Why Are My Tomato Leaves Curling or Yellowing? Feeding Fixes

Tomato plants tell you when something is wrong through their leaves — and the symptoms point to specific problems. This guide covers the four most common leaf problems UK gardeners see mid-season, the actual cause behind each one, and the feeding adjustments that fix them.

Diagnose First — Symptom Guide

Problem 1 — Blossom End Rot

Blossom end rot is the single most common tomato problem in UK greenhouses, grow bags, and pots. The bottom of the fruit develops a dark, sunken, leathery patch — sometimes when the fruit is still green, sometimes only as it ripens.

The cause is not what most gardeners think. Blossom end rot is rarely caused by low calcium in the soil. The cause is the plant failing to deliver calcium to the developing fruit, even when calcium is present in the soil. The two factors that cause this delivery failure are:

• Irregular watering. Calcium moves with water through the plant. A dry-out followed by heavy watering disrupts calcium delivery to the developing fruit.
• Excess potassium relative to calcium. Too much potassium in the feed blocks calcium uptake at the roots.

The fix:

1. Switch to a feed with chelated calcium — calcium in a chemical form the plant can actually absorb. Tom-Sol™ includes chelated calcium and magnesium in every dose.
2. Water consistently — same time of day, same volume, never let pots dry out then flood.
3. Mulch around plants to slow soil drying.
4. Stick exactly to the recommended feed dilution. Stronger feed does not help — it makes blossom end rot worse.

Problem 2 — Leaves Yellowing Between the Veins

Yellowing that appears between the leaf veins while the veins themselves stay green is a classic sign of magnesium or iron deficiency. It usually appears on older leaves first when magnesium is the cause, and on younger leaves first when iron is the cause.

In container-grown tomatoes, this is most commonly magnesium deficiency, often triggered by — again — excess potassium blocking magnesium uptake.

The fix:

• Switch to a feed that includes chelated magnesium. Tom-Sol™ delivers both chelated Mg and chelated Ca alongside the high-potash NPK.
• For severe cases, an Epsom salts foliar spray (1 tablespoon per 4.5L water, sprayed on leaves) can correct the immediate symptom while the root feed brings long-term balance.
• Avoid overfeeding with general-purpose high-potash feeds that don't include Mg.

Problem 3 — Leaves Curling Upwards

Upward-curling tomato leaves are usually a stress response, not a nutrient deficiency. The plant is essentially closing up its leaves to reduce water loss. Common triggers are:

• Heat stress — greenhouse temperatures over 30°C without ventilation.
• High light exposure — particularly during a heatwave.
• Excessive feeding — concentrated feed on dry roots can also trigger this response.

The fix:

• Ventilate greenhouses on hot days. Open doors and vents from early morning.
• Shade glass during heatwaves if leaves continue to curl.
• Check feeding rate — always 15ml per 4.5L water, never higher.
• Always water the plant normally before applying feed. Never apply diluted feed to dry compost.

Mild upward curling that occurs only in mid-afternoon heat and recovers by evening is normal and does not need intervention.

Problem 4 — Leaves Curling Downwards

Downward leaf curl (sometimes called leaf roll) is a different issue from upward curling. The most common causes are:

• Excess nitrogen — feeding nitrogen-led general-purpose feed during fruiting.
• Herbicide drift — particularly from lawn herbicides that contain growth-regulator chemicals like aminopyralid or clopyralid. Even trace amounts in compost made from treated grass can cause distinctive downward curl.
• Pruning shock — heavy de-leafing can briefly trigger curl.

The fix:

• If you've been using a general-purpose or lawn-style feed, switch to a high-potash tomato feed to stop the nitrogen overload.
• Check whether any nearby lawn or pasture has been recently sprayed. Manure or hay-based composts from treated land are a known source of contamination.
• Avoid heavy de-leafing in a single session.

Problem 5 — Lower Leaves Yellowing on Older Plants

Towards the end of the season, the lowest leaves on a tomato plant naturally yellow and die. This is not a problem and not a sign of feeding failure — the plant is reallocating nutrients from old leaves to the developing fruit higher up.

Remove the yellowed lower leaves to improve airflow and reduce disease risk. Continue feeding as normal.

The General Feeding Principles That Prevent Most Problems

• Use a feed that delivers chelated calcium and magnesium — these prevent the two most common tomato problems (blossom end rot and interveinal yellowing) at source.
• Never feed at higher than the recommended rate. Concentrated feed creates more problems than it solves.
• Water consistently. Calcium delivery to the fruit depends entirely on consistent watering.
• Water before you feed. Never apply diluted feed to dry compost.
• Start feeding at second-truss fruit set, not before. Early high-potash feeding wastes nutrients and can push the plant out of balance.