Winter gardening, and the practice of utilizing solar heat, was a common practice in botanical gardens hundreds of years ago.
As climate change and energy scarcity bring solar energy back to the forefront of public discussion, Let It Shine (New World Library, 2013) shows that this is not just a present-day concern. John Perlin offers the world’s only comprehensive history of humanity’s use of solar heat. From using the sun’s energy to facilitate winter gardening to stone-age Chinese building techniques, this work profiles fascinating characters and devices that helped make the solar revolution possible today. This selection, excerpted from “Heat for Horticulture,” details a fascinating history of winter gardening, from innovative greenhouse designs to a burgeoning understanding of solar heat.
Collecting solar heat for indoor horticulture enjoyed a revival during the sixteenth century. The tide of empirical science had begun to break the bonds of church dogma, and the growing wealth and stability of Europe contributed to an atmosphere that favored scientific exploration. With discovery and trade came an increased flow of money and an appetite for more comfortable living. Ships were now returning from Asia, Africa, and the New World with beautiful flowers, such as African violets, and delicious fruits, such as bananas, pineapples, and coffee. People wanted to grow these exotics at home and enjoy native fruits and vegetables in all seasons — just as the Roman emperor Tiberius had satisfied his year-round craving for cucumbers by raising them in glazed cold frames more than a thousand years before.
The Dutch and Flemish were the first modern northern Europeans to develop horticulture to a level equaling or surpassing that of the Romans. Perhaps their early independence from the authority of the church encouraged their pioneering efforts in the field of scientific gardening. Certainly their great success in world trade helped to provide the new bourgeois merchant class with the means to take up such gentlemanly pursuits as raising exotic plants. J. C. Loudon, a late-eighteenth-century horticulturist, observed that “horticulture . . . was in great repute in all the low countries during the seventeenth century.”
The French and English followed suit, trying to grow plants from the warmer regions of the world in the inhospitable climate of northern Europe. They also sought to improve the yield of native crops and to grow them out of season. Experts wrote books for the amateur gardener on how to accomplish this.
The unusually short growing seasons during this era were a major problem for gardeners. The period from the year 1550 to about 1850 has been called the “Little Ice Age” because Europe suffered extremely short summers and severe winters. In England, for example, the average temperature from 1680 to 1719 ranged from 58.6°F in summer to 37.9°F in winter. The cold was so extreme that on thirteen occasions during the seventeenth and eighteenth centuries the ice on the Thames could support the weight of people — a rare occurrence during the previous five centuries. Only if “nature was assisted by art,” as one gardening book put it, could a lover of plants “cure this great evil and dangerous enemy,” the cold. Solar energy became a favorite tool in this battle. Its power was extolled by Joseph Carpenter in 1717: The sun by its heat dissipates the cold and gross humours of the earth; it renders it more refin’d and easier for the vegetation of seed and fruit trees. Tis by the influence of this noble planet that the sap rises up between the wood and bark, producing the first buds, then the leaves and fruits; its beams serve not only to ripen the fruit, but it makes it large, beautiful to the eye, and pleasant to the taste.
The art of harnessing solar heat, of learning how to enhance its beneficent effects, was evolving once again.
In 1714 the Duke of Rutland tried to use sloping walls in his English garden during the winter, but he found that they did not provide enough solar heat to keep his plants alive. So he placed glass casements over the walls to keep the collected solar heat from dissipating quickly. The use of glass as a solar heat trap was not entirely new to England. Some forty years earlier, Sir Hugh Plat had suggested that his patrons use glass to protect new seedlings because it would “defend off the cold air and increase the heat of the sun.”10 Cold frames and glass greenhouses soon became immensely popular in England, as well as in Holland — where Europe’s first modern greenhouses had been built in the 1500s — and across the Continent. In fact, many have called the eighteenth century the “age of the greenhouse” because having a greenhouse became fashionable for nearly every person of means.
New glass-manufacturing methods allowed the production of large windows for greenhouses and homes. Previously, from the eleventh century to the end of the seventeenth century, windowpanes had usually been made by the crown-glass method. A craftsman blew hot glass into a bubble and then thrust a rod into the top of the bubble directly opposite the blowpipe. He detached the blowpipe, leaving an air hole where the tool had been, and as the glass began to cool he reheated the bubble and twirled the rod until centrifugal force caused the bubble to flatten into a disc. When the disc had cooled and hardened, the glass was cut into small, thin panes.
Locating crown-glass factories near fuel-rich areas helped to lower the price of glass and made windows more readily available to members of the ascendant middle class. Their growing demand for larger and thicker panes was finally met when the French developed the plate-glass process at the end of the seventeenth century. This process was strikingly similar to the Roman method. Glass was melted in a large cauldron, and several workmen carried the molten liquid to a casting table, where they poured it into a rectangular frame. With an iron roller, they flattened the glass to a standard thickness. After the plate had cooled and hardened, it was ground and polished. This plate-glass method produced windowpanes measuring up to 6 feet on an edge, and it quickly eclipsed the old crown-glass process — although crown glass remained common in England until the end of the eighteenth century.
Scientists sought novel greenhouse designs to enhance their solar heat collection and storage abilities, because they wanted to reduce the amount of fuel needed to keep plants warm at night, on cloudy days, and in the winter. They hoped to save fuel and believed that plants in a solar-heated greenhouse grew better than plants raised in artificial heat.
Like fruit walls, early greenhouses were built with a southern exposure. Scientists soon realized that greenhouse walls should also be sloped to capture more sunlight. Hermann Boerhaave, a seventeenth-century Dutch professor of botany, demonstrated that in Holland’s northern latitude the rays of the low lying winter sun would enter a greenhouse more directly if the glass walls were steeply inclined. He determined that an angle of 75.5 degrees from the northern horizon would be best for a latitude of 52.5 degrees. Most directors of botanic gardens in Europe who were interested in winter gardening took Boerhaave’s advice and built their greenhouses accordingly. Greenhouses intended primarily to encourage the growth of larger and more flavorsome fruit during the summer required a less acute angle of incline, because the sun passed more directly overhead. Using this strategy, one English gardener reported “the most abundant crops of grapes perfectly ripened with less time and effort and less expenditure on fuel than I have witnessed in any other instance.”
Michel Adanson, an eighteenth-century French scientist, recommended that the floor of the greenhouse, rather than its walls, be sloped. His approach was akin to placing a glass cold frame over a sloping fruit wall. Adanson wrote the first systematic treatise on the theory and construction of greenhouses. He presented rules, tables, and diagrams to be followed for building the most functional greenhouses in every possible location, from the poles to the equator.
Gardeners not only sloped the walls or floors of their greenhouses but also invented insulating techniques to increase heat retention. When the sun was not shining, they placed mats or canvas coverings over the greenhouse to conserve the solar heat that had been collected, as well as the heat generated by the fires that were often lit inside. The Dutch constructed greenhouses with two layers of glass — the dead air space between the layers acted as insulation.
Late in the eighteenth century, Dr. James Anderson took the idea of solar heat storage a step further. Normally when the sun was shining and the greenhouse became too hot, some of the solar-heated air was released by opening windows in the structure. In Anderson’s novel design, this hot air was captured and stored for later use. He divided his greenhouse into an upper and lower chamber. During the day, hot air collecting in the lower chamber rose through a pipe to the upper. At night, cold outside air was admitted to the upper chamber, forcing the stored hot air through a duct back into the lower chamber — to heat the plants. How well this storage system worked cannot be ascertained from the records, but it was an early attempt to store solar heat long enough that it could be used when the sun was not shining.
As personal wealth accumulated during the nineteenth century in England and other European countries, the greenhouse began to assume a more lavish form — the conservatory. In this glassed-in garden, the well-to-do could leisurely amble with their guests through lush, jungle-like foliage. The Gentleman’s House, an architectural guide for the wealthy landowner, pointed out the difference between the greenhouse and the conservatory: “The greenhouse is a structure in which plants are cultivated as distinguished from the conservatory in which they are placed for display.” The greenhouse was primarily functional, whereas the conservatory was a place where exotic plants were displayed for atmosphere. Gardening manuals described the fuel-saving features of a southern orientation for the conservatory.
Architects such as the British designer Humphry Repton brought the sunlit ambience of the conservatory right into the home by attaching this glass garden to the south side of a living room or library. On sunny winter days the doors separating the conservatory and the house were opened to allow moist, sun-warmed air to circulate freely into the otherwise gloomy, chilly rooms. Some contended that the conservatory also gave families a healthier way of spending their free time. As the nineteenth-century book The English Gardener exclaimed, “How much better during the long and dreary winter for daughters and even sons to assist their mother in a greenhouse than to be seated at cards or in the blubberings over a stupid novel!”
By the late 1800s the country gentry had become so enamored of attached conservatories that these became an important architectural feature of rural estates. According to John Hix, author of The Glass House, the conservatory was “no longer. . . seen as a simple extension to the dwelling, but an integral way of life.”
This fashion filtered down to members of the middle class, but on a much smaller scale. Modest conservatories, which took the form of a glass-covered room attached to the south side of the house, were common in urban areas. Rooftops of multistoried buildings also served as sites for greenhouses. As one writer remarked, “A warm greenhouse on the roof [is] a more pleasant thing than a dark parlor.” For the crowded city flat, a large window garden on the south wall had to suffice. Jacob Forst, a leading British horticulturist, envisioned that the south side of every urban building could be glassed over for growing grapes, figs, and cherries. “Such walls would never need paint,” Forst argued, and would offer an “admirable arrangement for house ventilation” by trapping sun-heated air and circulating it to the interior of the building.
As conservatories became popular, people grew indifferent to the direction in which they faced. Instead of the sun’s rays streaming in from the south, artificial heating systems now provided warmth for the garden houses, and conservatories became fuel consumers rather than fuel savers. One of the chief reasons for the demise of the conservatory in England was the institution of fuel rationing during World War I. The lesson of solar heating had been discovered and then lost.
Reprinted with permission from Let It Shine: The 6,000-Year Story of Solar Energy by John Perlin and published by New World Library, 2013.
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