AN ELDERLY BRITISH couple unwittingly grew a monstrous cannabis plant in their garden after buying what they thought was an innocuous green bush, and tending it carefully.

Police in Bedford, a commuter town 90km from London, posted a picture of a luxuriant cannabis plant growing as tall as the fence.

In a message on Twitter the force said the plant had been “seized today.”

“Elderly couple bought shrub at car boot sale, tended carefully-biggest cannabis plant we had seen!!” the message exclaimed.

Further details about the couple — or how either it or police got wise to the plant’s true nature — weren’t immediately available.