Root rot is one of the most common causes of houseplant death and almost always results from overwatering combined with inadequate drainage. Preventing it comes down to three things:
- Watering correctly (only when the compost is ready and not on a fixed schedule)
- Using free-draining compost and pots with drainage holes
- Checking roots periodically so problems are caught early.
Understanding how root rot develops makes it far easier to prevent.
What Causes Root Rot?
Root rot is caused by soil-dwelling fungi and water moulds (Oomycetes), most commonly Pythium and Phytophthora species, which proliferate in saturated, oxygen-depleted compost. Plant roots need both moisture and air to function: waterlogged compost excludes the oxygen that roots require, first damaging root cells and then creating ideal conditions for the pathogens to colonise the weakened tissue. The rot typically starts at root tips and works inward, turning roots from white and firm to brown, grey, or black and mushy.
The plant above ground may look normal for weeks while root rot progresses below the compost surface. By the time yellowing leaves, wilting, or soft stems become visible, the root system may already be significantly compromised. This is why prevention is far more effective than cure, and why checking roots during repotting is valuable even when a plant looks healthy.
Water Only When the Compost Is Ready
The most important root rot prevention habit is learning to read compost moisture correctly rather than watering on a fixed schedule. For most tropical houseplants, water when the top two to five centimetres of compost feel dry to the touch. For drought-tolerant plants (succulents, cacti, ZZ Plants, Sansevierias), allow the compost to dry out more completely before watering. For moisture-loving plants (Calatheas, Marantas, Ferns), water when the top centimetre is dry but before the compost fully dries out.
Pushing a finger into the compost is the simplest and most reliable method. A wooden chopstick or skewer inserted into the compost and pulled out clean (without compost sticking to it) indicates the lower portion of the pot is dry. Lifting the pot and comparing the weight to just after watering develops a useful intuitive sense of when watering is needed. See our guide on knowing when to water for more detail.
Use Free-Draining Compost and Pots With Drainage
Good compost structure and adequate drainage remove excess water after watering and maintain air in the root zone between waterings. A quality houseplant potting mix has appropriate moisture retention without excessive density.
For plants prone to root rot, mixing compost with perlite or pumice improves drainage and aeration significantly. Perlite is inert, does not compact, and holds air pockets in the compost even when wet.
Every pot should have drainage holes. Pots without drainage have no way to release excess water, making overwatering inevitable over time regardless of how carefully you water. If you want to use a decorative outer pot without drainage, use it as a sleeve for an inner pot with drainage holes and empty the outer pot of any standing water within an hour of watering.
Pot Size Matters More Than Many People Realise
Overpotting (planting into a pot significantly larger than the root ball) is a major contributor to root rot. In an oversized pot, the large volume of compost surrounding the small root ball stays wet for extended periods because the roots are not drawing moisture from it effectively. The damp, poorly aerated outer compost becomes an ideal environment for root rot pathogens. Choose a pot that is only slightly larger than the root ball, typically one to two pot sizes up when repotting.
Seasonal Adjustment
Root rot risk increases significantly in autumn and winter when plant growth slows, compost dries out more slowly, and light-driven evaporation from the surface decreases. Watering frequency should be reduced in line with the plant's reduced activity.
A plant that needed watering once a week in summer may only need watering every two to three weeks in winter. Not adjusting watering for the season is one of the most common causes of winter root rot.