Why Are My Plant Leaves Turning Yellow? Causes and Fixes
Yellow or burnt leaves usually come down to one of several things: too much or too little water, too much feed, a pH that locks the feed out, too much light or heat, pests, a real shortage, or simply old age. This guide shows how to read the leaf and which cause to rule out first, with tomatoes, cucumbers, chillies and houseplants in mind. Each fix links the meters, flush products and other kit we sell.
Read the leaf first: where the yellow is
| What you see | Most likely cause |
|---|---|
| Lowest leaf or two yellow, rest fine | Normal ageing |
| Pale, then yellow all over, from the bottom up | Short of nitrogen |
| Lower leaves yellow between green veins | Magnesium short or locked out |
| Newest leaves yellow between green veins | Iron locked out by a high pH |
| Brown, crisp tips on dark green leaves | Overfeeding (nutrient burn) |
| Yellow, limp leaves, heavy pot | Overwatering |
| Wilting, pot light and dry | Underwatering |
| Top leaves bleached, edges curling up | Light burn or heat |
| Fine pale speckling | Red spider mite |
| Curled, sticky new growth, often on chillies | Aphids |
| Silvery or bleached patches with tiny black specks | Thrips |
| White, powdery spots that spread | Powdery mildew |
Chase yellowing that climbs the plant or reaches new growth. Deficiency detail is in our cal-mag and deficiencies guide.
Overwatering: yellow, limp leaves and a heavy pot
In a pot that never dries back, roots run short of oxygen and take up less, and the leaves go yellow and limp. Fungus gnats, which breed in damp compost and turn up in many houseplant pots, are a common sign.
- Water by weight. Water again only when the pot feels light, and empty the run-off tray; our root rot guide covers watering soil and coco, and what to do if a plant wilts in a wet pot.
- Next time, open up the mix and the pot. Mix Perlite through before you pot up: its page says it does not become soggy and gets plenty of oxygen to the root zone. It will not fix watering too often, though. Air-Pots and fabric pots drain well: our pots guide compares them.
Overfeeding and nutrient burn
Too much feed typically burns from the tips in: tips and edges go brown and crisp on dark green leaves that claw downwards. Salts can build up in pots fed without run-off. Pre-fertilised compost catches people out too: the Biobizz All Mix page says it provides everything the plant needs for the first 2-4 weeks, and that mineral feeds usually start from around 3 or 4 weeks, guided by the plant, so hold a mineral feed back until then.
Measure first: the Bluelab Truncheon reads EC, CF and TDS, so you can check the feed against your chart. Advanced Nutrients say to cut the base nutrients by a quarter if you see tip burn (see our Advanced Nutrients feed chart). For a badly overfed plant in a pot, water it through with plain water or a flushing product, let it drain, and feed weaker once the pot feels light. In a tank, add water or remix weaker.
| Product | What its page says | Media |
|---|---|---|
| Canna Flush | Counters overfeeding; restores plants locked up by too much fertiliser | Hydro, coco or soil |
| T.A. FlashClean | Dissolves mineral salts; safe while plants are growing | All media, mainly hydro |
pH lock-out: the cause you cannot see
Lock-out is when the food is in the feed but the roots cannot take it up, often because the pH is out. It looks like a deficiency, so adding more feed can make it worse. As our cal-mag guide puts it, calcium and magnesium get harder to take up as the pH drops, and iron as it climbs.
- Measure what goes in with the Bluelab pH Pen or the economical NutSystems pH Test Kits (drops of indicator matched to a colour chart). In a tank, check every day or two.
- Aim for the range on your feed chart: Athena give 5.8-6.2 for coco, House & Garden 5.8-6.3 for soil and 5.7-5.9 for recirculating hydro. Our pH and EC guide has more.
- Adjust a few drops at a time with Growth Technology pH Down or Growth Technology pH Up.
- Only then suspect a deficiency. The Canna Elemental Additives page says an apparent shortage is frequently a feeding or environmental problem.
Light burn and heat stress
Light burn shows at the top: the leaves nearest the light typically bleach pale yellow or white and curl up while the lower ones stay green. LEDs run far cooler than HPS lamps, so there may be no heat to warn you. Heat stress looks much the same, so measure the temperature before you blame the light.
- Check the distance. Our LED wattage guide says to follow the maker's card, usually 30 to 45 cm from the canopy at full power, higher over seedlings. Raise or dim the light. The Maxibright PAR Meter page says it helps you avoid over-lighting.
- Check the temperature at the canopy. Our airflow, temperature and humidity guide gives 22-26°C for vegetative growth and 20-25°C in flowering, and says too cold slows growth and reduces nutrient uptake (see greenhouse and grow tent heaters). The ThermoPro TP358 records the 24-hour maximum and has high and low alarms.
- Very dry air can curl leaves too, the same guide says: see our VPD chart.
What to do: check in this order
Our advice: cheapest checks first, one change at a time, and judge the result by the new leaves, because marked leaves rarely recover.
- Read the pattern and look under the leaves. Sticky Fly Traps show early what is flying.
- Lift the pot. Heavy and wet: stop watering until it is light. Light and wilting: water it through (never let coco dry right out).
- Check the light height and canopy temperature.
- Check the pH going in, and in the tank. Put it right before you reach for any supplement.
- Check the EC. Too high: feed weaker (in a tank, add water). Burnt tips on a pot fed without run-off: flush, then feed weaker. Too low and pale all over: feed a little more.
- Then a calcium or magnesium shortage: see our cal-mag guide and the cal-mag additives we sell, starting at the bottom of the label rate.
Quick answers
Why are my tomato leaves turning yellow?
Check where. The bottom leaf or two yellowing with age is usually normal. Yellow and limp with a heavy pot points to overwatering; pale, then yellow all over from the bottom up, to feed too weak. Yellow between green veins low down points to magnesium: check the pH first, then see our cal-mag guide for what suits your water.
Why are my cucumber leaves turning yellow?
Start with the watering: cucumbers dislike cold, wet roots. The big lower leaves often yellow once the growth above shades them. Fine pale speckling points to red spider mite, and white powdery spots to powdery mildew, both common in greenhouses.
Why are my leaf tips curling down?
Often too much feed, with dark green leaves clawing down, or too much water, with limp leaves and a heavy pot. Edges curling up point to heat or light instead.
What does nutrient burn look like?
Brown, crisp tips and edges, usually on dark green leaves. Check the EC and feed weaker; burnt tips will not green up.
More Growers' Guides
- Cal-mag and deficiencies
- Root rot (pythium)
- Which nutrients? Soil, coco and hydro
- Hydroponics: the systems explained
Not sure what you need? Call the shop on 0117 971 3000 or pop in to the Bristol store and we'll help you pick.