British supermarkets have trained us to expect tomatoes, cucumbers and asparagus throughout the year. Their year-round presence can make seasonality seem obsolete. But the seasons have not disappeared. When British weather cannot provide the light and warmth a crop needs, growers must recreate those conditions or retailers must bring the crop from somewhere that has them.Both options carry costs, and this winter several of those costs are converging. That does not guarantee a shortage or a particular supermarket price. It does mean that some familiar out-of-season vegetables are unusually exposed to rising production and supply costs.Tomatoes show the problem particularly clearly. A tomato plant needs sufficient light and warmth to maintain photosynthesis, flowering, fruit development and ripening. In a British winter, a commercial glasshouse may therefore require heating, supplementary lighting, ventilation, humidity control. They also pump in extra carbon dioxide, which can boost photosynthesis and yields. Pumps, fans and irrigation systems also consume energy.These technologies make winter production possible, but not automatically cheap. The National Farmers’ Union warned in March 2026 that higher electricity network charges could add more than £1 million to the annual bills of some large glasshouse businesses. Growers cannot simply switch everything off. If temperature or light falls too far, growth slows, harvests are delayed and yield or fruit quality may suffer.British tomato production is not in uninterrupted decline. Around 79,000 tonnes of tomatoes were grown in the UK in 2025, up 4% on the previous year, helped by bright weather and energy prices that had softened from earlier peaks. Most British production, however, remains concentrated in the brighter part of the year. Maintaining comparable output through winter is a different technical and economic proposition.Cheap winter tomatoes depend on favourable conditionsDuring winter, more of the UK’s tomato supply comes from further south. The government’s Food Security Report shows Spain and Morocco becoming more important suppliers as British and Dutch production falls.Southern Spain in particular has vast areas of plastic-covered greenhouses, that are sometimes described as factories. Much of their economic advantage is simple: abundant sunlight and mild winter temperatures mean they need far less heating and artificial lighting than a British glasshouse. Almería’s 20-mile-wide mar de plástico (sea of plastic) produces millions of tonnes of fruit and vegetables each year, much of it for European markets. Google Maps, CC BY-SA That advantage brings other risks. Spain and Morocco are both water-stressed, while protected crops still need reliable irrigation. Excessive heat can impair tomato pollination and fruit quality, and storms, pests and plant diseases can mean less food reaches the market.In Almeria, Spain’s greenhouse capital, tomato output fell last winter while production costs rose. If several northern European countries then seek replacement supplies from the same regions, reduced supply can quickly push prices higher.The hot, dry British summer of 2026 adds another complication. Some UK vegetable growers needed more irrigation or harvested smaller crops, while some vegetables required imports unusually early. This does not determine the price of every winter tomato, but it leaves less room for domestic production to absorb disruption elsewhere.Asparagus depends on distant suppliesAsparagus reaches British shops by a different route. It is a perennial field crop with a naturally short spring harvesting season in Britain. Producing it cheaply in midwinter is not a realistic extension of Britain’s normal crop calendar. Winter availability depends instead on moving fresh spears quickly from regions whose production seasons occur at different times.In 2024, the UK imported approximately 10.5 million kilograms of fresh asparagus, roughly 95% of which came from Peru or Mexico. This concentration matters because a disruption to supplies from either country cannot necessarily be replaced immediately by British or European production.Peruvian figures for the first half of 2026 show the volume of fresh asparagus exports falling by nearly 8%, while the total value increased. This does not prove that British retail prices must rise – exchange rates, supermarket contracts and Mexican supplies also matter. It does show why consistently cheap winter asparagus should not be taken for granted. The product depends on distant production, refrigerated handling and fast, reliable transport before a highly perishable spear loses quality.Cheap winter vegetables will still be available at times. A mild winter, good harvests, lower energy prices and strong supplies from several countries can still produce supermarket promotions. Growers are also improving thermal screens, LED lighting, irrigation, climate control and the use of renewable energy.None of this removes the crop’s biological requirements. Nor can international trade guarantee that light, water, labour, energy and transport will all remain inexpensive at once. Consumers will continue to find tomatoes and asparagus outside the British season, but expecting them to be cheap every winter is becoming harder to justify.Kambiz Baghalian does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.