PlantRx

Begonia leaves turning yellow

By Karim DiabatePublished Updated How these guides are built

Yellow leaves narrow nothing down on their own, which is why guessing is expensive. What narrows it down on a begonia is not the colour but everything around it: which leaves went first, what the mix feels like two inches down, and what the roots look like.

A healthy Begonia, photographed for reference
A healthy begonia for comparison. This is a reference photo, not a picture of the problem. Photo by Cliff (CC BY 2.0), via Wikimedia Commons.

What is specific to this plant

  • Begonias want humidity but hate wet leaves, which is an awkward combination and the reason powdery mildew is so common on them.
  • Water on the foliage in a still room is the usual trigger. Airflow matters as much as watering.
  • Many begonias die back and rest seasonally, so a plant thinning in autumn may simply be resting.

What it could be, and how to tell

These look alike in a photograph, so each one below lists what you would ALSO see if that is the cause. Work down the list and stop at the one where the other symptoms match too.

most common

1. Seasonal dieback rather than decline

What else you would see

  • thinning through autumn with leaves yellowing in sequence
  • the same thing having happened at this time last year
  • firm growth points or tubers still present at the base
  • no spots, webbing or softness to go with it
How to confirm it: Check the season and the base of the plant. Many begonias die back and rest rather than growing all year, so leaf loss in autumn on a plant with firm growth points is a plant winding down, not one failing. Treating a rest as a crisis leads to more water and more feed, which is how a resting begonia is actually lost.

What to do

  1. Cut watering right back while it is resting and stop feeding entirely.
  2. Keep it somewhere cool and bright rather than warm and dark.
  3. Resume normal care only when new growth appears.
common

2. Overwatering, and the root rot that follows

What else you would see

  • stems collapsing at the soil line while the leaves above them are still firm
  • grey mould on fallen flowers or leaves left lying on the mix
  • the lowest leaves going soft and dark rather than crisp
How to confirm it: Slide the plant out of its pot and look at the roots. Healthy ones are fine and pale. Brown, soft, or hollow when you press them means rot, and that changes what you do next completely.

What to do

  1. Stop watering and let it dry down this far: the top inch.
  2. Check that the pot drains and empty any saucer or decorative outer pot it sits in. Water held out of sight keeps the mix wet whatever you change above it, and it stays invisible until you lift the pot.
  3. If the roots are soft and brown, trim them back to firm tissue with a blade sterilised first and repot into fresh light, airy, free-draining mix.
common

3. Underwatering

What else you would see

  • edges crisping on the largest leaves first
  • a fast wilt that reverses within an hour or two of watering
  • buds dropping before they open while the foliage still looks well
How to confirm it: Dry soil pulled away from the pot wall, with foliage that improves noticeably within a day of a thorough soak. If a thorough watering changes nothing in twenty four hours, stop assuming thirst and check the roots.

What to do

  1. Water thoroughly until it runs freely from the drainage holes, rather than a splash at the surface.
  2. If the mix has gone water-repellent and the water runs straight through, stand the pot in a basin of water for half an hour to rewet it properly.
  3. Water on dryness rather than on a fixed day of the week, and this is how far it should dry: the top inch.
less common

4. Not enough light

What else you would see

  • long bare stretches of stem between the leaves
  • each new leaf smaller than the last
  • lower leaves shed while the growing tip carries on
How to confirm it: Stretched growth and shrinking new leaves together, on a plant whose watering is otherwise right. Begonia wants bright indirect light.

What to do

  1. Move it to bright indirect light.
  2. Cut back the stretched growth. It will not compact again on its own, and cutting back is what triggers branching.
  3. Turn the pot every couple of weeks so it grows evenly.
less common

5. Too much direct sun

What else you would see

  • bleached, pale patches rather than an even colour change
  • damage only on the side or the leaves facing the window
  • leaves warm to the touch in the afternoon
How to confirm it: The pattern settles it: sun damage is one-sided and patchy, while watering problems and ordinary ageing are even across the plant.

What to do

  1. Move it back from the glass or filter the light. Begonia wants bright indirect light.
  2. Bleached tissue does not turn green again. Judge recovery by whether NEW leaves come in a healthy colour.
uncommon

6. Cold or a draught

What else you would see

  • damage that appeared within a few days of a cold snap, or of the heating or air conditioning coming on
  • the plant sits near a window, an exterior door or a vent
  • the damage is worst on the side facing the cold
How to confirm it: Sudden onset on the exposed side only, following a temperature change. A leaf resting against cold glass overnight is enough to do it.

What to do

  1. Move the plant away from the glass, the door or the vent.
  2. Damaged tissue will not recover. Leave it unless it turns soft, which is the point at which it can start to rot.
uncommon

7. Rootbound, or soil that is worn out

What else you would see

  • roots circling the inside of the pot or escaping the drainage holes
  • water running straight through with almost no absorption
  • two or more years in the same pot, and smaller new growth each season
How to confirm it: Lift the plant out and look. A dense mat of circling roots with very little mix left means the pot can no longer hold what the plant needs.

What to do

  1. Repot one size up in spring into fresh light, airy, free-draining mix, teasing the outer roots loose.
  2. Feed during the growing season, following the instructions on the label of whatever you buy.
uncommon

8. Ordinary ageing

What else you would see

  • one or two of the oldest, lowest leaves at a time and no more
  • healthy new growth continuing from the centre or the tip
  • no other symptom anywhere on the plant
How to confirm it: Oldest leaves only, while the plant is still producing new ones. A plant funding new growth from its oldest tissue is working normally.

What to do

  1. Remove the spent leaf at its base with a clean blade, purely for looks.
  2. Nothing else is needed.

Common questions

Will a yellow begonia leaf turn green again?

No. Once a leaf has yellowed, that leaf is spent and the plant has already withdrawn what it could from it. Fixing the cause stops the next leaves going the same way; it does not restore the ones already gone.

Should I cut the yellow leaves off my begonia?

Once a leaf is more yellow than green, cut it off at the base with a clean blade. While it is only edged with yellow, leave it: the plant is still reclaiming nutrients from it.

How do I tell overwatering from underwatering?

Feel the mix a couple of inches down and feel the stem at the soil line. Wet mix with a soft stem is overwatering. Dry mix pulled away from the pot with firm stems is underwatering. From above the leaves look much the same, which is how a drowning plant ends up being watered again.

Before you act on any of this

This is general plant care guidance for a plant, not advice about people or animals. We never tell you whether a plant is safe to eat or touch, and we never tell you a plant is safe for a child or a pet. If a child or animal may have eaten any part of a plant, contact Poison Control (US 1-800-222-1222) or a vet immediately rather than reading further.

Treatment advice here is deliberately category level. We do not give product names, mixing ratios or application intervals. Whatever the label on the product in your hand says is the instruction that counts, and following it is your responsibility.

Where this comes from

The general horticultural facts behind this differential come from published university extension and Royal Horticultural Society guidance. What is specific to this plant, and the order the causes are ranked in, is ours, and the methodology page says where that reasoning stops being reliable.

Other begonia problems

Not sure which one it is?

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