Citrus fruits (oranges, lemons, mandarins, tangerines, grapefruits, limes, pomeloes) are the highest-value fruit crop in terms of international trade. Citrus plants are not frost-hardy and can only be grown in tropical and subtropical climates – unless they are cultivated in fossil fuel heated glasshouses.
However, during the first half of the twentieth century, citrus fruits came to be grown a good distance from the (sub)tropical regions they usually thrive in. The Russians managed to grow citrus outdoors, where temperatures drop as low as minus 30 degrees Celsius, and without the use of glass or fossil fuels.
By 1950, the Soviet Union boasted 30,000 hectares of citrus plantations, producing 200,000 tonnes of fruits per year.