LONDON — Gone are the days when Celtic warriors painted their faces blue for battle with a mixture of crushed woad leaves soaked in urine. Gone too are the days when French courtesans used toxic belladonna to make their eyes sparkle and witches rubbed an ointment of reptile blood and mandrake on their bodies to enable them to fly. Yet in London’s Chelsea Physic Garden woad, belladonna, and mandrake flourish.

Some of the most important drugs used today come from seemingly common plants: morphine from the poppy, aspirin from the willow, a heart medication from the foxglove, a highly poisonous plant. Broad beans are used to aid in memory retention in Alzheimer’s sufferers. Extracts from plants keep many of us alive.

I recently visited the Chelsea Physic Garden, nestled in an upmarket residential area close to the heart of the capital city. In this 17th-century garden apothecaries and healers acquired herbal stock for curing all manner of ailments.

Confined within high walls, London’s “secret garden” ranks among the oldest of its type in the world, preceded only by a physic garden in Pisa, Italy, and the Oxford Botanic Garden.

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Anne Gordon for The Boston Globe

The garden's layout enables plant lovers to wander freely.

Laid out much like a monastery garden with mostly long narrow beds separated by grassy ribbons, this garden’s central feature is a rock garden pond, made with basalt spewed by volcanoes in Iceland.

In the garden’s far corner, Britain’s largest outdoor olive tree produces abundant fruit. They are tiny, bitter olives that require a brine soak to render them palatable. Overhanging a stony path nearby, a massive cork oak is adorned with a necklace of bottle corks. In the early 18th century nursing mothers believed that wearing cork necklaces would dry up their milk.

Edible, medicinal, and poisonous plants are all represented here. Until fairly recently outside guides were permitted to escort tours, but that practice has stopped. A Spanish guide reportedly allowed three members of her group to taste the round, almost black fruit of the deadly nightshade. All three were hospitalized.

Attesting to the danger of some herbal plants, poison from the husk of the castor oil seed was applied to an umbrella spike used to kill a Bulgarian dissident in Britain many years ago. I was surprised to see an apricot tree in the garden, then discovered that its kernel is a source of arsenic.

More delectable is chocolate from the cocoa bean. It also has medicinal properties. In 1742 Jean-Baptiste Labat, a Dominican priest and botanist, said dark chocolate quenched thirst and was flesh forming. In his opinion, it restored strength, encouraged sleep, helped digestion, and softened and purified the blood, preserving health and prolonging life.

Pesticides are unknown here. Thirty tons of manure arrive every winter from the Buckingham Palace garden. On this rich and nutritious diet the plants thrive. The resident bees are happy too with their pesticide-free environment. Beekeeper Peter James tends them and last year harvested 100 kilos of honey from a single hive.

Until the turn of the last century women were banned from entering the garden except for “weeding women” who were paid the paltry sum of sixpence a day. Fortunately pay and conditions have changed, but the rent for the garden — five pounds per annum when Sir Hans Sloane was the landlord in the early 18th century — remains frozen, a mere pittance for what today must be one of London’s most expensive pieces of real estate.

66 Royal Hospital Road, 011-44-20-7352-5646, www.chelsea
physicgarden.co.uk. Adults $14, children ages 5-15 and students about $9. Tangerine Dream Cafe open daily from noon; closed Mondays.