Ultimate Guide to Root Rot: The signs to watch out for and how to fix root rot fast

Ick. If you haven't smelled root rot before, I do not recommend it. Think rotten eggs and you're halfway there. Sadly though, even if you have escaped smelling root rot (lucky you), if you're anything like me, your plants may not have been as lucky to have escaped this common houseplant horror story. 

I started the hobby in such a textbook way. Although I grew up with a plant-loving mum (orchids were and still are her favourite), and have always had indoor plants for a good 30+ years as an adult (showing my age here), things got out of hand when I threw myself into the houseplant hobby with all my heart. I quickly went from 5 to more than 50 plants in a scarily short amount of time.

Then 100.

Then I stopped counting.

I wish I'd slowed down at the start. A lot of planty lives may have been saved. Not to mention a lot of plant pennies! I have lost my fair share of plants over the years. But on the upside, those losses came with a LOT of lessons (albeit the hard way). But back to the dreaded root rot. If you have been lucky enough to escape root rot so far, you might assume (like I did early on), that if you simply don't over-water, root rot won't come knocking. But I was wrong.

There are more causes of root rot than just over-watering. But to be fair, you'll find over-watering in the #1 position for the most common cause. That's because the fungi that cause root rot LOVE wet soil with low oxygen. Knowing what to look for, and acting fast when you see the signs, might save your plant. Plus, knowing the causes (and what never to do), could see you avoid having to deal with root rot in the first place. Everything from treatment, prevention, cure and causes are ALL covered in this Ultimate Guide to Root Rot, coming up...

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No time to wait? Fix root rot NOW

Already pretty sure you have root rot, don't care right now how it happened, or how to prevent it, and just want to skip to the immediate solution for what to do right now to save your plant (fingers crossed)? Here's what to do to fix root rot >

What are the signs of root rot?

Unfortunately, by the time your plant's foliage shows you what's going on with the roots below the surface, it may be panic stations. Root rot is very very sneaky (one for the Mr Deeds movie fans). It tends to start off slow and hidden below the surface.

The first sign of root rot: slow or stunted growth

The very first sign (that often goes unnoticed), is slow or stunted growth. But as root rot spreads, the signs start to show more dramatically above ground. If you do spot any of the symptoms below, act fast and it may not be too late to save your plant. 

Common signs of root rot in foliage: wilting and yellowing leaves

One telltale sign the roots aren't working like they should, is a plant that continues to wilt as if it's thirsty, even after it's been watered or if the soil is still wet. I see that more often on the top leaves, but it can happen anywhere on the plant or the entire plant can wilt. Another common one you may see happen alone or combined with wilt, is yellowing leaves, seen in the oldest leaves and leaves closest to the soil first. Left unchecked, leaves can brown, die and fall off. Sometimes it's only one or two yellowing leaves at a time, but the more you see, the more concerned you should be.

Both wilting and yellowing leaves can also be caused by under-watering. But what often gets missed is that under-watering can lead to root rot also. If you've left a plant dry too long, be careful what you do next when you do water. More about that coming up.

Another cause of yellowing leaves can be a nutrient deficiency. That can also be caused by damaged or rotting rots unable to uptake nutrients from the soil, or can simply be because you don't fertilse (or your fertiliser isn't complete and some essential minerals are missing). Pests can also be to blame. 

But what you do next is what matters. Because even if the cause turns out NOT to be root rot (fingers crossed), you must rule it out first, as root rot can be life or death, whereas the other causes don't normally need action taken as urgently to save the plant. 

If you didn't notice the early signs - and depending on the cause of root rot (as there are multiple fungi and bacteria that could be the bad guys involved) - you might also see water-soaked lesions, especially near the petiole or the crown or stem, often nearest to the soil.

What causes root rot?

No, I'm not talking about the wrong soil choice, or over-watering, or under-watering, sharing water, or even fungus gnats (yes, fungus gnats can spread root rot!), more about all of those and more coming up below. What I'm talking about first is who are the bad guys that cause root rot? 

A few different fungi and bacteria can cause root rot, and some plants get their own fungi and bacteria more common within their plant family. Such as fusarium in tomatoes. When it comes to our houseplants however, two of the common causes of root rot are different species of the fungi Pythium and Phytophthora, although others like Fusarium and Rhizoctonia may be to blame. You may also see these called 'water moulds'.

Pythium typically shows as as yellowing lower leaves and as it progresses, may also produce water-soaked lesions. Early on, root tips will die. As it progresses, roots appear water-soaked and the cortex (outer layer), of the root will pull off easily.

Phytophthora symptoms include wilting even when watering is adequate or the soil is still moist, lower stems going mushy or turning black'ish brown, and lower leaves yellowing then dropping. In some plants symptoms may appear like fertiliser burn as a result of infected roots losing the ability to regulate salts. 

Different causes of root rot however can share the same symptoms, so symptoms alone can help you diagnose root rot, but not necessarily pinpoint the fungi or bacteria behind it. 

How does root rot spread?

Depending on the species, the fungi that cause root rot can be soil-borne, water-borne, air-borne and spread by pests like fungus gnats. Using tools that have been in contact with the fungi can also spread it if not sterilised between plants, especially shears or snips used to cut plant tissue, and scoops or shovels used with infected soil. 

The fungi that cause root rot may also produce spores that can survive for long periods in the soil, dormant and waiting for the ideal conditions. Some species only become active in high temperatures, some in lower temperatures, but they all share the trait of preferring waterlogged conditions where the soil stays wet with low oxygen levels. 

Watch out when 'sharing' water

Being water-borne is another reason over-watering isn't the only cause of root rot. If you top-water a plant with active root rot, or dormant root rot spores in the soil, then re-use the water that drains out for the next plant, you can easily spread the spores.

The same can happen if you bottom-water your plants using the same water, plant after plant. Even a group of plants sat in the same saucer for water run-off to collect in can spread the spores.

BUT even if you have accidentally spread the fungi to every plant in your collection, that doesn't mean you will get root rot. The rest of the conditions still need to be ideal for the fungi to become active. 

Pests can spread root rot!

Fungus Gnats may be one of the most annoying pests for their habit of flying in your face (that's because they're attracted to the carbon dioxide in our breath), but are considered relatively harmless to plants. 

Which is true, until they get to higher numbers, or when the larvae run out of organic matter in the soil to eat, and start chomping on plant's root hairs instead. If conditions are nice and moist and you have a larger infestation of fungus gnats, the larvae that normally stay in the soil can also adventure above ground, leaving a slime trail that can be mistaken of that from small snails or a slug.

Firstly, just having fungus gnats can be a sign something's rotting in your soil. They also prefer moist soil conditions, so that can be another sign your plant's soil might be staying wet too long (conditions the fungi that cause root rot love too). But what's less well known is both fungus gnat larvae AND the flying adults can spread fungal diseases, including root rot and blight. 

What to do IMMEDIATELY when you suspect root rot

STEP 1: CHECK THE ROOTS

Firstly, check the roots! This part gets a bit messy. Gently unpot the plant and loosen the substrate so you can check how the roots look, feel and smell. You want to get those roots out of any wet soil right away also. Once unpotted, you may smell the telltale (horrible), smell of root rot but it will depend how far it's progressed.

Whether you do smell root rot, or see any roots that are darker than usual (black'ish brown), make sure to feel the roots also to confirm if they are firm or are soft or mushy. The outer layer (cortex) of the root may also pull off easily leaving the inside of the root behind. Soft, dark roots that you can 'squish' or 'pull off' all point to root rot (sorry).

How to treat indoor plants for root rot

STEP 2: WASH AND CUT 

If you do see any rotted / rotting roots and want to save the plant if you can, gently but thoroughly wash all the roots in lukewarm water (cold water can further shock the healthy roots), to remove the substrate and give you a better look at all the roots.

With sterilised shears or snips, cut off all rotting roots you can find. Make sure to cut above the rot, into the healthy root, leaving no rot behind. If you choose to skip step 3 up next, then it's best to sterilise your snips between each individual cut so you don't just spread the rot to healthy roots. Or, if you do step 3, you can sterilise all tools just once at the end after you finish.  

TAKE 'INSURANCE' CUTTINGS (OPTIONAL)

At this stage before I pot up what's left, I like to take a few 'insurance cuttings' from the growth tip/s so I can propagate them. That way if the plant doesn't survive, I can still keep it going through its babies which will be a perfect clone of the mother plant. See the Easy way to water propagate indoor plants > for tips on how and where to take cuttings from. 

STEP 3: TREAT THE ROOTS (OPTIONAL) 

Taking the extra precaution of treating the remaining roots before repotting is optional but I highly recommended it, especially because removing every bit of rot can be difficult, even more so in plants with finer roots and more dense root-balls. 

My go-to is a systemic fungicide, the one I use is called Buxus Blight (for those in New Zealand) which targets the two most likely fungal causes, Pythium and Photophthora. That one is a systemic, so rather than being short-acting and only working on the surface of the plant, it can be absorbed through roots and foliage to help kill the fungi and protect the plant from the inside out (and it keeps working for 30 days).

Buxus Blight can also then be used to treat the rest of the plant and the soil after repotting. A note that not every fungicide is effective against root rot (or against all fungi that cause root rot), or works in the same way, so if you can't get Buxus Blight, check the label before you buy and apply. 

Make up your fungicide as per the label and dunk the roots in the solution before you repot. For Buxus Blight it only requires a quick dunk of a few seconds in and out again to be effective. Save the solution you used for the 'root bath' for step 5, coming up.  

STEP 4: REPOT 

I personally prefer to repot the plant into fresh soil and a new pot since the spores that cause root rot can hang around for so long, but if that's not an option, you can sterilise the soil and the pot instead, or use a fungicide (or both, waiting 48 hours in-between). Even when reusing the same pot, I'd highly recommend fresh soil. 

Even when I repot into fresh soil and a new pot, I like to do step 5 and apply a fungicide again to the repotted plant afterwards. But you can just stop at step 4 and monitor the plant, keeping it somewhere warm, bright and with good airflow to help the plant recover. Keep that new soil lightly moist and let it dry out between watering according to the plant's needs.

Remember with fewer roots after removing the rotted ones, you'll very likely need to downsize the pot to reduce the risk of root rot getting established again from the substrate holding too much water. If you felt the soil was holding water too long or was too compact with enough air space, I'd also recommend picking a chunkier, more free-draining potting mix with lower water retention and more air space.

STERILISE THE POT & SOIL (OPTIONAL)

If you choose to re-use: I sterilise everything using hydrogen peroxide (H2O2). The brand I use is Oxygen Plus 3% (for those in New Zealand, or there's Essential Oxygen 3% available on Amazon for my overseas plant friends).

Hydrogen peroxide can both kill the fungi and bacteria that cause root rot, sterilise the pot and soil, and restore oxygen to help the remaining roots recover faster. It has so many other uses that I always have a bottle in my plant cupboard (you can use it for household cleaning too).

I use a 1:1 ratio of 3% hydrogen peroxide and water in a bucket or container (or you can spray it on). With gloves on, I put my pot in the solution and give it a good scrub then leave it to soak for 10 to 15 minutes. You can soak your tools in the same mixture to sterilise them also. Then give everything a quick rinse in plain water, and let them dry.   

You can also use hydrogen peroxide to sterilise the soil but I normally skip this step and use fungicide instead after potting up. If you want to do both, hydrogen peroxide breaks down after a day or two, so sterilise the soil, pot up your plant, and a couple of days later, treat with fungicide.

STEP 5: TREAT WITH FUNGICIDE (OPTIONAL)

This step's optional after repotting. It can kill remaining rot and also help the plant recover from transplant stress following repotting (depending on the product of course, but great if you get both benefits in one).

I use Buxus Blight again for this step (I just save the solution I dunked the roots in at step 3). It treats the rot and also helps recovery after repotting. Follow the directions of your choice of product. For Buxus Blight I spray the foliage only if no roots were left, or I spray both the foliage and the soil surface if some healthy roots remain. One treatment of Buxus Blight lasts 30 days, so depending on how the plant's doing 30 days later, I'll sometimes do a repeat spray a month after repotting.

How do you prevent root rot in houseplants? 12 ways to keep root rot away

#1 AVOID OVER-WATERING

The #1 thing houseplant hobbyists can do to prevent root rot is avoid waterlogged soil. You want free-draining soil that only holds the amount of water the plant prefers for the time needed, then to only water again when the plant needs it. Over-watering means watering again too soon. It doesn't matter how heavily you water when the plant is thirsty. Watering too lightly can cause root rot too! When you do water, water thoroughly so all of the substrate is evenly wet and no roots miss out (which can cause dry rot).

#2 CHOOSE THE RIGHT POTTING MIX

Ideally look for a potting mix with ingredients that keep space in the pot. Things like perlite, pumice, leca, pon and more can all help reduce compaction and increase airspace. Finer, soil-based or peat-based mixes tend to hold a lot more water and become compact when wet, leaving little space for air. When a plant needs higher water retention, like Peace Lilies and Maranta, look for a mix like Wildvine Houseplant Blend that includes those 'airy extras' into the mix. 

For more root-rot-prone plant families like aroids and hoyas, like Monstera, Philodendrons and more, which like to partially or completely dry out before being watered again, choosing a soil-free mix is a safer bet. Wildvine also has a lovely soil-free Aroid and Hoya Blend, or Bio Leaf's Aroid and Hoya Mix is a best seller too.

And of course for your cacti and other succulents and semi-succulents like Peperomia, where excess water causes big problems, soil-free is the way to go too. A finer aroid mix can work a treat, like the superb Bio Leaf Indoor Fine/Medium or a stonier blend like a Bonsai mix or Wildvine Succulent Blend are great options also, all higher in ingredients like bark, perlite, pumice, scoria and more. 

#3 DO USE POTS WITH DRAINAGE HOLES

(BUT AVOID A DRAINAGE LAYER)

It doesn't matter how fantastically free-draining your potting mix is if you have no way for excess water to escape. Putting plants in something with no drainage, or leaving water sitting in the bottom of a saucer or cover pot, puts out the welcome mat for root rot. No drainage holes also reduces airflow around the bottom of the pot where the roots (and root rot), hang out. It also increases the concentration of mineral salts from water and fertiliser over time which can kill roots and invite root rot in.

Another problem caused by keeping plants in a pot with no drainage, is dry rot. Under-watering to avoid root rot can lead some roots to miss out. When those root hairs die from drying out, and you water again, they can no longer uptake the water and instead end up rotting.

Adding a drainage layer does NOT increase drainage. That one's a bit of a myth. This is why drainage layer's increase root rot >

#4 DON'T SHARE WATER

The fungus that causes root rot can be water-borne, spreading the active or dormant fungi and spores from plant-to-plant (even when root rot isn't active). Whether top or bottom-watering, it's best not to save that water that's drained out of one plant, to water the next plant. Avoid plants sharing saucers also. Best to give them each their own saucer or cover pot to catch excess water.

When bottom watering, you also ideally want to change the water between plants. Keep in mind sharing water also increases the risk of other health issues like salt burn and fertiliser burn as once water's gone through a plant, excess salts get flushed out (and is best flushed down the drain). 

#5 GET RID OF FUNGUS GNATS

Fungus Gnats are known to spread root rot. Both the larvae and adults can pass it around, plant to plant, even on the feet of the adult flying fungus gnats! I used to ignore the occasional fungus gnat, but knowing that, now I don't wait to treat.

Keeping Sticky Traps always on hand at all time is a smart move (especially my favourites, the cute uBloomd Green Sticky Traps). Mozzie Bits are great also. Just add some into your water. As well as mosquito larvae, Mozzie Bits also kill fungus gnat larvae in the soil with no harm to plants, people, pets or beneficial bugs. If you're not New Zealand based like me, my overseas plant friends can get Mozzie Bits here from Amazon, as well as sticky traps.

#6 INCREASE AIRFLOW

Indoor plants tend to miss out on the airflow their outdoor buddies get in ample amounts. Good airflow is one factor that helps prevent most types of pathogenic fungi growing. It also helps soil dry out faster. Better airflow also increases plant transpiration. The rate at which plant's 'breathe' oxygen and carbon dioxide, as well as release water vapour, causing plants to use more water from the soil which helps soil dry out faster.

#7 INCREASE LIGHT

Plenty of light helps plants transpire better as well as grow faster, both which increases a plant's need for water, helping soil dry out faster. If you can't do much to increase natural light, get your plants a grow light instead.

#8 INCREASE WARMTH

Warmth is another way to help soil dry out faster. If I've overwatered or I'm  worried a plant's soil is taking too long to dry out, the first thing I do is pop it on a heat mat.

As warmth goes up, the transpiration rate goes up. For plants that prefer warmer temperatures (like the majority of indoor plants), warmth also helps them grow faster. Both which help the soil dry out faster as plants need for water increases.

Just keep an eye on those maximums. Always check what temperature range your plant prefers. You can also get a soil thermometer to keep an eye on soil temperature (those soil thermometers are also available here on Amazon for those not in New Zealand).

#9 DO USE TERRACOTTA POTS

Although I'm not personally a fan of terracotta pots (love the look but not the speedy drying out), they absolutely do help suck out excess moisture and dry out potting mix a lot faster. For me however, it's too fast (I tend towards being an under-waterer anyway these days). 

#10 DON'T USE OUTSIDE SOIL FOR INDOOR PLANTS

The spores that cause root rot are far more common in outdoor soil. Using or mixing in soil from outdoors for your indoor plants is like putting out the welcome mat for root rot. The soil from outdoors will more likely have dormant spores in it. Just add water and it could be hello root rot.

#11 AVOID BIG POTS

A pot that's a lot bigger than the plant's root system can hold so much water that it makes it difficult to avoid waterlogged soil. There are other benefits of avoiding big pots and not going up too much in pot size in one go. Find out why plants need small pots indoors >

#12 DON'T RE-USE OLD SOIL

When repotting, as wasteful as it feels, I prefer not to re-use my old potting mix. The spores from fungi that cause root rot can hang around a long time in soil. Plus old soil tends to get a build-up of salt over time, and also tends to change the pH over time as it breaks down. Once outside the healthy pH zone it can play havoc with root health, block some nutrients from being absorbed, and turn others toxic. Up to you of course at the end of the day. 

Keep learning

I love that you never stop learning when it comes to the houseplant hobby. If you're curious and want to learn more about watering, here are more free guides to keep learning...

Why plants rot in wet soil but not in water >

8 ways to water your plants when you're away >