D
Duncan Hilder
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- Thread Author
- #1
Water
By the time you find root rot, the watering mistake is already behind you. What matters now is what's left, and whether the pot it goes back into will do the same thing again.
Root rot is the plant problem most people find by accident. You tip a struggling plant out to repot it, expecting a tight ball of roots. Instead you get something dark, soft and smelling like a drain. It's a grim moment. It's also often fixable, as long as you act on what the roots show you rather than what the leaves are telling you.
Roots breathe. Every root needs oxygen as well as water, and in a healthy pot that oxygen sits in the small air pockets through the mix. When the mix stays saturated for too long, those pockets stay full of water, oxygen drops, and the roots weaken and start to die back. We go deeper on this in Root Health: Why Plant Roots Need Air, Not Just Water.
Wet mix also suits the organisms behind root rot. Water moulds such as Pythium and Phytophthora spread through saturated media, and they don't just finish off weak roots; they can attack healthy ones too. They usually arrive with an infected plant, reused mix, splashed water or unclean tools. Sometimes suffocation comes first, sometimes the mould does. Either way, both need the same thing: a root zone that stays wet.
Root rot isn't caused by water. It's caused by water that stays.
A plant with rotting roots can't take up water, so it wilts like a plant that's run dry. The natural response is to water it, which makes things worse.
The tell is the pot. A plant wilting in a light pot is probably thirsty. A plant wilting in a heavy pot is telling you to look at the roots first.
The Pot Weight Test: water thoroughly, let it drain, then lift the pot. That's your full weight, and it's the reference point for everything after. As the plant uses water, the pot gets lighter. The top of the mix dries first and can feel dry while the root ball underneath is still wet, which is why weight tells you more than poking the surface.
Other signs point below the surface too:
Ease the plant out of its pot. Squeeze or tap the pot to loosen it rather than pulling on the stem.
Healthy roots are firm and springy, and usually pale. Some plants carry naturally tan or brown roots, though, so go by feel rather than colour. Rotted roots are dark and mushy and collapse when you pinch them. Often the outer layer slides straight off, leaving a thin wiry string behind. If it smells rotten, that's your answer.
It comes down to two things: how much firm root is left, and whether the base of the stem is still solid.
Tease it away from the roots, or rinse the root ball in room-temperature water so you can see what you're working with. Don't reuse the old mix. It may have broken down, it may be carrying rotted root material, and either way it's hard to read. Fresh mix gives what's left a clean, airy start.
Use clean, sharp scissors, and wipe the blades with rubbing alcohol between cuts. Cut above the rot into firm tissue. Be decisive, because any soft root you leave behind is the problem you'll be dealing with again in a month.
The root ball will probably be smaller than it was, so go back into the same size pot or one size down, not up. Extra mix with no roots in it just holds water nobody's drinking. Make sure the pot has a drainage hole, and scrub it in hot soapy water if you're reusing it.
Choose the right mix for the plant (more on that below).
Water through to settle the mix, let it drain fully, and lift the pot. That's your new reference weight, because the old one no longer applies. With fewer roots, the pot may lighten more slowly than it used to, so forget the old rhythm and read this pot from scratch. Water again when it's dried back to the usual point for that plant.
The instinct is to rest a sick plant somewhere quiet and shady. But lower light means slower water use and a wetter pot, which is the last thing recovering roots need. The Shadow Test will tell you whether it's getting enough. Just don't move it into harsher sun than it's used to.
Feeding a plant with damaged roots adds stress, not strength. Wait until it's pushing new growth, then feed every second watering at the label rate.
Both get recommended in every rescue thread. Neither replaces the cut. Household peroxide can injure healthy root tissue if it's too strong. Cinnamon does contain compounds with antifungal activity, but there's no good evidence that dusting it onto a houseplant's roots reliably treats root rot. Cut away the dead tissue and fix the root zone. That's the treatment.
Drainage gets talked about as if it's the hole in the bottom of the pot. The hole matters (here's why). But drainage really comes down to how quickly the whole pot gets back to air after watering, and four things decide that.
Light. Light sets how fast the plant uses water. A plant in low light drinks slowly, so its pot stays wet longer. This is why light comes first in the Method: if a pot never dries, check the light before anything else.
The watering cycle. Water when the pot has dried back to where that plant wants it, not when the calendar says so. The dos and don'ts of watering covers the rhythm.
The mix. Mix breaks down over time, and broken-down mix packs tight and holds water in the space where air should be.
The Finger Probe Test: loose and crumbling apart rather than packed into a dense mass means the structure is still working.
If it's packed and dense, the mix is part of the problem. Why is my potting mix staying wet? goes further on this.
The pot. A layer of gravel or pebbles in the base doesn't help water leave the mix (we looked at the research). A pot that's too big holds water no root can reach. And a decorative outer pot quietly collecting run-off will keep the base of the root ball sitting wet, so tip it out after every watering.
Light, water, mix, then feed. When a pot won't dry out, work through them in that order.
If the rescue needs fresh mix, match it to the plant. Indoor Potting Mix suits most everyday indoor plants. Aroid Mix is for monstera, philodendron, pothos and other aroids: its chunky structure keeps air around the roots, while the coir and composted bark hold the moisture they use. Cacti & Succulent Mix is for succulents.
Indoor and Aroid Mix carry no added fertiliser, which suits a recovering plant, because you decide when feeding starts.
Cut back to what's firm, give it fresh mix in a pot that fits, and let the weight of the pot tell you when to water. Most plants that come through root rot don't need anything clever. They need the conditions that caused it to stop.
Root health: why plant roots need air, not just water
Am I overwatering my indoor plants?
Why is my potting mix staying wet?
Potting up: the when, the why and the how
By the time you find root rot, the watering mistake is already behind you. What matters now is what's left, and whether the pot it goes back into will do the same thing again.
Root rot is the plant problem most people find by accident. You tip a struggling plant out to repot it, expecting a tight ball of roots. Instead you get something dark, soft and smelling like a drain. It's a grim moment. It's also often fixable, as long as you act on what the roots show you rather than what the leaves are telling you.
What root rot actually is
Roots breathe. Every root needs oxygen as well as water, and in a healthy pot that oxygen sits in the small air pockets through the mix. When the mix stays saturated for too long, those pockets stay full of water, oxygen drops, and the roots weaken and start to die back. We go deeper on this in Root Health: Why Plant Roots Need Air, Not Just Water.
Wet mix also suits the organisms behind root rot. Water moulds such as Pythium and Phytophthora spread through saturated media, and they don't just finish off weak roots; they can attack healthy ones too. They usually arrive with an infected plant, reused mix, splashed water or unclean tools. Sometimes suffocation comes first, sometimes the mould does. Either way, both need the same thing: a root zone that stays wet.
Root rot isn't caused by water. It's caused by water that stays.
Why it looks like thirst
A plant with rotting roots can't take up water, so it wilts like a plant that's run dry. The natural response is to water it, which makes things worse.
The tell is the pot. A plant wilting in a light pot is probably thirsty. A plant wilting in a heavy pot is telling you to look at the roots first.
The Pot Weight Test: water thoroughly, let it drain, then lift the pot. That's your full weight, and it's the reference point for everything after. As the plant uses water, the pot gets lighter. The top of the mix dries first and can feel dry while the root ball underneath is still wet, which is why weight tells you more than poking the surface.
Other signs point below the surface too:
- Lower leaves yellowing and dropping
- A pot that takes far longer to lighten than it used to
- A sour smell from the mix
- A stem that's gone soft right at the mix line
How to check the roots
Ease the plant out of its pot. Squeeze or tap the pot to loosen it rather than pulling on the stem.
Healthy roots are firm and springy, and usually pale. Some plants carry naturally tan or brown roots, though, so go by feel rather than colour. Rotted roots are dark and mushy and collapse when you pinch them. Often the outer layer slides straight off, leaving a thin wiry string behind. If it smells rotten, that's your answer.
Can it be saved?
It comes down to two things: how much firm root is left, and whether the base of the stem is still solid.
- Plenty of firm root and a solid stem: good odds. This is a trim-and-repot job.
- Only a handful of firm roots: possible, but slow. Take off dead or badly damaged leaves, but keep the healthy ones, because they're what the plant uses to rebuild.
- Rot has reached the stem or crown: the roots aren't coming back. If it's a plant that grows from cuttings, take them from healthy growth and start again.
The rescue, step by step
Get the old mix off
Tease it away from the roots, or rinse the root ball in room-temperature water so you can see what you're working with. Don't reuse the old mix. It may have broken down, it may be carrying rotted root material, and either way it's hard to read. Fresh mix gives what's left a clean, airy start.
Cut back to firm tissue
Use clean, sharp scissors, and wipe the blades with rubbing alcohol between cuts. Cut above the rot into firm tissue. Be decisive, because any soft root you leave behind is the problem you'll be dealing with again in a month.
Match the pot to what's left
The root ball will probably be smaller than it was, so go back into the same size pot or one size down, not up. Extra mix with no roots in it just holds water nobody's drinking. Make sure the pot has a drainage hole, and scrub it in hot soapy water if you're reusing it.
Use fresh mix
Choose the right mix for the plant (more on that below).
Water once, then wait
Water through to settle the mix, let it drain fully, and lift the pot. That's your new reference weight, because the old one no longer applies. With fewer roots, the pot may lighten more slowly than it used to, so forget the old rhythm and read this pot from scratch. Water again when it's dried back to the usual point for that plant.
Give it the light it normally wants
The instinct is to rest a sick plant somewhere quiet and shady. But lower light means slower water use and a wetter pot, which is the last thing recovering roots need. The Shadow Test will tell you whether it's getting enough. Just don't move it into harsher sun than it's used to.
Hold the food
Feeding a plant with damaged roots adds stress, not strength. Wait until it's pushing new growth, then feed every second watering at the label rate.
What about hydrogen peroxide and cinnamon?
Both get recommended in every rescue thread. Neither replaces the cut. Household peroxide can injure healthy root tissue if it's too strong. Cinnamon does contain compounds with antifungal activity, but there's no good evidence that dusting it onto a houseplant's roots reliably treats root rot. Cut away the dead tissue and fix the root zone. That's the treatment.
The drainage truth
Drainage gets talked about as if it's the hole in the bottom of the pot. The hole matters (here's why). But drainage really comes down to how quickly the whole pot gets back to air after watering, and four things decide that.
Light. Light sets how fast the plant uses water. A plant in low light drinks slowly, so its pot stays wet longer. This is why light comes first in the Method: if a pot never dries, check the light before anything else.
The watering cycle. Water when the pot has dried back to where that plant wants it, not when the calendar says so. The dos and don'ts of watering covers the rhythm.
The mix. Mix breaks down over time, and broken-down mix packs tight and holds water in the space where air should be.
The Finger Probe Test: loose and crumbling apart rather than packed into a dense mass means the structure is still working.
If it's packed and dense, the mix is part of the problem. Why is my potting mix staying wet? goes further on this.
The pot. A layer of gravel or pebbles in the base doesn't help water leave the mix (we looked at the research). A pot that's too big holds water no root can reach. And a decorative outer pot quietly collecting run-off will keep the base of the root ball sitting wet, so tip it out after every watering.
Light, water, mix, then feed. When a pot won't dry out, work through them in that order.
Starting again with fresh mix
If the rescue needs fresh mix, match it to the plant. Indoor Potting Mix suits most everyday indoor plants. Aroid Mix is for monstera, philodendron, pothos and other aroids: its chunky structure keeps air around the roots, while the coir and composted bark hold the moisture they use. Cacti & Succulent Mix is for succulents.
Indoor and Aroid Mix carry no added fertiliser, which suits a recovering plant, because you decide when feeding starts.
The takeaway
Cut back to what's firm, give it fresh mix in a pot that fits, and let the weight of the pot tell you when to water. Most plants that come through root rot don't need anything clever. They need the conditions that caused it to stop.
Keep reading
Root health: why plant roots need air, not just water
Am I overwatering my indoor plants?
Why is my potting mix staying wet?
Potting up: the when, the why and the how