From the front door of the glass-walled gift shop at the Alnwick Garden in the far northeast of England, the scene looks innocent enough. A sapphire green English lawn slopes gently downward, toward traditional, ornamental gardens of rose and bamboo. Across the small valley, water cascades down a terraced fountain.

But a hundred or so plantings kept behind bars in this castle's garden are more menacing — and have much to tell visitors about poison and the evolutionary roots of medicine.

"These Plants Can Kill" warn two signs on a locked, iron gate that's also marked with a skull and crossbones.

The Duchess of Northumberland (aka Jane Percy) started the Poison Garden in 2005 as part of the 12-acre, elaborate garden on the grounds of her family's home, Alnwick Castle.

Many of England's cities and towns have apothecary gardens — historical plots containing plants turned into treatments centuries ago by doctors, herbalists, religious folks and shamans. Most such gardens exist today to teach visitors about the history of medicine.

Plants evolved poisonous properties to keep from being eaten, explains Dr. Henry Oakeley, who has studied the medicinal effects of plants as a Garden Fellow at the Royal College of Physicians. Some of those same chemicals that began as plant protection, he says, are now used to help people — by killing cancer cells, say, or quieting an overactive muscle or a painful nerve cell.

Still, Percy's goal wasn't to explore the medical benefits of plants.

"The line between kill and cure is what I'm interested in," she says. "The story of how plants can cure, I find pretty boring, really. Much better to know how a plant kills."

Visitors to the Poison Garden are allowed to look but not touch, says head gardener Trevor Jones. "They're not allowed to stand too close to the plants. They're not allowed to smell them or touch them or taste any of them, because they do all have the ability to kill you. We only take 20 people in at a time so that the guide can see everybody, and see exactly what they're up to and can take control."

Some of the plants in the Poison Garden might surprise you. Percy has many times watched backyard gardeners walk through the grounds and spot plants that they recognize from their yards back home. Those can't be poisonous, the visitors figure.

The Duchess of Northumberland knows better. "Well, I've got that in my garden," she says, "and I could tell you how to kill with it."

She doesn't actually offer up poisonous recipes. But she does confide that a powerful poison can be made from Helleborus species — very much like the ones growing in my garden in Seattle. Extracts from Helleborus were once used in low doses as a purgative to help children expel intestinal worms, Percy explains. Overdoses killed.

And it doesn't take many berries from Atropa belladonna (deadly nightshade) to kill either — the plant is common in England and growing happily in the Poison Garden. The duchess also warns that that merely brushing up against the bushy green Ruta graveolens (commonly called Rue) or touching the sap from Euphorbia (the Dr. Suess-like plants sometimes called spurge) can give a person a nasty rash.

The duchess is particularly interested in telling children about the killing power of plants.

"If you're a child, who cares that aspirin comes from the bark of a tree?" she asks. It's far more interesting, she says, that in Victorian times, children had something called killing jars. The jars held laurel leaves, which killed spiders or butterflies but left them intact — great for collectors.

The garden has a special permit to grow coca plants and marijuana plants (both are enclosed in metal cages, with closed-circuit TV cameras pointed at them). Thousands of schoolchildren have learned from the guides here about the power of these and other plants to affect their own biology.

Percy doesn't have any research on whether the anti-drug message she intends gets across, but several tour guides say the kids listen carefully — after poking each other and giggling.

Through the Middle Ages and into the Renaissance, plants were a popular source of poisons used in palace intrigues. Members of the Medici family in Florence were said to be extremely adept, and the poisoning didn't stop there. In 1978 a member of the Bulgarian secret police used an umbrella tip to inject ricin — a powerful poison extracted from the beans of a castor plant — into the leg of a political dissident, as he walked down a London street. (And yes, a castor plant grows in Alnwick's poison garden.)

Jones, the head gardener, says he enjoys the poison aspect of this unusual place, but he also has a soft spot for the lower-dose, medicinal uses of the plants he tends. He has a favorite plant — a low-lying vine with periwinkle flowers. It's a Madagascar periwinkle, sometimes known as Vinca rosea, and, in a sense, the plant saved his life.

Years ago, Jones had leukemia, and the drug used to cure his illness was originally derived from Madagascar periwinkle. He makes sure that this particular pretty little vine is weeded all around, and gets a bit of extra plant food.

Joanne Silberner, a former health policy correspondent for NPR, is an artist in residence at the University of Washington's department of communication.

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