A lemon tree is a subtropical plant, and in its natural habitat, its fruit is green and only slightly acidic. For lemons to develop their tart flavor and yellow color, the temperature must dip below 50 degrees F/10 degrees C but remain above freezing. (If temperatures drop below freezing mature lemons on a tree will die.) Thus, maintaining the window of acid-development opportunity is a top priority to lemon growers.
Modern growers generally use wind machines and oil-burning lamps to steady the environment around their trees. At Lake Garda in northern Italy, the northern most citrus growing spot in world, lemon growers have used the same method of temperature maintenance for centuries.
Lemon growing around Lake Garda dates back to at least 1500. Because of the warming influence of the lake itself and the protection of the surrounding mountains, the region is mild, and lemon cultivation is viable. Still, for about three weeks a year, the temperature dips below freezing, too cold for the persnickety lemon tree.
So, high walls were built around the lemon gardens of Lake Garda. Then, stone and concrete pillars were put up at regular intervals (thirteen feet/four meters) between the trees. When November turned cold, gardeners brought out matting and wooden screens to put on top of the pillars so protect the trees. Glass panels built into the wooden screens allowed enough light to seep in.
When it got very cold, fires and stoves were lit inside. In April, the structure was dismantled. In this fashion, each tree would produced about 600 lovely, tart lemons every year for about a century.
For more on lemons, see Much Depends on Dinner: The Extraordinary History and Mythology, Allure and Obsessions, Perils and Taboos of an Ordinary Meal, by Margaret Visser (pages 259 to 284). New York: Collier Books, 1986.