A spokesman for Wyevale Garden Centres, which has 106 UK branches, said: "At first, it was just a trickle of inquiries, but now stores are virtually being besieged each weekend."
"We have had men buying dozens of the plants and, at one store in Croydon, there were men old enough to know better fighting over the last remaining trays."
The latest gardening craze was triggered by a discovery by a 55-year-old furniture restorer, Michael Ford, on his allotment.
He said: "The effect was almost immediate. I had to stay in my potting shed for an hour or so before I could decently walk down the street."