Tresco sits at 49.95 degrees north, the same latitude as Vancouver Island and Labrador. By rights almost nothing in Tresco Abbey Gardens should survive an English winter. The Gulf Stream is the partial answer; the island's geography is the rest. Tresco rarely sees frost. Summer sunlight is long and clear because there is little continental pollution this far west. Smith and his successors planted shelterbelts to break the wind: Monterey cypress and Monterey pine from California, which grow fast in coastal conditions and tolerate salt. Inside the wind shadow, microclimates form, each terrace warmer and stiller than the last. Plants that would die in Devon thrive here. The garden is one of the most extraordinary horticultural experiments in Britain, sustained by walls, geography, and 190 years of stubbornness.

The bones of the old priory are still here. A Benedictine abbey was founded on Tresco in 964 AD, and most of what remains today is from the Priory of St Nicholas, established by monks from Tavistock Abbey in 1114. Pirates damaged it badly in 1351, and it never recovered from the Dissolution of the Monasteries under Henry VIII. By the time Augustus Smith arrived, the priory was a roofless arch and a scatter of stone. Rather than clear it away, he made it the centrepiece. Walk through the gardens today and you pass beneath the medieval arch that once led monks to vespers, surrounded by king proteas and Echium that the monks could never have imagined.

Augustus's grandson, Arthur Dorrien-Smith, expanded the collection on a scale his grandfather could barely have imagined. In 1907 he joined the Sub-Antarctic Islands Scientific Expedition to the Auckland and Campbell Islands, then travelled through New Zealand and Australia gathering plants. He returned in 1909 for more, then again to the Chatham Islands. By the time he was done he had amassed roughly 2,280 specimens. The plant labels at Tresco still mark these origins: Chathams, Tristan da Cunha, the Cape, the Canaries. Every garden tells a story about empire. Tresco's is a quieter and more curious version of that story, told in latitudes and soil types rather than flags.

Two things at Tresco are not plants but feel like they should be. The Valhalla Museum, inside the gardens, displays about thirty carved wooden figureheads salvaged from ships wrecked on Scilly: the Greek god Boreas possibly carved by Pierre Puget in the 17th century, Tsar Alexander I from the Alessandro II Grande that struck the Mare ledges on New Year's Day 1851, and a French bronze cannon raised in 1970 from HMS Association, lost in the Scilly naval disaster of 1707. The figureheads stare out from the foliage with the expressions of people who saw the sea kill the ships they belonged to. The other unexpected residents are red squirrels, introduced in 2012 and reinforced in 2013 by a colony flown in by helicopter from RNAS Culdrose. Tresco has no foxes, no grey squirrels, no squirrel pox. The babies appeared in the Abbey Woods in 2014, and the squirrels have been breeding successfully ever since.