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Palm House, Royal Botanic Gardens, Kew
Kew Gardens, Richmond, Greater London, UK
Palm House, Royal Botanic Gardens, Kew
associated engineer
Richard Turner
date  1844 - October 1848
UK era  Victorian  |  category  Building  |  reference  TQ185770
ICE reference number  HEW 366
photo  Jane Joyce
Structurally the most important building in the Royal Botanic Gardens at Kew. The wrought iron rolling techniques used to construct its frame were borrowed from ship building and employed architecturally for the first time here. The curved profile of the greenhouse maximises light for the plants, and the climate control system employed was state-of-the-art technology for the period. The Palm House is Grade I listed and has been refurbished.
The Royal Botanic Gardens were created in 1759 for Augusta, Princess of Wales, who lived at Kew Palace (TQ184774). In 1841, botanist Sir William Jackson Hooker (1785-1865) was appointed Director of the gardens and he opened them to the public. The grounds have been extended and now cover more than 49 hectares.
Hooker recognised the need for more greenshouses, to display tropical plants and palms from around the world. In 1843, Richard Turner (1798-1881) of the Hammersmith Ironworks in Dublin visited London in connection with a winter garden at Regent’s Park, going on to meet Hooker in January 1844 and impressing him with plans for a palm house at Kew.
Architect Decimus Burton (1800-81) also had ideas for a palm house here and proposed an elegant arched cross-shaped building framed in cast iron. His proposed two-tier curvilinear building profile was similar to that of the Chatsworth Conservatory (completed 1840, dem. 1920) designed by Joseph Paxton (1803-65), and required a large number of internal columns.
By contrast, Turner suggested wrought iron for the arch ribs — its lightness and tensile strength enabling the roof to span 15.2m between columns. And so the Palm House at Kew became the first architectural building to use the newly patented Kennedy & Vernon rolled wrought iron I-beams, designed originally for ship decks.
The building's design was finalised in March and April 1844. Its appearance was refined by reducing the length of the middle section and adding flanking wings. Burton moderated the exuberance of Turner’s ornamental detailing and Hooker selected thes location, west of The Pond, in an area that previously held a lake.
The Palm House is 110.5m long, with 5,500 components and 16,000 panes of glass. Its central upper vault is rectangular in plan, 41.9m long, 30.5m wide and 19.2m high. A galleried walkway between the two curved tiers is accessed from spiral stairs at either end of the upper vault. Each of the two wings is a single barrel vault 10.1m high ending in a hemispherical apse. Like the upper vault, the wings include clerestory windows, around 1m in height.
The structural wrought iron ribs were rolled at Malins & Rawlinson's works at Millwall under licence to Kennedy & Vernon of Liverpool. They left Millwall as straight I-beams 229mm deep, were shipped to Turner's works in Dublin for joining and rolling to the correct curve and then shipped back to Kew. The first one was erected on 15th October 1845.
The ribs are spaced at 3.8m centres, with diagonal arch ribs defining the corners of the upper vault. The ribs are braced by horizontal purlins consisting of cast iron tubes containing post-tensioned wrought iron ties 38mm in diameter, patented by Turner in 1846. The upper tier is supported on 20 tubular cast iron columns, 10 each side of the central aisle, which take rainwater to cisterns in the building.
Turner also designed the 50mm deep T-section sash bars that run from the base to apex of the vaults, with swivelling sashes at clerestory and gallery levels and sliding sashes on the roof. The curved and lapped glass panes between the sashes are 914mm long and 241mm wide, held in place with putty.
The curvilinear shape of the Palm House maximises daylight — the sun’s rays are perpendicular to some part of the glazing whatever the season. Though useful for hothouses designed for fruit growing, large areas of clear glass are not ideal for light-sensitive rainforest plants. Shielding them from leaf burn with roof blinds was not considered an option. In 1845-7, experiments by Robert Hunt (1807-87) showed that manganese oxide-free glass, tinted green with copper oxide, diffuses solar gain and approximates the light conditions in a tropical forest.
To provide the warm humid atmosphere the plants require, a heating plant, powered by 12 coal-fired boilers patented by Burbidge, Healy & Co, was installed in the basement cellars. The hot water produced fed steam radiators and flowed through some 8.5km of underfloor pipes laid under 1.2m square cast iron gratings.
The fuel was delivered through a 170m tramway tunnel from a coal bunker near a 32.6m Italianate brick chimney (TQ187768) used to expel flue gases. The chimney also contained a water tank that fed perforated pipes in the glasshouse for the watering of tall plants. In the early days, all the plants were presented in large timber containers or clay pots.
A contemporary commentator described the building as "one of the very finest plant-houses in the world. Its graceful lines and admirable proportions make it as pleasing to the eye as it is possible for a structure of glass and iron to be". Burton later designed the Botanic Garden's Temperate House (TQ184764) to the south west, completed in 1862 and extended by 1899. It was the world’s largest glasshouse at the time.
The boiler room in the Palm House flooded in November 1848, just a month after completion. Pumping failed to prevent water reaching the supply tunnel by May 1849, so the boiler room floor was raised in 1853, reducing the flue draught. In any event, the heating system proved ineffective in winter and the palms had to be moved to other glasshouses at Kew each season.
In 1860, two large beds were dug in the centre of the floor to accommodate the tallest palms, with more beds added later. In 1865, Sir Joseph Dalton Hooker (1817-1911) succeeded his father as Director of the Royal Botanic Gardens, holding the post for 20 years.
In 1867, to raise the indoor temperature, flue stacks were constructed in the flanking wings, bypassing the chimney and saving a quarter of the fuel. Ten years later, in 1877, the boilers were replaced by six new tubular boilers, and a circuit of heating pipework installed at gallery level to mitigate draughts. In 1894-6, the original pipework in the wings was renewed and new pipes added at clerestory level.
From the 1890s onwards, the tinted glazing of Kew’s various greenhouses started to be gradually replaced by clear glass, as London's smog was obscuring the sun and soot was settling on the panes. By 1904, the Palm House's glazing had deteriorated. Some panes lost their green colour altogether or turned pink, largely resulting from their manufacture. The specification had not been followed and they were tinted using iron protoxide rather than copper oxide. In the 1930s, the building was re-glazed with clear glass.
The boilers were renewed in 1934-5 and extra pipework added. In 1950, the tunnel railway supplying the boilers was electrified, a new chimney flue installed and the interior ironwork repainted. The glasshouse structure was Grade I listed in January 1950, restored in 1955-7, and re-opened in 1959.
In 1961, the boilers were moved from the basement to a yard at the end of the supply tunnel, which was now only used for the heating pipelines. The boilers were converted to burn oil, re-using the original chimney. The two chimney stacks in the wings were removed later in the 1960s.
Surveys of the Palm House in 1980-3 revealed extensive ironwork corrosion exacerbated by the humidity. The building's heritage listing was amended in October 1983, and it closed to the public in September 1984. Full restoration commenced in autumn 1985. It was dismantled almost completely and rebuilt, restoring damaged components with like materials.
Ductile iron was used for castings and mild steel for strengthening the arch ribs. All 16km of glazing bar sashes were replaced in stainless steel, the clear toughened glass panes set in silicon rubber mastic and specialist paint applied to the metalwork. A new gas-fired heating system, including atomised humidification, was installed. The original floor grilles were retained. The building was officially re-opened on 6th November 1990, by Queen Elizabeth the Queen Mother.
In July 2003, the Royal Botanic Gardens at Kew were awarded UNESCO World Heritage status.
In recent times, the Palm House glass has been cleaned around once a decade. The last operation took place in 2014, and required six weeks of rope access work, and elevated mobile platforms and bespoke cleaning equipment.
The building houses one of the oldest potted plants in the world, a huge Jurassic cycad (Encephalartos altensteinii) collected by plant hunter Francis Masson (1741-1805) in the early 1770s during Captain James Cook's second global voyage. Cycads predate flowering plants and can live for more than 500 years.
Architect: Decimus Burton
Frame fabrication and erection: Richard Turner, Hammersmith Ironworks (Dublin)
Ironwork: Malins & Rawlinson of Millwall
Foundations: Thomas Grisell
Boiler supply: Burbridge, Healy & Co
Boiler supply (1877): E.G. Rivers
Research: ECPK
"The use of scientific experimentation in developing the glazing for the Palm House at Kew" by Henrik Schoenefeldt, Construction History, Vol.26, pp.19-39, 2011
"Kew: The History of the Royal Botanic Gardens" by Ray Desmond, Random House, 1998
"Houses of Glass: A Nineteenth-Century Building Type" by Georg Kohlmaier and Barna von Sartory, MIT Press, 1991
"Who Designed the Palm House in Kew Gardens?" by R.G.C. Desmond, Kew Bulletin, Vol.27 (2), pp.295-303, 1972
reference sources   CEH Lond

Palm House, Royal Botanic Gardens, Kew