The first garden at Alnwick was laid down in 1750 by the 1st Duke of Northumberland, who hired Capability Brown to landscape the parkland next to Alnwick Castle. Brown was a Northumberland man, born just up the road, and he gave Alnwick the sweeping, naturalistic grounds for which he became famous. The 3rd Duke filled hothouses with pineapples and brought seeds back from continents most of his neighbours had never seen. By the 1850s the 4th Duke had laid out an Italianate garden with a vast conservatory, and by century's end the grounds were at their grandest: yew topiary, avenues of limes, acres of beds. Then came the twentieth century, the Dig for Victory campaign of the Second World War, post-war austerity, and a slow collapse. The working garden closed in 1950 and rotted quietly for half a century.

When Jane Percy became Duchess of Northumberland in 1995, she could have left the wreckage alone. Instead she set about reviving it. In 1997 she brought in the Belgian landscape architects Jacques and Peter Wirtz to design what would become the most ambitious new garden built in the United Kingdom since 1945. The first phase opened in October 2001, centred on a stepped water cascade that fires twenty thousand litres a minute down the hillside. A 6,000-square-foot tree house complex with a cafe opened in December 2004, eventually claiming to be the largest in the world. A pavilion and visitor centre with a barrel-vaulted gridshell roof followed in May 2006, designed by Michael Hopkins and Buro Happold. The total bill ran to forty-two million pounds. The 12th Duke gave the 42-acre site and contributed nine million towards the redevelopment. The garden now belongs to a charitable trust, separate from the Northumberland estates that birthed it.

The Duchess wanted a hook, something that would distinguish Alnwick from every other restored country garden in Britain. She found it in plants that hurt. The Poison Garden grows Strychnos nux-vomica, the source of strychnine. It grows Ricinus communis, which yields harmless castor oil and lethal ricin from the same seed. There are foxgloves, deadly nightshade, hemlock, and angel's trumpet, Brugmansia, which is hallucinogenic and corrosive. Laburnum drips its yellow chains over the path. The garden also serves a drug-education brief, with managed plantings of cannabis, coca, and the opium poppy Papaver somniferum. Visitors enter only with a guide, do not touch, do not lean in to inhale. The plants that visitors find most surprising are usually the ones already growing in their own gardens at home, quietly waiting to be misidentified.

Walking the Alnwick Garden today, you move from the cascade through rose gardens, ornamental gardens, a serpent garden of water sculptures by William Pye, and a bamboo labyrinth. The cherry orchard holds 329 Tai Haku cherries, one of the largest collections of that variety in the world, and it foams white every spring like spilled milk. The tree house above shelters a restaurant in the canopy, reachable by wobbly rope bridges. The Poison Garden remains the headline, but the duchess has built something larger than a single dark joke. She built a garden that argues, repeatedly, that horticulture can be theatre, that plants can be dangerous, and that the public will pay good money to watch both.