Hot Gardening!

Hot walls and floors enclosed by a glass house were artificially heated to augment the warmth of the sun in ripening the fruit.  They seem to have become popular in England by the end of the 17th century.  As early as August 7th 1685 John Evelyn wrote of his meeting with Mr Watts, the keeper of the Apothecaries’ garden of Simples at Chelsea [later named the Chelsea Physic Garden]:

"What was very ingenious was the subterraneous heat, conveyed by a stove under the conservatory, all vaulted with brick, so as he has the doores and windowes open in the hardest frosts, secluding only the snow."

Figure 1.  Dutch forcing-frame with back wall flue, Holland, 1737.

Queen Mary (1662-1694), who took an active interest in plant cultivation, had 3 new glass houses erected at Hampton Court soon after her arrival there in 1689. The Dutch carpenter Hendrick Floris designed the south-facing stove-heated buildings in the Pond Garden to house the Queen’s collection of citrus trees and exotic plants and Christopher Hatton described them as ‘much better contrived and built than any other in England’.  However, they fell into disrepair upon her death in 1694 and were replaced in 1701 by the Lower Orangery.

In Essex there is an account of what must have been one of the early examples of this technology by the Reverend William Holman of Halstead, who died in 1730. In a description of the gardens at Blue Bridge House, in his parish of Halstead he noted:

“Next this is an artificial wall built for the ripening of grapes, sooner than ordinary, by a fire kindled in the foundations.”

Before the advent of cast iron piping, which allowed the use of circulating hot water to heat the walls, either straw was burnt in a cavity at the base of the wall or hot smoke from an external furnace was drawn through serpentine flues in the walls.  Philip Miller, who took over the Chelsea Physic Garden in 1722, visited to Holland in 1727 to learn about the techniques involved in cultivating exotic plants and fruits, such as the pineapple. He would have seen systems such as that illustrated in Figure 1.

Figure 2.  The accumulated soot and the wall collapse, Christopher Dingwall, Scotland’s Garden and Landscape Heritage gives scale.

On his return he wrote detailed instructions, quoted here from the Gardener’s Dictionary of 1748, on how to make and manage such hot walls – for example ‘The flues should be one foot wide in the clear, that they may not be too soon stopped with the soot, as, also for the more convenient cleaning of them’ Miller also recognised the dangers of smoke leaking into the hot house and destroying the plants

‘the thickness of the wall in front of these flues need not be more than four inches, but it must be well jointed with mortar, and pargitered within side to prevent the smoke from getting into the house, and the outside should be faced with mortar, and covered with a coarse cloth, to keep the mortar from cracking…..’

The composition of these walls seems to have been inherently weak, possibly because of the intense heat, and many collapsed.  The heated flues in floors probably have a better survival rate since they were protected from the weather and the effects of gravity but they are usually buried or built upon. However, hot walls was still being constructed into the 19th century.  Figure 2 shows an example from c.1801/2 in the walled garden that Repton designed at Valleyfield, near Edinburgh.

Figure 3.  The Abridgement of the Gardener’s Dictionary, London 1771 (not paginated), Philip Miller.

The drawing in Figure 3 shows Miller combining two techniques – the hot wall and the hot bed.  Tanner’s bark was added to the bed in order to raise the temperature of the soil by its slow breakdown, thus allowing the cultivation of exotic plants in cool climates.  The date at which the hot bed technique was introduced to England is unclear.  The famous portrait of the Royal Gardener John Rose (d. 1677) presenting Charles II with a pineapple is dated c.1677 since Charles was known to have shaved off his moustache by that date.  It has been suggested this proves hotbeds were in use by that date - but the fruit may have been imported rather than grown by Rose.  William and Mary certainly popularised this technique some 20 years later.

Figure 4.Site of the 2 sunken boilers at Audley End.

At Audley End there are examples of both smoke and water heating.  Sir John Griffin Griffin c. 1766 enclosed Lady Portsmouth’s kitchen garden with a wall.  On the south-facing aspect he built heated glasshouses, which are recorded as producing exotic flowers as well as melons, pineapples, grapes and figs throughout the year.  In 1802 the southern part of the west wall of the Kitchen Garden was rebuilt as a hot wall with serpentine flues. In common with examples at Belsay, Raby Castle and elsewhere, this wall seems not to have had glass in front.

Sir John’s glasshouses were replaced in 1811 by a vine house which in the later 19th century was heated from the rear by 2 duplex boilers each with a fire burning within a ring of upright water-filled pipes. The boilers were fired on alternate weeks to allow time for them to cool so they could be cleaned. Both boilers were positioned below ground level (Figure 4) to allow the heated water to rise and circulate by gravity.

The management of artificial plant forcing was a skilled job and mastery of it added to the prestige of the Head Gardener as well as to that of his employers by being able to offer out of season produce to their guests. Hot gardening indeed!

I am very grateful to Michael Leach for suggesting this topic and for providing the material from Philip Miller and on Bluebridge House.

Images: Fig 1 Dutch forcing-frame with back wall flue, Holland, 1737, courtesy of Kate Harwood, Herts, Gardens Trust; Fig 2 The accumulated soot and the wall collapse at Valleyfield, author’s photograph; Fig 3 The Abridgement of the Gardener’s Dictionary, London 1771 (not paginated), Philip Miller, courtesy of Michael Leach, Essex Gardens Trust; Fig 4 Site of the 2 sunken boilers at Audley End, author’s photograph.