Why Are My Houseplant Leaves Turning Yellow?

Yellow leaves on a houseplant usually mean the roots are unhappy, and the most common reason is too much water. Light that is too weak, a cold draught or a pot that is too big come next. The way to find out which is to lift the pot, check the compost at depth, and, if you still can't tell, slide the plant out and look at the roots.

October is when this question gets typed most. The sun is lower, the radiators are about to come on, and plants that sailed through summer begin to look tired. The clocks go back at 2am on Sunday 25 October, which will make the dim corner of the sitting room that much dimmer in the evenings. It is a good week to look properly at every plant you own.

Stop and read the leaves first

Yellowing is a symptom with several causes, and where the yellow appears narrows it down. The RHS gives a useful rule of thumb for water. If the plant is too dry, the older leaves at the base yellow and fall first. If it is too wet, old and new leaves yellow and drop at the same time.

That difference is easy to miss when you're standing over a plant that looks generally miserable. So make it a habit to look at which leaves have gone, not only how many. Ask:

• Are the yellow leaves the oldest ones, at the bottom? That points towards thirst, or simply ordinary ageing.
• Is yellowing spread across old and new growth together? That points towards too much water.
• Are the leaf edges brown and crisp as well? The RHS lists brown and crispy edges with wilting as a sign of underwatering.
• Is there rot at the base of the stems? The RHS lists stem rot and dark, mushy roots as signs of overwatering.

If a plant has lost one lower leaf in a month and looks otherwise fine, relax. The RHS says it is normal for the odd lower leaf on a mature plant to yellow and fall.

The usual culprit: too much water

Overwatering kills more houseplants than everything else combined, and the houseplants course spends a good deal of time on it for that reason. Constantly wet compost has no air in it. Roots need oxygen, so in waterlogged compost they suffocate and rot, and a plant with rotten roots can't drink however much water surrounds it.

Which leads to the nastiest feature of the whole business: an overwatered plant wilts and yellows exactly like a thirsty one. The RHS makes the same point, noting that it is easy to mistake overwatering for a lack of water and drown a plant, especially over winter when growth slows. The kindly beginner sees the droop, waters again, and finishes the job.

So diagnose from the pot, not from the leaves:

1. Lift it. A pot that is heavy and drooping is too wet. A pot that is startlingly light is dry.
2. Push a finger 2–3cm into the compost. Damp at that depth means leave it alone. Dry at depth, with a light pot, means water.
3. Check the drainage. If the pot has no hole in the bottom, that is your answer. Water has to be able to leave.
4. Tip away anything standing in the saucer. A plant sitting in a puddle for a week is overwatered however carefully you watered it.

If the compost is soaked, stop watering entirely and let it dry out. The RHS suggests lifting the plant out of its pot to see whether water has collected in the base, and tipping away any excess. Look at the roots while you're there: the RHS lists dark, mushy roots as a sign of overwatering. If that is what you find, repot into fresh, free-draining houseplant compost and go easy on the water afterwards.

Then, and this is the hard part, wait. A dry plant forgives you within hours of a soak. A rotted one takes weeks to show whether it will recover.

Why autumn makes it worse

You water on the habits of summer, but the plant is working to the rhythm of autumn. Less light means slower growth, slower growth means less water drawn up, and the compost stays wet for longer. A pothos that wanted a drink every week in July may want one every ten to fourteen days in October. Snake plants and ZZ plants, which want the whole pot dry before the next soak, can go a month in a British winter.

This is why the course teaches watering by weight, not by calendar. The pot tells you what the plant needs. The calendar only tells you what you did last time.

Too little light

Water gets blamed first, but light is the plant's food. Poor light, the RHS says, produces weak, spindly growth and pale leaves. It also lists "too shady a position" among the causes of yellowing, and recommends moving a plant to a brighter position, especially in winter when light levels drop.

Distance matters more than people think. Light falls away fast as you move from a window, and your eyes adjust so well that they mislead you. A spot two metres back from the glass may get a fraction of the light on the sill. The course's test is simple: if you can read a paperback there at midday without reaching for a lamp, most of the forgiving plants will manage. If you can't, that corner belongs to the ZZ plant or to nothing.

So in October, do this:

• Move plants closer to the window, within a metre if you can.
• Prefer east and west windows for most plants. A south-facing sill is fine for snake plants and succulents.
• Rotate the pot a quarter-turn each week so it doesn't lean.

Moving a plant closer to the light is better than putting it on a stricter watering regime. Pale, leggy new growth is a light problem, not a thirst problem.

Draughts, radiators and the cold

The RHS lists too cold a temperature, a sudden drop in temperature and draughts among the causes of yellowing, and says draughts can cause leaf yellowing and dry leaf edges. It also warns that plants kept near direct sources of heat, such as radiators, can end up with scorched leaves.

October is when both problems arrive together. The sill that was pleasant in August becomes the coldest place in the house on a clear night, with a cold pane of glass behind the leaves. The plant moved onto the shelf above the radiator gets dry heat from below.

A few practical points:

• Don't leave plants between the curtain and the window on cold nights. Pull them into the room, or move them off the sill.
• Keep them clear of doors that open onto the outside.
• Keep them off the top of radiators and out of their direct heat.
• Avoid moving a plant between very different rooms in one go. A sudden drop in temperature is on the RHS list.

A pot that is too big

The RHS lists overpotting, which means potting up into too large a container, as a cause of yellowing. A small plant in a huge pot sits in a mass of wet compost its roots can't reach, which is overwatering by other means.

If you repotted in a hurry and the plant has sulked ever since, check the pot size. The rule from the course is to go up one size only, 2–5cm wider, and to do it in spring, once the plant is root bound. Roots circling the pot wall or escaping from the drainage holes are the sign. If a plant is badly overpotted, the RHS suggests repotting into a smaller container. Do that gently, and don't rush to feed it afterwards.

What not to do

Resist the urge to treat a yellowing plant with feed. The course is blunt about this: houseplant compost carries enough nutrients for months, and from October to March growth stops, so unused feed just salts the compost. Half-strength liquid feed once a month through spring and summer is plenty, and nothing at all in winter. Missing a month costs nothing. Doubling the dose burns the roots.

Don't repot in October either. A plant that is already stressed doesn't need the second shock. The exception is a plant whose roots are sitting in sodden compost, where fresh, free-draining compost is the better risk.

And don't keep changing things. Move the plant to a better spot, correct the watering, then leave it alone for a few weeks. Leaves that have gone fully yellow won't turn green again, so judge recovery by new growth. Once a leaf is fully yellow you can pinch it off, which tidies the plant and removes something that might go mouldy.

A ten-minute check for every plant you own

Do this once, this week, and again after the clocks change:

1. Lift each pot. Note which are heavy and which are light.
2. Water only the light ones, thoroughly, at the sink, and let them drain fully.
3. Tip out every saucer.
4. Move anything standing on a cold sill or above a radiator.
5. Check the underside of the leaves for webbing or white fluff. Sooner or later something moves in, and the course covers fungus gnats, spider mites and mealybugs without panic.
6. Pinch off fully yellow leaves.
7. Write the date of each watering down. The gaps will surprise you.

If you have only one plant and have been wondering whether the hobby is for you, it is. A single pothos teaches you more than a jungle you're guessing at. The houseplants course takes you from one plant to a rooted cutting in six weeks, and the growing food course is the natural next step when the windowsill starts to feel small. If you want to see how it all fits together outdoors, the composting course is where your peelings go next.

The short version

• Yellow leaves usually mean too much water, too little light, a draught or a pot that is too big.
• Where the yellow appears helps. Old leaves at the base only suggests thirst. Old and new together suggests too much water.
• Diagnose from the pot, not the leaves: lift it, then test the compost 2–3cm down.
• If the pot is soaked, stop watering and let it dry. Check the roots: dark, mushy ones mean overwatering.
• In October growth slows, so plants need less water than in summer. Water by weight, not by the calendar.
• Move plants closer to a window, away from cold glass, draughts and radiators.
• Don't feed between October and March, and don't repot a stressed plant. Spring is for repotting.
• One yellow lower leaf now and then is normal. Judge recovery by new growth, not old leaves.
• The clocks go back on Sunday 25 October, so do your check before then and again after.