<P> Redevelopment of the garden was instigated by Jane Percy, Duchess of Northumberland in 1997, and has been led by Belgian landscape designers Jacques and Peter Wirtz . It is the most ambitious new garden created in the United Kingdom since the Second World War, with a reported total development cost of £ 42 million . </P> <P> The first phase of redevelopment, opened in October 2001, involved the creation of the cascade and initial planting of the gardens . In 2004 a large 6,000 sq ft (560 m2) tree house complex, including a cafe, was opened . It is one of the largest treehouses in the world . A pavilion and visitor centre designed by Sir Michael Hopkins and Buro Happold opened in May 2006, with capacity for 1,000 people . The pavilion and visitor centre feature a barrel - vaulted gridshell roof . The gardens include several water features as well as architectural landscaping, topiary and decorative gates . </P> <P> A garden featuring intoxicating and poisonous plants was added in February 2005 . Species of the Poison Garden include Strychnos nux - vomica (source of strychnine), hemlock, Ricinus communis (source of harmless castor oil but also deadly ricin), foxglove, Atropa belladonna (commonly called Deadly Nightshade), Brugmansia and Laburnum . The mission of the Poison Garden also includes drug education, with featured plantings of cannabis, coca and the opium poppy Papaver somniferum . </P>

Poisonous garden at alnwick castle in the county of northumberland
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