appears, usually 4–8 weeks.
When the roots are gone: take cuttings
If the stem base is soft, or fewer than about a quarter of the roots are sound, save the plant through propagation while it still has healthy top growth. Pothos and heartleaf philodendron root easily from single-node cuttings in water or moist perlite, usually in 2–4 weeks at 20–25 °C. Monstera roots from a stem section with a node and an aerial root. Snake plant can be divided if some rhizomes are firm, or propagated from leaf sections, though leaf cuttings of the variegated ‘Laurentii’ lose their yellow edge. Individual ZZ leaflets will root, but slowly, over months.
Fungicide drenches cannot revive dead roots or overcome the conditions that caused the rot; if you use one, follow your local label.
Keeping it from coming back
Prevention is mostly about pot and water. Use pots with drainage holes and a size only 2–5 cm larger than the root ball, skip the gravel layer (it raises the saturated zone rather than removing it), and empty cachepots and saucers within 15–30 minutes of watering. Check the mix before every watering and stretch the intervals as light falls in autumn and winter. PlantPal reminders work well as a prompt to check, rather than an instruction to water.
Give extra care to the plants most prone to rot. Snake plant, ZZ and aloe in a cool winter room may need water only every 3–6 weeks, and a single extra watering in December can be enough to start rot. Plants potted up too far at once, or sitting in a decorative pot without a drainage hole, are the other usual victims.
