Hippies would have thought they were hallucinating. The horses would have failed a dope test. Geoffrey Kibb was amazed but stone cold sober and knew a scientific phenomenon when he saw one.

Mr Kibb discovered the holy grail of the 1960s hippie culture, the most potent magic mushrooms known to science, growing in a vast carpet on a racetrack in the south of England. He estimated there were 100,000 of them, enough to blow the mind of an entire town.

He reported his findings to fellow researcher Peter Shaw, an expert in fungi. To him the field of wavy-capped magic mushroom (Psilocybe cyanes-cens) was confirmation of an astonishing colonisation of Britain by exotic species of mushroom - an invasion innocently caused by gardeners anxious to keep weeds at bay.

Dr Shaw says that by spreading wood chips over the ground gardeners create the perfect habitat for fungi of all sorts. The wavy-capped magic mushroom, "a particularly aggressive species" and a native of the Pacific north-west of America, is now firmly established in gardens, parks and any other place where wood chips are used for weed control.

About 10 species of mushroom are eaten for their hallucinogenic qualities. Psilocybe cyanescens is identified by its wavy cap, purple brown spore print and rapid blueing of stem and cap on bruising, although there are poisonous species of similar appearance.

The blueing reflects the high psilocin/psilocybin content of the fungus (which as any old hippie will tell you is the bit that makes you fly). Dr Shaw is presenting his findings today at the British Ecological Society meeting in Birmingham where he is inviting amateur mycologists to hunt down other rare species.

On his way to work at the University of Surrey Dr Shaw passes a roundabout in Leatherhead. "The roundabout was mulched in 1999 and in May 2000 a flush of creamy-yellow fungi came up."

They turned out to be four different exotic varieties growing in the wood chips - one of which, Agrocybe putaminium, had only been recorded once before in Britain, at Kew gardens. "The wood chips were bought from a commercial supplier in Essex, but how they acquired their strange fungal flora is still unclear."

The bad news for magic mushroom hunters - deliberate growing of magic mushrooms for use is illegal but mere possession is not - is that this is not the time of year for fruiting.

But out of sight the roots from which they grow are spreading rapidly. Dr Shaw's theory is that suppliers keep vast heaps of wood chips in nurseries, allowing aggressive fungi to colonise..

Wood chips and bark chips make a better habitat than the original decaying wood on which the fungi grow. This is because the root structure does not have to force its way through a hard surface but glides between the chips.

Because of his Leatherhead experience and other discoveries Dr Shaw says there are bound to be exotic species not thought to exist in Britain growing happily in gardens, and some have already transferred to the wild - the magic mushroom has been been found growing on trees in Burnham Beeches, Buckinghamshire.