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The main problem with diagnosing plant problems is all the problems typically show up the same symptoms – so how are you meant to know how to treat it?
There’s no quick solution, but I’ve listed the main symptoms of under and overwatering below, and I’ll try to explain the differences between, e.g yellow leaves caused by overwatering and yellow leaves caused by underwatering. Oh, and also if the symptom suggests one but not the other – for example, you’re unlikely to see mushy roots if underwatering is your main issue.
Ok, let’s crack on.
- Yellow leaves
- Brown/black leaves
- New growth is rotting before it unfurls
- Droopy/wilting leaves
- Brown stem
- Mushy roots
- Soil smells musty
- Soil never dries out
- Soil takes a long time to dry out
- The plant is stunted/not growing
- Leaves are falling off the plant
- An abundance of fungus gnats
Yellow leaves are usually a sign of overwatering rather than underwatering.
Overwatering makes the soil heavier, and therefore denser. Dense soil can’t retain enough oxygen to keep roots healthy, and over time the oxygen-less environment causes an increase in the volume of the anaerobic bacteria that cause root rot.
So, why the yellow leaves?
Well, as the volume of healthy roots diminishes, the plant can no longer sustain as many leaves, and starts to make sacrifices – this is why the older leaves start to go first*
Leaves are green because they are filled with chlorophyll, a compound that allows plants to convert light into energy during photosynthesis. As the leaf dies, some of the chlorophyll dies too, though the plant recoups as much as it can by reabsorbing it.
Basically, the leaves are yellow because all the green has gone.
However, yellowing leaves can be caused by a number of issues – how can we tell that overwatering is the problem, rather than nutritional deficiencies?
It’s actually pretty simple – the most important thing is to go in with no biases. Just because you haven’t watered your plant in a while doesn’t mean that it isn’t overwatered.
So once you’ve cleared your mind of any preconceived notions, check the soil – really check it. Lay down a towel, or clear your floor or whatever and pull the plant out of its pot. Look at the soil – is it broadly staying together or crumbling apart easily?
Generally speaking, the crumblier the soil, the less likely it is that the soil is really compacted.
Now touch the soil – is it dry? Don’t ask yourself if it’s wet, because there are varying degrees of wetness – ask if it’s dry.
If the soil is dry and crumbly, it’s unlikely to be overwatered – it might even be hydrophobic, where the soil has dried out to the point that it struggles to absorb water. Grab a bowl, put in some water, and add your soil. After 15 minutes you can reassemble your plant.
Your plant is likely to be underwatered – keep a close eye on how often it dries out. Bottom watering is a great solution to hydrophobic soil.
If the soil is wet (like, wet wet) then overwatering is likely your issue. If you haven’t watered in a while then you need to add some orchid bark or something to your soil to make it chunkier.
Your issue could also be that your plant isn’t growing, in which case you need to work out why (check the light, temperature, and humidity first).
Black leaves that look like this:
…tend be a result of overwatering. The marks are darker due to excess moisture in the leaf – the cells burst, and part of the plant dies. Simply put, it’s dark because it’s wet (and dead).
Brown leaves tend to be the result of underwatering – they also tend to be crispy, like this:
If you suspect that underwatering is your issue, but you feel like you’re watering a lot, then make sure that your plant isn’t rootbound. If it is (i.e. you take it out of the pot and all you can see are roots) then you can either repot it, trim the roots, or simply water it more often (bottom water is usually best for this, because you can be sure your issue isn’t hydrophobic soil.
New growth is rotting before it unfurls
It can be quite tricky to diagnose why the plant is dying off before it’s even finished unfurling, but in general, if the end is crispy it’s underwatering, and if it’s black (and often a bit deformed) it’s usually overwatering.
Again, it’s all a case of checking the soil and seeing if the soil is drier or wetter.
There are a couple of other reasons your leaf isn’t unfurling properly:
- Water has got trapped in the leaf – this is usually a case of too high humidity plus too cold of a temperature.
- A water quality issue – if just the leaf tip is brown but the leaf does eventually unfurl and looks ok (brown tip notwithstanding) then water quality may be your issue.
Edema is quite common in houseplants (where the plant takes up too much water and the cells burst) but isn’t that big of a deal unless the leaves actually go transparent.
Drooping leaves are one of the first signs you’ll spot if your plan is underwatered. And overwatered.
Luckily, it’s usually pretty easy to tell which it is – again, the most important thing you can do is disregard any information you think could be pertinent with regards to when you last watered it. Deal with the situation as it is, not what you think it should be.
Just check the soil. Is it dry? If it is great, but also check the root ball – especially if you’ve just repotted it. Sometimes there can be a bit of demarcation between the old and new soils – especially if you’ve gone from a dense mix to a chunky mix or vice versa.
Check the moisture level of the root ball. If it’s super wet, then try and remove as much soil as possible and mix up the new and old soils.
I don’t usually recommend doing this unless you need to because it can cause unnecessary stress but sometimes it helps if you do it a couple of weeks after repotting.
If the root ball is super dry and the rest of the soil is moist, then try soaking the root ball in a bucket of water for half an hour before repotting.
A brown stem is a sign of root rot BUT it can also be a sign of corking, which is a perfectly normal part of plant, er, life. I have an article on corking here – there are a tonne of pictures you can compare to, but it’s usually something like this:
If the stem is dark brown and a little bit squishy, then I suspect root rot. Take the plant out of the soil and check that the root descends right to the base – if it does, then the stem has started to rot.
Stem rot means that the connection has been severed and the leaves can’t get any water and nutrients. The roots and stem won’t recover, but you can propagate the parts above the rot and recover the plant that way.
A brown stem is unlikely due to underwatering, because the leaves would most likely be totally dead before the stem dried out. If the entire plant has indeed dried out, then you can try putting the roots in rehab, but a brown stem due to dryness is, erm, very dry indeed.
There are some instances when brown stems are caused by other issues. We have my Rhaphidophora Tetrasperma to thank for this image because it decided that a heat lamp was its best friend:
Most likely the Rhaphidophora Tetrasperma will just decide to grow from another node, but also it will 100% do exactly the same thing.
Mushy roots is caused by overwatering, not underwatering, and is usually a sign that root rot is fairly advanced. Snip off any mushy bits and reroot it as best you can. I find that rerooting in water is the easiest way, so treat the whole plant like you’re propagating it in water.
Soil smells musty
Soil smelling musty is often a sign of overwatering, because the lack of oxygen in the soil causes an increase in bacteria. Mould and mushrooms CAN be a sign that your soil is too damp, but often it’s harmless – check your soil anyway though.
If your soil smells but it’s dry, that’s rarely bad news for your plant, but often it’s a sign that your potting mix is high in organic matter (it’ll probs smell a bit…farmy). It’s fine but can cause an uptake in the number of fungus gnats, which are a PITA.
Soil never dries out
I’ll go into soil that takes a while to dry out due to its density, but first I want to discuss user error. This is one of those things that new plant people struggle with, because of the dissonance between plant care consists of, and what plant care should be.
Many, many people water their house plants once a week, and when they get issues like drooping, they immediately assume it’s underwatering.
Now, it could be – the only way to tell is to check the soil and see if it’s wet. If the soil around the roots is moist, then underwatering isn’t the issue. If it’s dry, then go ahead and give it a good soak.
My point is that a lot of people that subscribe to the ‘water once a week’ ethos never let their plants dry out.
Some plants, like Calathea , like to be evenly moist all the time. They hate being both over and underwatered, which is why I tend to keep them in terrariums where the conditions remain consistent.
But other plants like to dry out quite a lot – not for long, maybe a couple of days (though they’ll survive much longer),but they like to have dry soil before they’re watered again.
Not only do they prefer a dry day or so, much some plants actually need a period of drought to do the things they want to do. Hoya, for example, like to stay pretty dry in the early Spring because it helps them prepare for blooming.
Soil takes a long time to dry out
Soil should take a couple of weeks to dry out, but there are dozens of external factors which can influence how it takes. If your soil takes longer to dry out than that, then you’re likely to end up overwatering your plant, even if you rarely actually water them.
The easiest way to remedy this is to add something like perlite or orchid bark to the soil, but you could also try moving the plant to a brighter, warmer spot.
(By the way some cactus growers like to dry their potting mix out with a hairdryer. I’m not advocating this, but if you’re really struggling with soaking soil then it’s an option.
The plant is stunted/not growing
This is equally likely to be caused by overwatering or underwatering, so all you need to do is think logically:
- Do you check the soil before you water it?
- Do you water on schedule regardless of…anything else?
- Do you forget to water?
Overwatering tends to manifest in the leaves before it noticeably slows and stunts new growth.
In general, dry soil is underwatered, wet soil is overwatered. There’s no trick.
That being said, there are a plethora of things that cause stunted growth, so check for:
- Time of year – we often get slow, stunted growth in winter
- Pot too big or small
- Lack of nutrients
- Low light
- Low humidity
- Too cold
Leaves are falling off the plant
Leaf drop happens faster in case of an overwatered plant because the moist tissue, rotten or not, is pretty good at holding itself together, even if it’s dead.
Underwatered leaves crisp up and drop off – they may not drop off on their own, but the gentlest touch can do the job.
Look for the colour and texture of the leaf – if it’s dry and crisp then it’s probably underwatering, if it’s black and mushy then it’s probably overwatering.
An abundance of fungus gnats
A load of fungus gnats is a classic sign of overwatering – if you’re underwatering then your plants won’t have much for them to feed on so they won’t bother you.
There are a ton of ways to deter fungus gnats, but you need to ensure that your plants are drying out properly as well as working in eradicating the gnats.
It can be tricky to ascertain whether you’ve been over or underwatering your houseplants, but you should be able to work out which it is by checking your plant’s soil. Overwatered soil shouldn’t be dry, underwatered soil shouldn’t be wet, and you should know how often you’re watering.
If you’ve been watering every day and your soil is dry, then you need to add something like coir into your soil to help it retain water. If you never water and the soil won’t dry out, then you need to add bark, perlite, or sand to your mix. Replace about a quarter of your soil with the amendments – you can always add more later.
If you have any plant problems and you can’t seem to get to the bottom of them, feel free to leave a comment here or DM me on Instagram.