Why Are My Tomato Leaves Turning Yellow?

Yellow leaves on a tomato plant usually mean one of a handful of fixable things: too much water, too little food (nitrogen or magnesium), a run of cold nights, or — less often — disease. The good news is that none of these is hard to put right once you know which one you're looking at, and most plants bounce back quickly once you do.

Below are the likely causes, ranked from most to least common in a UK garden or greenhouse, each with the fix. Then there's a quick guide to telling them apart by the pattern of yellowing, and how to stop it happening again.

Most likely causes (ranked, each with the fix)

1. Overwatering or poor drainage

This is the single most common cause, especially for tomatoes in pots and grow bags. When compost stays soggy, the roots can't get the oxygen they need, they start to struggle, and the leaves go pale and yellow — often the lower ones first. It's easy to do with the best intentions, particularly after a wet UK spell when you keep watering out of habit.

The fix: stop watering and let the top 2–3cm of compost dry out before you water again. Push a finger in — if it's damp below the surface, leave it. Make sure every pot has drainage holes and isn't sitting in a saucer of standing water. For grow bags, cut generous drainage slits in the base. Once the roots can breathe again, new growth usually comes through green.

2. Underfeeding — nitrogen and magnesium shortage

Tomatoes are hungry plants. Once they're growing strongly, and especially once they start fruiting, they draw heavily on the compost and quickly run short of nutrients — most visibly nitrogen and magnesium.

A nitrogen shortage shows as an even, overall paling and yellowing that starts on the oldest, lowest leaves and works upwards, because the plant moves what nitrogen it has up to the new growth.

A magnesium shortage looks different and very distinctive: the leaf yellows between the veins while the veins themselves stay green (this is called interveinal yellowing). It's extremely common in tomatoes grown in pots and grow bags, particularly mid-season when they're cropping hard.

The fix: start a regular liquid feed if you haven't already — a high-potash tomato feed once flowers appear keeps fruiting plants fed. For magnesium specifically, a foliar feed of Epsom salts works quickly: dissolve about 1 tablespoon (roughly 20g) of Epsom salts in a litre of water, put it in a spray bottle, and mist the leaves on a dull day or in the evening, every couple of weeks. You'll often see the difference within a week or two. There's more detail on getting the routine right in our guide to watering and feeding tomatoes.

3. Cold nights early in the season

In a UK spring, plants put out too early often yellow simply because they're cold. Tomatoes dislike night temperatures below about 10°C, and a sharp dip can leave leaves pale, purplish or yellow and growth stalled — even though nothing is "wrong" with the plant.

The fix: keep tomatoes warm and don't rush them outside. Indoors, on a windowsill or in a heated propagator, hold young plants until nights are reliably mild. Outdoor tomatoes shouldn't go out until after your last frost — check the planting calendar for timings where you are, and in a cold spring be prepared to wait until late May or even June. A cloche or fleece buys you a few degrees on chilly nights. Once it warms up, the plant usually grows away and the new leaves come through healthy.

4. Natural ageing of the lower leaves

Sometimes a few yellow leaves at the very bottom of the plant are nothing to worry about at all. As a tomato grows tall and focuses energy on flowers and fruit higher up, its oldest, lowest, most-shaded leaves naturally die back and yellow. This is normal — not a deficiency, not a disease.

The fix: simply snip off the oldest yellow leaves at the stem with clean scissors or secateurs. Removing them tidies the plant, improves airflow around the base (which helps prevent disease), and lets it put its energy into the growth that matters. As long as the new growth at the top is green and healthy, a little yellowing at the bottom is fine.

5. Disease — early blight or late blight

Less common, but worth ruling out. Early blight typically shows as yellowing around dark brown spots with concentric rings, again starting on the lower leaves. Late blight — the serious one in damp UK summers — usually appears as fast-spreading brown-black blotches, often with a pale yellow halo, sometimes with white mould underneath, and can take a plant down within days.

The fix: remove and bin affected leaves immediately — do not compost them. Improve airflow, water at the base (never over the leaves), and avoid wetting the foliage. If you suspect late blight, act fast: our guide to tomato blight covers how to confirm it and what to do, including saving any unaffected fruit. If you keep seeing yellowing with dark patches and sunken ends on the fruit, also check our piece on blossom end rot, which is a watering issue rather than a disease.

How to tell which it is

The pattern of yellowing is your best clue. Look at where it is and what shape it takes:

• Lowest leaves yellow first, whole leaf, evenly pale — most likely overwatering or a nitrogen shortage. Check the compost: soggy points to water; bone-dry-then-yellow points to hunger.
• Yellow between the veins, veins staying green — classic magnesium deficiency. Very common in pots and grow bags mid-season. Reach for the Epsom salts foliar feed.
• Pale, stalled, maybe purplish, after a cold snap — cold, especially on plants put out too early. Warm them up and wait.
• Just a few of the very oldest bottom leaves, plant otherwise thriving — natural ageing. Snip them off and move on.
• Yellowing around brown spots or fast-spreading dark blotches — disease. Remove affected leaves, improve airflow, and check the blight guide.

One quick sanity check: look at the newest growth at the top of the plant. If the tips are green, lush and growing, the plant is fundamentally healthy and you're dealing with something minor — usually feeding or a few old leaves. If the new growth is also sickly, take it more seriously.

How to prevent it

Most yellowing comes down to a few habits. Get these right and you'll rarely see it:

• Water consistently. Aim for evenly moist compost — never waterlogged, never bone dry. The finger test beats a fixed schedule every time. Steady watering also helps prevent split fruit and blossom end rot.
• Feed weekly once flowering starts. Switch to a high-potash tomato feed when the first flowers appear and feed once a week. This keeps fruiting plants supplied with the nutrients that prevent both nitrogen and magnesium shortages. Our watering and feeding tomatoes guide sets out a simple routine.
• Keep them warm. Don't plant out before nights are reliably above about 10°C — in much of the UK that means late May at the earliest. Use a cloche or fleece for cold nights.
• Give them airflow. Remove the lowest leaves as plants grow, space pots so air moves between them, and water at the base. Good airflow and dry foliage are your best defence against blight and other diseases.

Yellow leaves look alarming, but on tomatoes they're usually the plant simply telling you it's too wet, too hungry or too cold — all easy to put right. For the full picture on growing healthy, productive plants from sowing to harvest, see our main tomato guide, and browse the rest of the problem-solving section if something else isn't quite right.