Look at where the yellow sits before you reach for the feed. Cucumber leaves turning yellow on the oldest growth near the base means one thing; yellow on the brand-new leaves at the top means quite another. The plant is telling you which nutrient it has run short of, or whether the trouble is water, light or a mite on the leaf underside. Read the position first, and most of the guesswork disappears.
Yellowing of the leaf is what gardeners call chlorosis, and on greenhouse cucumbers in a British summer it is usually a feeding or watering story rather than a disease. Helpfully, the cause writes itself across the plant in a fairly readable pattern. This guide walks the pattern with you, top to bottom, so you can name the cause and fix it the same afternoon.
Why leaf position tells you the cause
Cucumber plants move some nutrients around inside themselves and lock others in place. Nitrogen, magnesium and potassium are mobile, so a hungry plant strips them out of its old lower leaves and ships them up to the new growth. That is why their shortages show on the oldest leaves first. Iron and manganese cannot be relocated, so their shortages appear on the youngest leaves at the top.
So the single most useful question is: which leaves went yellow first, the old ones or the new ones? Pair that with a second look, whether the yellow sits between the veins or covers the whole leaf, and you have narrowed five possible causes down to one or two. The RHS notes that interveinal yellowing points to manganese, iron or magnesium, while nitrogen gives a more general yellowing of the older leaves.
Old lower leaves going pale and yellow: nitrogen
If the oldest leaves near the base turn an even, pale yellow and the plant looks spindly and slow, suspect nitrogen first. The RHS describes nitrogen shortage as spindly, stunted plants with pale yellow leaves, sometimes with pink tints, the older leaves affected first. University extension growers report the same order: the lower leaves turn pale yellow before they drop.
The yellow is uniform across the whole leaf rather than picked out between the veins. That whole-leaf wash is the tell that separates nitrogen from magnesium. A fast-growing cucumber carrying a heavy crop runs hungry, and it burns through nitrogen first.
The fix
Give a general-purpose liquid feed, or work in a nitrogen-rich fertiliser such as poultry manure pellets. If you have been feeding only tomato food, that is the likely gap, since high-potash feeds are deliberately low in nitrogen. Colour returns to new growth within a week or two once the plant is fed.
Yellow between the veins on older leaves: magnesium
Magnesium shortage paints a different picture. The yellow appears between the veins while the veins themselves stay green, sometimes with reddish-brown tints, and the oldest leaves can develop brown dead patches. British greenhouse growers meet it more than any other deficiency, and it has an awkward habit of arriving exactly when the plant works hardest.
Here is the link worth knowing: a high-potash feed can cause it. Plants take up potassium in preference to magnesium, so heavy tomato feeding during fruit swelling can lock out magnesium even when there is plenty in the soil. The symptom often shows up during rapid growth and fruit enlargement, the very moment you push the potash hardest.
The fix
- Fast, foliar: spray Epsom salts at 20 g per litre of water onto the leaves. Magnesium absorbs through the leaf surface, so this greens the plant up quickest.
- Long term, to the soil: apply magnesium sulphate, Epsom salts or Kieserite, at 30 g per square metre.
- Ease off the potash a little while the plant recovers, then resume your feeding rhythm.
Yellow on the newest top leaves: iron
When the youngest leaves at the growing tip go yellow between the veins while the lower leaves stay green, the nutrient is iron. Iron is immobile in the plant, so it cannot be pulled from old leaves to rescue new ones, which is precisely why the shortage strands the top growth. The veins hold their green against a yellowing background, the classic look of interveinal chlorosis on the newest leaves.
Iron lockout in a greenhouse usually traces to cold, wet compost or a high pH that makes iron unavailable rather than a true absence of it. Warmer, evenly moist compost often resolves it on its own. Where it persists, a chelated iron feed corrects the colour, and getting the watering even, covered below, removes the root cause.
Could overwatering be turning my cucumber leaves yellow?
Yes, and it is a common trap because the instinct when leaves yellow is to water more. Waterlogged compost fills the air pores with water instead of oxygen, so the roots cannot breathe or take up nutrients, and the leaves yellow despite plenty of water around them. Yellowing is often the first visible sign of roots drowning.
Even moisture fixes it, not more or less water. The RHS advice for cucumbers is to keep the compost evenly moist, watering at the base of the plant to keep the foliage dry, and in hot spells that can mean watering daily. Erratic swings between bone-dry and sodden stress the roots and show as yellow leaves and bitter, misshapen fruit. Push a finger into the compost before you water: damp an inch down means wait.
Heat plays into this too. Cucumbers are happiest at 18–25°C, and a greenhouse that runs much hotter dries the compost fast and tips you into a water-stress cycle. Our guide to keeping a greenhouse cool through a UK summer covers shading and venting that take the edge off both problems. If you are heading off for a week, the greenhouse watering while away guide keeps that even-moisture going without you.
Fine pale speckling: glasshouse red spider mite
Some yellowing is not feeding at all. Glasshouse red spider mite makes fine pale mottling on the upper leaf surface, a stippled, speckled look quite unlike the broad washes of a deficiency. Turn the leaf over and you may see many tiny yellowish-green mites and white cast skins; in a bad case there is fine silk webbing across the leaves, and they lose most of their green and dry up.
The mites pierce leaf cells one by one and drain the chlorophyll, leaving pale dots that spread into general yellowing or bronzing as feeding goes on. They thrive in warm, dry conditions and are usually only a problem from March to October, which lines up neatly with the hot, dry greenhouse air of a British summer. Damping down the floor to raise humidity makes the greenhouse less welcoming to them.
The fix
For a real infestation, the biological control Phytoseiulus persimilis is the one most growers reach for. It is a predatory mite about 0.5 mm long that feeds on every life stage of the pest, and it works best introduced early while numbers are still low. Check the undersides of new leaves with a hand lens before you blame the feed; speckling plus webbing means mites.
The high-potash feeding rhythm that prevents most of this
Most yellowing on a greenhouse cucumber is a feeding-rhythm problem, and a steady rhythm prevents it. The RHS routine is straightforward: feed every 10–14 days with a general liquid fertiliser, then once the plants start flowering, switch to a high-potash liquid feed such as tomato food. Some growers go weekly once the fruits are swelling, so weekly to fortnightly once flowering begins both sit within sensible practice.
Potash drives flowering and fruiting, which is why the switch matters once the first flowers open. Watch the magnesium link above: lean too hard on potash and you can starve the plant of magnesium and bring on the very yellowing you were feeding to avoid. Balance solves it: a high-potash feed for the fruit, with an occasional Epsom-salts spray to keep magnesium topped up.
| What you see | Where | Likely cause | First move |
|---|---|---|---|
| Whole leaf pale yellow | Old lower leaves | Nitrogen | General liquid feed |
| Yellow between veins, veins green | Old leaves | Magnesium | Epsom salts, 20 g/litre foliar |
| Yellow between veins | Newest top leaves | Iron | Warm, even moisture; chelated iron |
| Edges brown and scorch first | Mature leaves | Potassium | High-potash tomato feed |
| Fine pale speckling, webbing | Any leaf, check underside | Spider mite | Phytoseiulus, damp down |
| Lowest leaves only, top vigorous | Base of plant | Normal ageing | Remove the old leaves |
When yellow lower leaves are nothing to worry about
Before you treat anything, rule out plain old age. If the lowest, oldest leaves yellow while the new growth up top stays green and vigorous, that is natural senescence, the leaf reaching the end of its working life, and it is no cause for alarm. Commercial cucumber growers actively remove the lower leaves to a height of 50 cm from the soil to improve airflow and reduce disease, so a tidy strip of the yellowed bottom leaves is good housekeeping rather than a rescue.
The test is simple. Healthy new leaves at the top plus yellow ones only at the very base reads as ageing. Yellow creeping up into the new growth reads as a problem you need to name. A related stress, blight, never touches cucumbers but it does ruin a tomato crop in the same greenhouse, and the tomato blight guide is worth a look if your tomatoes share the space.
The growing advice here works whatever greenhouse you have. That said, even moisture and steady warmth are easier to hold in a structure that ventilates well. The NORDIC greenhouse in Swedish pine and twin-wall polycarbonate opens front and back, with a rear opening window opposite the door for cross-ventilation as standard, which helps keep the air moving and the spider mites less at home on hot days. It is a help, not a requirement; your cucumbers will tell you the same story in any greenhouse.
A two-minute diagnosis to run today
Go out to the plant and start at the bottom. Find the first yellow leaf, note whether it is old or new, then check whether the yellow is whole-leaf or trapped between green veins, and turn one leaf over to look for speckling. Old and uniform points to nitrogen; old and interveinal points to magnesium at 20 g of Epsom salts per litre; new and interveinal points to iron and even watering. Feed every 10–14 days, switch to high-potash once the flowers open, and keep the compost evenly moist at 18–25°C, and most cucumber yellowing never gets started.
Yellowing that feeding will not fix
If you have fed, watered evenly and the leaves keep yellowing, the cause is usually below the soil or it is a disease. Cold roots and a locked-out root zone stop a plant taking up nutrients however much you add, and two cucumber diseases mimic hunger closely enough to fool most growers. Run through this before you reach for more feed.
| What you see | Likely cause | The fix |
|---|---|---|
| Whole plant pale and sulking, growth stalled after a cold night | Cold roots: cucumbers take up far less nitrogen once the root zone drops to about 16°C, and the RHS sets 12°C as the floor for planting out | Hold a steady 18–25°C, lift pots off cold paving, and do not feed a cold, wet root ball |
| Yellowing despite feeding, in very limy tap water or fresh bagged compost | pH lockout: iron and manganese become unavailable when the root zone drifts too alkaline | Water with rainwater where you can and use a chelated iron feed that stays available at higher pH |
| Yellow and green mottling, leaves curling down, narrowed and distorted, fruit warty | Cucumber mosaic virus, spread by aphids and on hands and tools | No cure exists. Destroy the plant promptly, wash hands and tools, and grow a resistant cultivar next year |
| Angular yellow patches between the veins on the upper leaf, grey or purple mould underneath in humid spells | Cucurbit downy mildew, driven by warm damp air | Improve airflow, water the soil not the leaves, remove affected foliage, and cut humidity |
The quick verdict: if a mottled or distorted leaf does not match any nutrient pattern, treat it as virus and remove the plant rather than risk the rest of the crop. For the rest, warm the roots and steady the watering before you blame the feed.
