The lost gardens : an English garden mystery / Anthony Eglin.

Enhanced descriptions from Syndetics:

Hidden within the derelict gardens of abandoned Wickersham Priory, a deadly secret is waiting. But when an unsuspecting young Californian named Jamie Gibson finds herself the new owner of the estate, through a surprise bequest from a total stranger---the secret begins to stir.
Jamie, fired with enthusiasm to restore the gardens to their 1930s glory, seeks the help of Lawrence Kingston, a retired professor of botany, eccentric bon viveur, and amateur sleuth. Lawrence soon unearths an old chapel, which leads to an ancient Healing Well, which in turn yields a human skeleton. And as the police pursue their inquiries, Kingston begins his own investigation---following a baffling trail of clues that wind down through the centuries, from the battlegrounds of World War II to the depths of the Middle Ages.
It is a trail marked by misadventure, revenge, compassion, and murder when finally Kingston unlocks the secret of Wickersham Priory, he and Jamie must confront a reckoning that neither of them could have ever imagined.
As with the highly acclaimed "The Blue Rose, " Eglin brings his botanical and literary skill to this new mystery.