Both monarda, above, and the Venus flytrap, below left, found their way to British shores from the US

Travel sometimes broadens the mind but it certainly broadens gardens. The idealised “English garden” owes most of its charms to plants imported from faraway sources. We are still in the “Chinese phase” of transformation that began in the 19th century and has far to go at the level of small hardy plants. Earlier there were European phases and the crucial impact of western Asia, source of tulips and shrubs as lovely as the better sorts of philadelphus. In the US, at least on the east coast, gardening has paid homage traditionally to the example of the “English style”. In fact, English gardening owes an inestimable debt to the US, to a degree historians do not always realise.

Matters have now changed with the vigorous book The Brother Gardeners: Botany, Empire and the Birth of an Obsession by author-broadcaster Andrea Wulf. I have been using my own travels for the time to appreciate it. The theme and the main players are not new but she has brought out details and exploited letters and documents that have not been so powerfully pulled together before. Her thesis is that imports from the US had an immense impact on English gardening from the 1730s onwards and changed the face and range of English planting in the next 50 or so years. Simplified garden histories tend to concentrate on these years as the birth of the English landscape garden through the genius of William Kent and Capability Brown. There was a parallel shift going on at the level of planting and smaller gardens. Hundreds of new varieties were being propagated by a new wave of nurserymen and outside the big landscape parks the “English” garden was being reinvigorated by an American transfusion.

It was above all the work of two men, John Bartram in Philadelphia and Peter Collinson, a respected cloth merchant in London. Both men were Quakers but Collinson was socially more polished than Bartram, the working farmer. Many of their letters to and fro survive, built up from their shared trade in plants, packed and shipped by the tireless Bartram across the Atlantic.

From the 1730s onwards the contact between these two men changed the scope of English gardening. New plants poured in to Collinson and his paying subscribers, bringing Magnolia Grandiflora or the curious Venus flytrap to a new and avid clientele. Wulf is at her best in tracing the impact of the new American flora on English gardeners whose woodlands, shrubberies and greenhouses took on a new aspect. There was no shortage of garden mania. In 1741 the Duchess of Queensberry appeared at court in a white satin ballgown embroidered with American nasturtiums and honeysuckle, “brown hills covered with all sorts of weeds” and even an old tree-stump among climbing plants. She looked like a walking landscape garden, a tribute to high society’s favourite fashion.

The new American imports caught on first among titled society and Wulf gives a fascinating sketch of their impact on the young Lord Petre at his Essex country seat. Virginian junipers, American sycamores and pines and hemlock trees enriched his artistic landscape, which was planned for varied colours of green and above all for autumn effects. It was all more “new wave” than the clumps of beech trees favoured by Capability Brown. It brought a touch of America to Essex until the gifted Lord Petre died at the age of only 30, killed by smallpox. His model landscape had 30 new types of coniferous tree, far beyond the basic holly and box of previous generations.

The fashion of a few nobles soon rippled outwards and encouraged emulation by more modest garden owners. “If Petre had lived,” Collinson remarked sadly, “all around him would have been America in England.” Instead, nurseries multiplied to spread the masses of new possibilities. Red-flowered monarda and spring-flowering Dodecatheon Media, the American Shooting Star, were some of the new beauties that spread from Collinson’s own garden. Meanwhile the brave, methodical Bartram was scouring ever more unsettled parts of the American wilderness to feed the English demand.

Even outside London new nurseries proliferated. In his more general study of Man and the Natural World: Changing Attitudes in England, 1500-1800, the historian Keith Thomas noted this growth and linked it to the social competition of the well-off and a relentless lust for new fashions. Wulf well connects the boom to the much wider range of fine plants that Collinson and others were bringing in. Bartram’s boxes of new plants reached about 100 new growers in only a decade and then filtered outwards to gardens of all sorts and sizes, giving them an American scope that amazed European visitors. Thieves even began to rob the beds in Collinson’s own garden, stealing slipper orchids and rare magnolias. He lobbied for a parliamentary act, which duly prescribed deportation to the American colonies for any plant thieves caught with stock.

“Poor ragged shoemakers, weavers and bakers,” Collinson remarked, became swept up with the “itch” to pay half a guinea or more for the latest flower. Wulf draws the threads of her story compellingly together and lights up an “American connection” in Georgian garden growth as never before. My one caution is that she has to skim over the preceding 50 years when American plants were already known by other routes. Even before Collinson, a Bishop of Fulham had been using one of his missionaries in North America to send up to 1,000 new plants from his territory. The order was eased by the fact that North America was a part of the Bishop’s titular diocese. Collinson and Bartram were supremely important but they were not the first to join England to the American flora. The earlier collections of the two Tradescants had opened English eyes to what could be found and much of it had gone back to the Cecil family, their employers at the palatial site of Hatfield House. The great growth of the English garden trade seems to come in the years from 1730 to 1790 but even in 1710 the famous Brompton nursery of London and Wise was selling plants by the million during its heyday. Bartram and Collinson did not wake up English gardening for the first time.

Later in the 18th century the range of plants grew yet again with imports from the Antipodes and parts of the far east. Wulf neatly observes how property advertisements for houses in suburban settings begin in the 1780s to cite the number of shrubs and exotic plants as a selling point in newspaper particulars. She tells a fascinating story of change and growth on so many fronts and takes garden history beyond the usual story of “English” landscape-parks. The theme is clear enough. English gardens have never been insular. Today’s hectic array of global planting is only the latest chapter in a long history.

To buy Andrea Wulf’s ‘The Brother Gardeners: Botany, Empire and the Birth of an Obsession’ at a discounted price of £7.19, plus postage and packing, contact the FT’s ordering service. Tel: +44 0870-429 5884; www.ft.com/bookshop