Help! Why are my plant's leaves turning yellow? (and what to do before it's too late)

I feel like this question should win some sort of award for 'Most Popular'. The reason it's asked so often is because there seem to be SO many possible causes of plant leaves turning yellow. And sure, if you judge it by everyone's vastly different answers, you'd be forgiven for thinking this is one of the most complex plant issues to resolve. 

BUT you'd be wrong. There is actually only ONE main cause of yellowing leaves in our indoor plants... 

However, what there ARE so many of, is solutions! When you think about it, that's not a bad thing. Let's take a look at the one main cause of yellowing leaves in house plants, and how to work out what solution is right for your plant, so you can correct it before it's too late.

And that's not being dramatic! Yellow leaves could be a warning of imminent death, so never ignore this cry for help in case yours is more dooms-day than a subtle hint something's wrong. It may also be nothing to worry about, but should not be ignored either way.

The one cause of yellow leaves

The #1 main reason for an indoor plant's leaves turning yellow all stem from the same issue: a lack of nutrients. Yep, that's THE answer. But let's find out what the solution is to why YOUR plant is suffering a lack of nutrients and what you can do to fix it quick smart.

Here are the top 5 reasons why your plant's leaves are turning yellow, and how to get your plant well again before it's too late. But a quick warning first... 

Warning: Never cut off yellow leaves!

Whatever the cause, the first thing NOT to do is cut off those yellow leaves. If you do trim yellowing leaves, you're cutting off your plant's temporary life support! Until you find out why and fix what's wrong, put down the snips. More about why coming up.

Plus if you do yank the plug early, the plant won't have had time to create a scab. Pull that leaf off before it’s ready and you'll create an open wound that could let nasties in like bacterial and fungal infections :( Plants don’t clot and scab as speedily as we do. Best to let nature take its course on timing for when the leaf is ready to come out.

That leaf is also NOT doing your plant any harm. Plants don't 'waste energy' on dead or dying leaves because they don't heal themselves like we do. They just grow a new leaf!

Before we jump to reason #1, if you're getting a yellowing leaf at the same time as your plant's putting out a new leaf, that usually means you need to fertilise right away. New growth increases your plant's demand for nutrients like nitrogen. Since nitrogen is a mobile nutrient, if your plant isn't able to get enough from the soil through its roots, it will take it from an old leaf to support that new leaf. Definitely don't cut that yellowing leaf off as it's currently supporting that new leaf, and do give your plant a nutrient boost right away. More coming up about fertiliser.

Reason #1 for yellow leaves: Overwatering

If leaves are turning yellow from the bottom of your plant up, and it's spreading fast, the most likely reason is overwatering or underwatering. Overwatering means roots can't breathe, so they stop doing their job transporting water and other nutrients to the leaves, which can cause death from oxygen deficiency. Yep, your plant is quite literally drowning.  

The word 'overwatering' can be a bit misleading though. It's not about how much you water, but how often. In fact you SHOULD fully drench your potting mix every time you water. If you don't completely, evenly saturate the soil, that can cause roots in the dry patches to die off, which can also cause root rot. Overwatering actually means when you water again too soon and your soil's still plenty moist below the surface. Roots need a balance of moisture and air. 

What are the signs of overwatering?

Signs of overwatering often start with lower leaves going yellow first, then brown. Leaves become wilted, soft and limp (not dry, brittle or crispy). As it progresses, leaves may start falling off with concerning speed (when that happens, not all leaves will go yellow or brown first before falling off).


You might notice the soil is still wet when it would normally be dry by now. Under the surface, roots will likely be turning soft, slimy or mushy depending on how soon you caught it. If you didn't catch it early, roots may also smell unpleasant, quite literally like rotting vegetation (because that is what's happening).

Very early signs you might be over-watering including moisture-loving bugs like Fungus Gnats setting up home in the soil, spotting little white fuzzy balls or mushrooms growing in the soil (which are a type of fungus). Both are signs your soil may be staying wet longer than it should, or that you're watering again too soon.

This cause deserves the #1 spot because yellow leaves caused by overwatering is a common sign of the dreaded root rot and can foreshadow imminent death if not caught in time.

How to fix overwatering 

Healthy roots should be pale and firm. Sometimes what's in your potting mix stains them brown. Some plants like Philodendron Pink Princess can also have matching pink roots. Potting into clear pots helps you know what's normal. But what they should never be is stinky, soft or slimy. If you suspect overwatering, slide your yellow-leafed patient out of their pot and check those roots. If you find a tangle of soft, mushy roots, treat for root rot quick, before it's too late.

How to prevent overwatering

If you were 100% sure your plant needed watering because the surface was bone dry, don't be sucked in. You have to check below the surface. Play it safe and grab a soil moisture meter instead. The colour-changing Sustee, or digital Crew Soil Sensor are my go-to's. Or if your budget's been spent on plants (you and me both), and there's not much left for their care, at least get an el-cheapo 3-in-1 water meter. Cheap as chips - although not as accurate as a Sustee or digital meter - but still a goodie for peace of mind on a budget to avoid overwatering (and underwatering). 

PS: a warning about water meters

Before you live and die by what your water meter tells you, remember no water meter tells you when to water, they only tell you the moisture level of the soil. You still have to decide if it's time to water, based on what level of moisture your plant prefers.

Reason #2 for yellow leaves: Underwatering

This should be a no-brainer to diagnose for you. Unless it's a new arrival, you'll usually know if you've been neglecting your plant and either watering too little, or going too long between watering.

If you're a worried waterer, so scared of root rot from over-watering that you end up under-watering, or you're simply too busy to check every jungle member and some inevitably get missed, definitely get yourself a Sustee or digital water meter. Honestly, those little things are total game changers. 

The good news - if caught early before it's dried to a crisp - is it's an easy fix for this one, and much less serious than overwatering. It's time to water. And yes, you may still lose those yellow leaves, but if you've caught it in time, it shouldn't get worse, and recovery should be around the corner. 


A warning though: underwatering can also cause root rot. That's because under-watering can cause root hairs to die when a plant is either left too dry for too long, or not watered heavily enough meaning some roots miss out. So when you do water again, the roots can no longer absorb the water and end up rotting instead. Tips to prevent root rot after underwatering are coming up below.

Signs of underwatering 

Like this example below, common signs include lower leaves going yellow then brown, often starting at the edges. The plant will look wilted. Leaves become curled, dry, brittle and crispy (depending on how advanced it is). Stems may be also shrivel, be dry, brittle and turning brown if not caught early.

Soil may also pull away from the edge of the pot, leaving a gap around the edge. If you lift the pot it will feel much lighter than usual, and the plant will usually lift out easily from the pot, soil and all. 

Reason #3 for yellow leaves: Nutrient deficiency

If you're dealing with yellowing of older leaves first, you might have had the advice 'not to worry, it's just an old leaf dying'.

So firstly, yes, that's technically correct - it IS dying, and it IS an old leaf. However I can't agree with the 'don't worry' bit. You should worry. And I certainly wouldn't ignore it and just put it down to 'old age' or being 'normal'. It's not, on both counts. And definitely don't cut it off!

This reason yellowing happens to older leaves first is because your plant is sacrificing them. It's taking mobile nutrients out of older leaves - like nitrogen, potassium and phosphorus - and moving them to newer leaves to stay alive and keep growing. That yellowing leaf is currently life support for your plant. 

If you don't fix the cause - or if you just take the internet's advice and 'cut it off' because it's 'normal' - another leaf will start turning yellow. And another. And another, until you fix what's causing them to turn yellow in the first place. And if you've ruled out over-watering or under-watering, then a nutrient deficiency should be next on your list to check.

How to fix a nutrient deficiency

If you don't fertilise at all, stop being a meanie and start 'feeding' your plants. Fertiliser isn't technically food for plants (it's more like the ingredients your plants need to grow), but you get the idea. 

Think of growth like putting your foot down on the accelerator in your car. At some stage you'll need to re-fuel or you'll eventually be running on fumes, then stop all together.  

When you first got your plant, the nursery would have been fertilising to speed up growth and get your plant bigger, sooner, ready to sell. Organic goodies in your potting mix might have kept it keeping it going (and growing) for a while also.

That's why it's quite common for all to be well for the first growing season when you get a new plant. Nothing but light and water needed. But come next growing season, nutrients start to run low, then run out. Growth slows down. Leaves aren't as big and healthy. Something's just 'not right'. Then those first yellow leaves start to appear as your plant starts to take from its old leaves, what it can no longer get from the soil.

Which fertiliser is best for indoor plants?

The good news is there are lots of indoor fertilisers available. You're spoiled for choice. Start by looking for one that's both complete and balanced. Check the label. You'll want to see more than just 'NPK' listed (that's Nitrogen, Phosphorus and Potassium).


Look for a fertiliser formulated for indoor plants, as unlike outdoor plants, our house plants are stuck in pots, unable to escape excess mineral salts (which can otherwise cause fertiliser burn), and unable to get the benefit of organic goodies outdoors.

Outdoor plant fertilisers often use urea as their nitrogen source. Fine for outdoors, but indoors it can build up and burn roots. You'll find a small but growing number of fertilisers available these days formulated for indoor plants that use a nitrogen source that's free of urea.

Chlorides is another common ingredient in fertilisers that has a high salt index. Although finding a fertiliser without both urea and chlorides is a big ask, they do exist (one option is GT).

Keep in mind if you top water, and water heavily enough to fully, evenly saturate your potting mix until water is freely pouring out of the drainage holes - then even if you don't have the best fertiliser - every time you water you'll be flushing out excess mineral salts, reducing the risk of fertiliser burn.

Also look for a fertiliser with calcium included so you don't have to add calcium separately. Calcium is NOT a mobile nutrient. That means it's one your plant can't take from older leaves. A calcium deficiency can cause stunted, deformed growth, split, cracked leaf edges, new growth that browns and dies early, and buds that form but fall off before flowering.

If your chosen fertiliser doesn't list calcium on the label, don't panic. Very few fertilisers include calcium. Instead, you can also sprinkle on a little dolomite lime once a season. 
 

Reason #4 for yellow leaves: Soil pH gone bananas

Another reason for yellowing leaves is soil pH. Your soil could be too acidic or too alkaline. Both are cause for concern as they can cause nutrient deficiencies, or worse, nutrients can turn toxic. 

Although different plants have their sweet spots, for most potted indoor plants a healthy soil pH zone is between 5.5 to 7. That's the range where most nutrients in the soil can be accessed by our plants. A small number of fertilisers these days are pH buffered, so just by feeding your plants you're also helping maintain a healthier soil pH.

When the pH goes out of that healthy zone, even if all essential nutrients are in the soil - and no matter how wonderful your fertiliser is - the roots can't access the nutrients, causing yellow leaves from (you guessed it), that same single cause, nutrient deficiency.

At pH extremes, instead of a nutrient deficiency, you can end up with nutrient poisoning

For example, if your soil pH level drops too low, the essential nutrient manganese can turn toxic. At the other end of the scale, molybdenum becomes toxic. Even if you do fertilise, the wrong soil pH can stop your plant taking up and using those goodies. It's called 'nutrient lockout'.

See each nutrient in the soil nutrient pH chart below? Where each nutrient's line is thickest shows you the ideal soil pH for plants to access that nutrient. Below around 5.5 or above 7 deficiencies can occur, cutting off your plant's acces to the essential nutrients they need. A good rule of thumb for indoor plants is to aim for a soil pH of 6.

Bananas and worms

There are a few reasons your soil pH can go bananas. And yes, one is actual bananas. As fun as it is (and the feel-good factor is great too), DIY plant food can be bad news for our indoors plants. I'm looking at you too worm tea.

Bananas tend to be pretty neutral (around 5), but banana peel can be way up around a pH of 9. If you're determined to try making 'banana peel tea' though, go ahead, but I'd test the pH of your concoction first to be safe, and then dilute to get the pH into a safer zone. 

Worm tea is another outdoor goodie to say no to for indoor plants in my books. The pH of 'worm tea' averages a whopping 8.5! That's another one that needs to be tested and diluted more than you might think to get to safer levels for indoor plants.

Maybe I did something wrong, but I ended up with fruit flies to add to my fungus gnats the first and last time I tried banana peel tea on my plants. No thank you! 

If you're determined to give plants a growth boost from all that goodness in bananas, go for HB-101 instead. It's all natural and made from bananas (plantain to be exact - the big brother to bananas and far richer in potassium).

Keep in mind when going the DIY route, that indoors our potted plants are stuck. Any excess nutrients can't leach away from delicate roots, and small changes can make a much bigger difference compared to outdoors. Even outdoor gardeners are cautious of overloading on banana peels or banana peel tea for the shock it can cause a plant from a sudden change in soil pH (that's also why they say not to chuck too many banana skins on your compost by the way).

You can get soil pH test kits at garden centres or online. But DIY fertiliser is one I personally avoid for my indoor jungle. Consider a complete and balanced fertiliser instead. One that's pH buffered is GT (pH buffered to 6 once diluted).

Like most of us, I'm on a budget, but with how much I've spent on my plants, buying good plant food is a tiny price to pay compared to replacing a loved member of my jungle. Bye bye bananas for me.

Signs your soil pH is too acidic or alkaline

This one can be trickier to diagnose, because although the symptoms you'll see are still from a nutrient deficiency - or a toxic level of a nutrient - the cause can be due to the too-alkaline or too-acidic soil. Adding more fertiliser won't fix this one.

That's why a soil pH test is the most reliable way to find out what's going on. You can also get a cheat meter that has a pH tester built-in, but they won't anywhere near as accurate. 

Reason #5 for yellow leaves: Fertiliser burn

If you show your plants a little too much love, you might have overdosed and have a case of fertiliser burn to deal with. Most plant foods give you a  general maintenance dose, meaning even if you stuff up and accidentally give a lot more than usual, your chance of burning your baby is pretty low. 

The reason fertiliser burn is lower down the list of common causes of yellowing leaves, is because it's a less common cause, but still something to consider if you've ruled out other reasons. A variety of frustratingly vague symptoms can indicate over-fertilisation. What makes this one trickier still is that fertiliser burn doesn't often happen immediately, but instead happens over time as excess mineral salts build up in the soil.

Common signs of fertiliser burn

One sign to look for is a 'crust' of minerals on the soil surface, or around the drainage holes underneath. Those white, salty deposits are mineral salts, excess minerals the plant can't absorb that have built up on the surface of the soil. 

Another sign is browning 'burned' leaf tips and edges. That's also a sign of irregular watering or underwatering. The reason that's on both lists is because both fertiliser burn and underwatering mean water and other nutrients aren't reaching the leaves.

Some plants actually get rid of excess minerals by sending it to their leaf tips, so you can end up with an entire plant with brown leaf tips over time. There are plants prone to this more than others, with Peace Lily (above), Spider Plants and Draceana all members of the brown leaf tip club. 

It's not just excess minerals from fertiliser that can cause brown leaf tips, the minerals in your tap water can also, such as flouride and chlorine. 

Weirdly, another sign of over fertilising is the same as not fertilising at all - and that's no or slow growth.

Another to add to the list is leaf drop (this one can be so dramatic when the whole plant does this in one go - eek).

All of those symptoms are because fertiliser burn damages the roots, causing nutrients to be unable to reach the leaves. Too much salt in the soil can also turn soil pH toxic and can 'steal water' from the roots, causing roots to die. Above the surface (you guessed it), that causes yellow leaves and eventually, plant death.

How to fix fertiliser burn

If you're worried fertiliser burn is the cause, and if the roots are okay, first thing is carefully scrape off any obvious salt build-up off the soil surface. Never water 'through' that crusty, salty layer. Next step - bye bye dead leaves. Remove the wilted 'burned' leaves if they have completely dried up. Now it's time for a bath. Sort of. A good flush through with plenty of plain water will help wash out excess mineral salts.

It can take a good two to three drenches to do this. Let your plant fully drain. Don't let it sit or soak in water. No wet feet. After all that is done, don't fertilise for a good month or so while your plant (hopefully) recovers. 

And of course, when you do fertilise, choose the 'safest' fertiliser you can find. Look for one that's complete and balanced, made for indoor plants, and feed as the label says.

How to prevent fertiliser burn

Look for lower-salt fertilisers with no urea, no chlorides and no sodium. No matter what fertiliser you use, top watering is recommended to reduce the risk of mineral salt build-up causing fertiliser burn, by flushing out excess salts every time you water. Do keep in mind it takes a lot to over-fertilise unless you have a major stuff up, so remember to rule out the other causes of yellowing leaves first.

It's worth noting also that some plants are more salt-sensitive than others. Variegated plants for example are more prone to booth root rot and fertiliser burn because they grow slower than their all-green buddies.

That doesn't mean they should be starved. They still need fertiliser, just less (I use the Weakly Weekly Method for mine). Choosing a free-draining substrate and a suitable indoor plant fertiliser is extra important for your variegated jungle members. If you want to learn more about the care specific to variegated plants, read The Ultimate Guide for Variegated Indoor Plants >

The one exception

There is one exception for when you should cut off that yellowing leaf. If you see signs of an infection (look for brown or mushy spots on leaves). Those you should trim off asap with sterile snips in the hope you get it before it spreads to the rest of your plant.

The yellow leaf nutrient chart

 
If your leaves match one of the descriptions below, don't grab the fertiliser just yet. Keep in mind even if you use the best fertiliser ever, if your watering isn't ideal, you use too much fertiliser, or your soil pH is bananas, just adding more fertiliser won't help. You still need to work out what the root cause is (pun totally intended), for why your plant's not getting the nutrients it needs.

Not all nutrient deficiencies show up as yellowing leaves, but here are the more common ones that do...

YELLOWING OF OLDER, INNER LEAVES FIRST 

Nitrogen deficiency usually shows up as a general yellowing with older, inner leaves turning yellow first. As it gets worse, the yellowing moves outward through the plant, reaching young leaves last.

YELLOW LEAF EDGES, INNER LEAF GREEN

Potassium deficiency starts off with leaf edges turning yellow, but the inner leaf stays green. Again, older leaves will show the symptoms first, and if not fixed, those leaf edges turn brown next.

YELLOW MOTTLED OR MARBLED PATCHES BETWEEN VEINS ON OLDER LEAVES FIRST

Magnesium deficiency starts as yellow mottled or marbled patches between the veins on older leaves first. The yellow moves from the leaf centre outwards, with edges turning yellow last.

YELLOWING BETWEEN LEAF VEINS ON YOUNGER GROWTH FIRST

Iron deficiency can also show up as yellowing between leaf veins, but it shows in young growth on the top and tips first.

NEW LEAVES TURNING FULLY YELLOW

Sulphur deficiency starts with the newest leaves first, turning the entire leaf yellow.

There you have it. The top 5 most common causes of yellowing leaves and how to fix each one. Remember what it usually comes down to is lack of nutrients, whether it's water or minerals. And until you work out what's causing that for your plant, keep those yellowing leaves on! They are your plant's temporary life support right now.

If you correct any watering issues, and feed a complete and balanced food with all the essential nutrients, those two actions alone often sort out yellowing leaves quick smart. You will likely still lose the leaves that are currently turning yellow, but that will often stop prevent it 'spreading' to the rest of your plant.