For centuries before modern greenhouse glass, urban farmers harvested solar energy using walls with thermal mass.
These pear trees benefit from the warm microclimate created by the brick fruit wall.
From the 16th to the 20th century, urban farmers in Europe grew Mediterranean fruits and vegetables as far north as England and the Netherlands. They grew these crops by planting them next to massive “fruit walls,” which stored the heat from the sun and released it at night, creating a microclimate that could increase the temperature near the walls by up to 22 degrees Fahrenheit as compared to the surrounding area. Later, greenhouses built against the fruit walls further improved yields by capturing and storing energy from the sun even more effectively.
It was only at the very end of the 19th century that the greenhouse turned into a fully glazed and artificially heated building. Modern glass greenhouses require massive inputs of energy for heating, especially because they’re often located in temperate climates with cold winters, plus many require artificial lighting and humidity control. Heating a building that’s entirely made of glass is very energy-intensive because glass has a limited insulation value. Even if it’s triple-glazed, glass loses much more heat than an insulated wall.
The design of the modern greenhouse is strikingly different than the technology it evolved from. Initially, the quest to produce warm-weather crops in temperate regions (and to extend the growing season of local crops) didn’t involve any glass at all. In 1561 (during the apex of the so-called Little Ice Age, a period of exceptional cold in Europe that lasted from about 1300 to 1850), Swiss botanist Conrad Gesner described the effect of sun-heated walls on the ripening of figs and currants, which matured faster than when they were planted farther from the wall.
Gesner’s observation led to the emergence of fruit walls in Northwestern Europe. By planting fruit trees close to these specially built walls with high thermal mass and southern exposure, a microclimate was created that allowed the cultivation of Mediterranean fruits in temperate climates. The French quickly started to refine the technology by pruning the branches of fruit trees so they could be attached to a wooden frame on the wall. This practice, known as “espalier,” allowed them to optimize the use of available space and to further improve upon the growing conditions.
These fruit walls reflected sunlight during the day. They also absorbed solar energy, which was then slowly released as heat during the night, preventing frost damage. Consequently, a warmer microclimate was created on the southern side of the wall. Fruit walls also protected crops from cold northern winds, and protruding roof tiles or wooden canopies were sometimes used to shield the fruit trees from heavy rain, hail, and bird droppings.
Initially, fruit walls appeared in the gardens of the wealthy, such as at the Palace of Versailles in France. However, some French regions later developed an urban farming industry based on fruit walls. The most spectacular example was Montreuil, a suburb of Paris, where peaches were grown on a massive scale. Montreuil had more than 370 miles of fruit walls by the 1870s when the industry reached its peak. The 740-acre maze of jumbled-up walls was so confusing for outsiders that the Prussian army went around Montreuil during the Siege of Paris in 1870.
Montreuil produced up to 17 million fruits per year. Building many fruit walls close to each other further boosted the effectiveness of the technology because more heat was trapped, and wind was kept out almost completely. Within the walled orchards, temperatures were typically 14 to 22 degrees higher than they were outside.
The 8- to 10-foot-high walls (some of which can still be found in Montreuil) were approximately 15 inches thick and coated in limestone plaster. Insulated mats suspended from the tops of the walls could be pulled down over the plants to protect the fruits on very cold nights. Further from the walls, crops were grown that tolerated lower temperatures, such as apples, pears, raspberries, vegetables, and flowers.
In 1730, a similar industry formed for grape cultivation in Thomery, which is about 37 miles southeast of Paris — an area much farther north than where grapes were typically grown at the time. At the peak of production in the early 20th century, about 900 tons of grapes were produced along approximately 200 miles of fruit walls packed together on only 370 acres of land. The walls, built of clay with a cap of thatch, were 10 feet high, up to 300 feet long, and spaced about 30 feet apart. They were all topped with tile copings, and some even had small glass canopies.
Because grapevines prefer a warm, dry climate, most of these fruit walls had a southeastern exposure. A southern exposure would have been the warmest, but then the vines would have been more exposed to the damp winds and rains coming from the southwest. The western and southwestern walls were used to produce grapes of lower qualities.
The Dutch also contributed to the development of the fruit wall. They started building fruit walls during the first half of the 18th century, initially only in gardens of castles and country houses. Many of these walls had unique forms. Most remarkable was the serpentine or “crinkle crankle” wall. The alternating convex and concave curves in the wall provided stability and helped to resist lateral forces. Furthermore, the slopes gave a warmer microclimate than a flat wall. This was obviously important for the Dutch, who were (and still are) located nearly 200 miles north of Paris. Although it was actually longer than a linear wall, a serpentine wall saved on materials because the wall could be constructed only one brick thick while still remaining strong and stable.
In Britain, though no large-scale urban farming industries appeared, the fruit wall became a standard feature of country house gardens from the 1600s onward. The English developed heated fruit walls in the 18th and 19th centuries to ensure that fruits weren’t killed by frost and to help ripen fruit and mature wood.
In these “hot walls,” horizontal flues ran back and forth, opening into chimneys on top of the wall. Initially, the hollow walls were heated by fires lit inside of the wall, or by small furnaces located at the back of the wall. But during the second half of the 19th century, heated fruit walls were increasingly being warmed using hot water circulated through pipes within the walls.
Large transparent glass plates were hard to come by during the Middle Ages and early modern period, which limited the use of greenhouses for growing crops. Window panes were usually made of hand-blown plate glass, which could only be produced in small dimensions. To make a large glass plate, the small pieces were combined by connecting them with rods or glazing bars.
Nevertheless, European growers made use of small-scale greenhouse methods starting in the early 1600s. The simplest forms of greenhouses were the cloche (a bell-shaped jar or bottomless glass jug that was placed on top of the plants) and the cold frame (a small seedbed enclosed in a glass-topped box). Modified cold frames, called “hotbeds,” were also used, which took advantage of the heat produced by decomposing horse manure buried beneath the seedbed.
In the 1800s, some Belgian and Dutch cultivators started experimenting with the placement of glass plates against fruit walls and discovered that this practice could encourage crop growth. This method gradually developed into the building of greenhouses against the fruit walls. In the Dutch Westland region, the first of these greenhouses were built about 1850. By 1881, some 14 of the 111 miles of fruit walls in this region were under glass.
These greenhouse structures became larger and more sophisticated over time, but they all fundamentally benefitted from the thermal mass of the fruit wall, which stored heat from the sun for use at night. In addition, many of these structures were provided with insulating covers that could be rolled out over the glass at night or during cold, cloudy weather. In short, the early greenhouse was a passive solar building.
The first all-glass greenhouses were built in the 1890s, first in Belgium, and shortly afterward in the Netherlands. Two trends played into the hands of the fully glazed greenhouse. The first was the invention of the plate glass production method, which made larger window panes much more affordable. The second was the advance of fossil fuels, which made it possible to keep a glass building warm despite the large heat losses. Consequently, at the start of the 20th century, the greenhouse became a structure without thermal mass, and the fruit wall that had started it all was largely forgotten.
Kris De Decker is the founder of, and main writer for, Low Tech Magazine, a blog that promotes traditional knowledge and low-tech solutions to modern problems.
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