Even during the winter snows, dining tables in northern China can be seen laden with tomatoes, cucumbers and watermelons, fruits and vegetables that were once only seen in summer. You can buy all types of vegetables in the supermarkets at any time of year, and this means vegetables that are not in season. This has been made possible by the widespread use of polythene tunnels, which have become common in Chinese farming over the last 20 years or so. Out-of-season vegetables have given urban residents more choice and farmers greater income; some say it benefits both. The industry that has grown up around growing, processing and selling unseasonal vegetables has helped one county in east China’s Shandong province rank among the 100-richest in the country.

But while enriching our lives, out-of-season growing can cause serious environmental and food-safety issues. And that means we need to take a closer look.

Farmers know that to cultivate vegetables out of season, they must alter the micro-environment in which crops grow. This is normally done by covering the ground with a plastic membrane, which increases temperature and humidity. This method, however, results in a lack of ventilation and creates ideal conditions for pests and harmful micro-organisms to breed, which then need to be tackled with pesticides.

A polytunnel covering a seven to eight fen (around 500 square metres) typically needs 300 to 800 yuan (US$43 to $115) worth of pesticide every growing season. And to keep costs down, farmers will use the most powerful pesticides, such as the fast-acting insecticide methamidophos, which is 20% cheaper than the alternatives. One type of pest, the root maggot, lives inside the root of Chinese chives, meaning external spraying is ineffective. Farmers therefore cover the roots of the plant with methamidophos. The phosphorous in the pesticide acts as a fertiliser, helping the plants to grow and giving an attractive colour and large leaves. Methamidophos is now commonly used to grow out-of-season vegetables. As a result, Chinese chives are known to be toxic when not in season, and better-informed consumers tend to stay away.

The repeated planting of a single crop under a greenhouse tunnel will also result in poorer harvests. To reduce this loss, farmers use greater quantities of fertiliser. Believing that more fertiliser means bigger harvests, they often use two to five times the recommended amount. Besides presenting food safety risks, this also causes pollution of the soil and groundwater. Using fertilisers usually means 60% remains in the fields; even more remains under polytunnels, which are untouched by the wind and rain. In one area of north China, the nitrate levels in groundwater 80 metres deep are 100 times the safe level as recognised in the United States. In recent years, cancer rates among farmers in areas growing out-of-season vegetables have risen significantly. This is not unrelated to the pollution caused by fertiliser use.

In order to get to market first, farmers often pick their crops before they are ripe, and ripen the vegetables with growth hormones. Some use additives to ensure their crops look colourful and attractive to consumers. The heat and humidity in polytunnels mean cucumbers do not flower, so some enterprising farmers even use drugs to encourage flowering – contraceptive drugs. The authorities urgently need to take action against this, since long-term consumption of cucumbers containing female hormones can cause early puberty in children and infertility in adults.

Out-of-season growing also requires large quantities of agricultural membrane to build the polytunnels that are springing up across the countryside. This causes major pollution issues. Half a million tonnes of plastic membrane are abandoned in the fields every year. In north China’s Hebei province, membrane used in tunnels, vegetable fields and grain fields accounts for 5.63 kilograms, 7.37 kilograms and 2.82 kilograms of waste per hectare respectively. This will inevitably affect crop yields. More conscientious farmers remove it from the fields, but even then there is nothing to do but burn it, releasing persistent organic pollutants (POPs) into the atmosphere. 

Growing vegetables in China can generate four to 10 times the income of grain crops, and land is increasingly used for this purpose. Currently, 270 million mu (180,000 square kilometres) – or 15% of China’s arable land – is used to grow vegetables. Thirty percent of this growing uses facilities like polytunnels. High-return vegetable crops have encouraged both farmers and the government, with villages and towns making claims to be the nation’s leading producer of garlic, watermelons, celery, pumpkins and so on. In Shandong province, the average farmer plants 0.63 mu (420 square metres) of vegetables and gourds – 20.7% of the land used for grain – making Shandong one of the most intensive growers of vegetables in China (2.3 times the national average). The quantities of vegetables produced cannot be consumed locally; 70% of the harvest needs to be exported to other provinces or overseas. As grain planting becomes less profitable, more farmers are planning the switch to vegetables. This expansion will inevitably threaten grain production and worsen the food-security crisis.

The move away from seasonal vegetables is no good thing. The risks of contamination, environmental pollution, plastic pollution and food security are too great. The authorities should consider this matter; it is an issue that affects both the national economy and the people’s well-being. The scale of out-of-season growing and the use of polytunnels should be controlled; organic and environmentally friendly farming should be encouraged; and crops should be grown as naturally as possible.