Key takeaways
- Oedema is a watering problem, not a pest or disease, so it does not spread
- Bumps form when roots take up more water than the leaves can lose
- Pelargoniums, tomatoes, peppers and cabbages are the usual sufferers
- Wet compost plus cool, dull, still air is the main trigger
- Water less and in the morning, open vents and improve the light
- Marked leaves stay marked, but new leaves come through clean
Corky little bumps on the undersides of your leaves? It is very likely oedema (pronounced “ee-dee-ma”). It is a watering problem, not a pest or disease. The plant has taken up more water than its leaves can get rid of, and some leaf cells have burst. Water less, open the vents, and new leaves come through clean.
It is easy to mistake for an infestation, so lots of people reach for a spray. Sprays do nothing, because nothing is attacking the plant. This guide shows you how to recognise oedema, which plants get it, and how to stop it coming back.
Why do corky bumps form on leaves?
Bumps form when the roots take up water faster than the leaves can lose it. Leaves normally let water out through tiny pores. When they can’t, water builds up in the leaf cells until some swell and burst. The plant seals the damage with corky tissue, which leaves the raised bumps you see.
This happens when the compost is wet but the leaves are losing very little water. A bright, breezy day keeps things in balance. A dull, cool, damp day in a closed greenhouse does the opposite. The leaves lose almost nothing, while the wet roots keep pushing water up.
The bumps usually show on the undersides of lower leaves and on leaf stalks. They start as pale, watery blisters, then turn brown and corky.
Up close, oedema bumps are firm and corky and part of the leaf itself. No insects, no webbing, no powder.
How to tell oedema from pests and disease
Oedema has no insects, eggs, webbing, spores or fuzzy growth. If you find any of those, the problem is something else. Three quick checks settle it.
- Look for anything living. Use a magnifying glass on the leaf underside. Oedema shows only bumps.
- Check the pattern. Oedema turns up on lower leaves of several plants that share the same damp, still spot. It does not spread outwards from one sick plant.
- Try to scrape a bump. A scale insect lifts off with a fingernail. An oedema bump is part of the leaf and stays put.
Oedema is often mixed up with rust, downy mildew or scale. Rust leaves orange or brown powder on your finger. Downy mildew shows grey or purple fuzz under the leaf. Scale insects lift off. Real fungal diseases, such as powdery mildew, also spread from leaf to leaf, which oedema never does.
Cabbages and other brassicas get oedema too. The bumps are scattered and corky, with nothing crawling.
Which plants get oedema most?
Pelargoniums (the bedding and ivy-leaved “geraniums”) are the classic sufferers. Among edibles, tomatoes, peppers and aubergines get it in a damp, dull greenhouse, as do brassicas such as cabbage and cauliflower. Camellias, begonias, succulents and grapevines can show it too.
Greenhouse, conservatory and indoor plants get it far more often than plants outdoors. Outdoors, the air usually moves enough to keep leaves losing water. Pots on a windowsill in winter, sitting in wet compost, are another common spot.
If your pelargoniums are prone to it, the pelargonium growing guide covers watering them through the year. In short: let the compost almost dry out between waterings.
On tomatoes, oedema shows as small pale bumps along the veins. It’s a sign the plant is too wet for the weather.
How to fix oedema and stop it coming back
Balance the water going in with the water going out. Water less, water in the morning and ventilate. No spray, feed or fungicide treats oedema, because nothing is attacking the plant.
| Trigger | Why it matters | The fix |
|---|---|---|
| Wet compost from overwatering | Pushes too much water into the leaves | Let the top of the compost dry before watering |
| Still, damp air | Stops leaves losing water | Open vents and doors whenever the weather allows |
| Dull, low light | Slows water loss from the leaves | Clean the glass and space plants out |
| Cool weather | Leaves lose very little water | Skip watering on dull, cool days |
| Evening watering | Leaves roots wet overnight | Water in the morning instead |
Morning watering works because the plant then has a full day of light and moving air to use the water. The guide to greenhouse ventilation and humidity shows how to keep air moving, even in spring and autumn. For watering outdoors as well, read about the best time of day to water.
Open vents and a propped door keep air moving, so leaves can lose the water the roots take up.
How long does oedema take to clear?
The bumps already there never go away. They are permanent scars on that leaf. But the leaves still work, so leave them on the plant. Pulling them off only weakens it.
The old leaves keep their bumps, but smooth new growth at the top shows the watering fix has worked.
Once you’ve changed how you water, new leaves should come through smooth. That clean new growth is the sign the fix has worked. If the plant looks untidy, you can remove the worst-marked leaves later, once plenty of new ones have grown.
When is oedema most likely? A year in the greenhouse
Oedema is most likely in spring and autumn. That is when greenhouse air is damp and dull but plants are still being watered as if it were summer. Indoor plants are most at risk in winter.
| Month | What to do |
|---|---|
| January to February | Water sparingly. Open vents on mild days. Keep the glass clean |
| March | Water in the morning as growth starts again. Open vents by day |
| April to May | Watch tomatoes, peppers and pelargoniums after dull spells. Ventilate well |
| June to August | Low risk in warm, bright weather. Keep air moving and water to need |
| September | Risk rises as nights cool. Stop watering in the evening |
| October | Cut back watering. Ventilate whenever it is mild and dry |
| November to December | Water only when compost is nearly dry. Never water on dull, cold days |
Common mistakes with oedema
Spraying for pests or fungus. Nothing is attacking the plant, so sprays do nothing. Fix the watering instead.
Stripping off marked leaves. Scarred leaves still feed the plant. Leave them and watch the new growth.
Watering in the evening. Wet compost overnight in cool weather is the classic trigger. Water in the morning.
Keeping the greenhouse shut. Still, damp air stops leaves losing water. Open up whenever the weather allows.
Watering by the calendar. Water when the compost is drying, not because it is Saturday. Dull weeks need far less.
Frequently asked questions
What causes oedema on plant leaves?
Roots take up water faster than the leaves can lose it. The leaf cells swell, burst and heal as corky bumps. It happens when the compost is wet but the air is cool, dull and still.
Is oedema a disease and will it spread?
No, oedema is a watering problem, not a disease. Nothing is attacking the plant, so it cannot pass from plant to plant. Fix the watering and airflow and no new bumps form.
How do I get rid of oedema on my plants?
Water less, water in the morning and open the vents. Hold off watering on dull, cool days and give plants as much light as you can. New leaves should come through clean, but existing bumps stay.
Which plants get oedema most?
Pelargoniums, tomatoes, peppers and cabbages get it most. Camellias, begonias, succulents and grapevines can show it too. Greenhouse and indoor plants get it far more often than plants outdoors.
Will oedema-damaged leaves recover?
No, the corky bumps are permanent on those leaves. The leaves still work, so leave them on the plant. Judge recovery by the new growth, which should be smooth.
Is oedema the same as a pest or fungus?
No, no insect or fungus is involved. There are no eggs, webbing, spores or fuzzy growth to find. If you can scrape a bump off or see powder, it is something else.
Browse the problems hub for more leaf troubles. If your leaves are turning yellow rather than bumpy, read the guide to chlorosis and yellow leaves next.
Our team produces practical UK gardening guidance from source research, contributor records and established horticultural practice. Editorial standards are overseen by Matt W.
