Five Reasons Your Tomatoes Are Not Performing and What to Do About It Now

The tomatoes are in, the trusses are forming, and somewhere across Britain a gardener is standing in front of a struggling plant with a slight frown, trying to work out what it is trying to say. The leaves look a little wrong. The fruits are there but few. Something is off, and it is not the weather, or at least not only the weather. Tomatoes have a reputation for being difficult in the UK, but that reputation is only partly deserved. Most of the problems are diagnosable, most of the causes are fixable, and almost none of them require an unusually warm summer to resolve.

What they require is attention to a handful of specifics that experienced growers learn, often the hard way, over several seasons. If your plants are underperforming this July, read on, because the window to turn things around is narrower than you might think, but it is not yet closed.

Inconsistent Watering Is Doing More Damage Than You Realise

This is the one. If there is a single cause behind more tomato problems than any other, it is the irregular watering regime that most of us fall into without meaning to: generous on Sunday, forgotten on Wednesday, a thorough drenching on Friday when we notice the compost has pulled away from the sides of the pot. Tomatoes do not forgive this pattern.

Blossom end rot, that dark, sunken patch on the base of the fruit that looks like disease but is not, is caused almost entirely by inconsistent moisture at the roots. Calcium deficiency is often cited as the culprit, and technically that is true, but the reason the plant cannot take up calcium is usually that the soil moisture has been swinging between wet and dry. Calcium travels in water; interrupt the flow and the fruit suffers. Split skins, particularly on varieties like Gardener's Delight or Sungold, are the same story in reverse: a long dry spell followed by heavy rain or watering causes the fruit to expand faster than the skin can accommodate.

The fix is tedious but not difficult. Water regularly and deeply, ideally at the same time each day, and aim to keep the compost consistently moist rather than alternately waterlogged and parched. In a greenhouse or polytunnel in July, that will likely mean watering every day, sometimes twice for plants in smaller pots or growing bags. Mulching around the base of outdoor plants with a few centimetres of garden compost helps slow moisture loss significantly between sessions.

Feeding Has Fallen Behind the Plant's Needs

Once the first truss has set, a tomato plant is working very hard. It is building fruit, maintaining foliage, and pushing out new growth all at once. At that point, the nutrients in a bag of proprietary compost or a well-prepared bed are no longer enough on their own. The plant needs feeding, and it needs feeding consistently.

A high-potassium liquid feed, applied weekly once the first fruits have set, is the standard approach, and it works. Most commercial tomato feeds are formulated with this in mind. What often goes wrong is that gardeners start well, miss a week, and then quietly abandon the routine. The plant does not collapse immediately. It simply produces fewer, smaller, less flavourful fruits, and the lower leaves begin to yellow in a way that is easy to mistake for something more alarming.

Yellowing that progresses steadily upward from the base of the plant is generally a sign of nitrogen deficiency, often because feeding has been inconsistent. Yellowing that appears between the veins of older leaves suggests magnesium deficiency, for which a dilute solution of Epsom salts, applied as a foliar spray or soil drench, is a reliable and inexpensive remedy. Neither of these is a disease, and neither requires drastic action. They require you to feed the plant, and to keep feeding it, every week until the end of the season.

Poor Ventilation Is Creating Problems You Cannot See

Tomatoes in a greenhouse or polytunnel need air movement, and in a British summer, good ventilation is the difference between a healthy crop and one that lurches from one fungal problem to the next. Botrytis, or grey mould, thrives in the humid, warm, still air that accumulates inside an unventilated structure on a July afternoon. So does tomato leaf mould, a common UK greenhouse problem that begins as pale yellow patches on the upper surface of leaves and a greyish-brown growth beneath.

The practical answer is to open every vent and door you have as soon as the temperature rises in the morning, and to keep them open until the evening cools. On still days, even with everything open, the air inside a polytunnel can feel thick and warm in a way that does nothing for the plants. If you have the option to introduce a small fan on warm, humid days, do it.

Ventilation also supports pollination. Tomatoes are self-fertile, which means each flower contains both male and female parts, but they rely on vibration, usually from wind or insects, to release pollen effectively. In a closed, still greenhouse, that vibration does not happen reliably. Gently shaking the flowering trusses by hand, or running a fan during flowering hours, encourages pollen release and improves fruit set.

Removing the lower leaves from the plant, up to the first or second truss, helps enormously with airflow around the base. It also removes older foliage that is more likely to harbour fungal problems and does nothing productive for the plant anyway.

Temperature Fluctuations Are Disrupting Pollination

British summers are not, as a rule, consistent. A week of proper July warmth can be followed abruptly by a cool, grey period that feels more like late April, and tomato flowers are sensitive to exactly this kind of change. If the temperature during flowering drops below around ten degrees Celsius at night, or climbs above around thirty degrees during the day, pollen becomes non-viable and fertilisation fails. The flower opens, nothing happens, and it drops without setting fruit.

Outdoor tomato growers in the UK deal with this more than greenhouse growers do, and it is one of the genuine arguments for choosing the right variety. Varieties like Ferline, Crimson Crush, or the old reliable Alicante have been selected partly for their ability to set fruit in cooler, less predictable conditions. If you are growing a variety bred for a warmer climate and finding fruit set disappointing in a cool British summer, the variety may simply be the wrong choice for the situation.

If you are growing under glass and experiencing temperature spikes, consider shading the structure during the hottest part of the day. Shade paint, applied to the outside of the glass, reduces the temperature inside significantly and can make a real difference to fruit set during a hot spell. For outdoor plants, there is less you can do about a cold night, but ensuring the plants are in the most sheltered spot available, and not in a frost pocket or exposed to a north wind, gives them the best chance of riding out a difficult period.

You Have Let the Plant Get Too Tall Without Pinching Out

This is perhaps the most common mistake among gardeners who are newer to tomatoes, though experienced growers are not immune to the self-inflicted optimism of deciding to leave one more sideshoot, just to see. Cordon tomatoes, which are the upright, single-stemmed varieties most commonly grown in UK gardens and greenhouses, need their sideshoots removed regularly. This is not optional if you want a productive plant rather than a spectacular but fruit-light bush.

Sideshoots grow in the angle between the main stem and a leaf branch, and left to their own devices they become new stems in their own right, each one drawing energy away from the fruit that has already set. A plant that has been allowed to develop multiple strong sideshoots is not a plant producing more tomatoes; it is a plant producing less.

Pinching out sideshoots when they are small, no more than a centimetre or two, is far easier and less damaging to the plant than removing thick, woody sideshoots that have been growing unchecked for several weeks. Get into the habit of checking every week, running your eye along every leaf joint before you do anything else. A sharp pair of bypass secateurs makes cleaner cuts on larger sideshoots and heals faster than a torn or crushed stem.

For plants that have grown beyond the height of their support, it is also worth stopping the growing tip once four or five trusses have set. In the UK, with the season shortening toward August, a sixth or seventh truss is unlikely to ripen before the cold arrives. Stopping the plant directs its remaining energy toward the fruit already on it, which is rather more useful than producing flowers that will never reach maturity.

If you are looking at your plants now and seeing a combination of several of these problems at once, which is common enough, resist the urge to do everything in a single afternoon. Water consistently, begin feeding this week if you have stopped, remove the worst of the sideshoots and any congested lower foliage, and give the plants a few days to respond before you assess what else needs attention. Tomatoes are more responsive than their temperamental reputation suggests. They need consistency, not heroics.