Cannabis farms have been found hidden among rhododendron bushes, prompting experts to warn the drug is being grown across the Scottish countryside.

The remains of a substantial cannabis factory was found in the Torridon area in Wester Ross hidden in rhododendron bushes.

Contractors bought in to cut down the invasive rhododendron ponticum alerted the landowner when they stumbled upon DIY polytunnels made with hosepipes and sheet plastic.

In the four "microdomes" were dozens of two-litre bottles filled with compost, moisture gel and dead stalks from what appeared to be cannabis. With a few ounces of cannabis fetching hundreds of pounds, the street price could be up to thousands of pounds.

Duncan Gray, owner of Ben Damph Estate where the plants were found, was shocked by the discovery.

He said: "We've found deer bones, old cars and crash helmets but we didn't expect to find a cannabis farm in the rhododendrons.

"It was entirely hidden from the road or even by helicopter. I guess the rhododendrons would also protect the plants from deer and sheep that do not come into the plantation."

The rhododendrons are on the south side of Loch Torridon and close to the road, making transport easy.

A dog walker, who preferred not to be named, also reported finding a cannabis farm on a smaller scale in a wood in East Lothian. In this instance the drug was being grown in bottles with just the tops sawn off to create a similar greenhouse effect.

Dr Neil McKeganey, of the Centre for Drug Misuse Research in Glasgow, pointed out two-thirds of cannabis is now believed to be home-grown. Most of the cannabis farms discovered are indoors in flats, but it has also been found on rooftops, derelict farms and wasteland.

"Production of cannabis is a serious problem," said Dr McKeganey. "It is an extra­ordinarily difficult problem to tackle because of the range of areas where it can be found. Often you only found out about those areas when you stumble upon them."

Online manuals give clear instructions on how to grow cannabis outdoors using everyday materials.

Dr McKeganey said he had heard reports of cannabis being grown in the countryside but it was seldom discovered.

"I have seen periodically reports of cannabis in unusual places," he said. "I suspect it is quite common, though not commonly identified."

Possession of cannabis is a criminal offence, although possessing the means of growing cannabis is not.

Horticulturalist Anthony Winch said cannabis was "very hardy and idiot-proof" and the right variety may do well in the Scottish Highlands or elsewhere in the countryside.

Sources said the farms could be used as a nursery for growing more vigorous plants that would then be bought on indoors, and suggested the Highland air would give a more pure "high".

However, Mr Winch said it was more likely to be for home use and producing a poor-quality drug.

"The THC level is almost ­exclusively dependent on warm temperature. This would probably produce very low-quality weed."

Police Scotland declined to comment.

l A man and woman have been reported to the procurator-fiscal after the discovery of a cannabis farm in Lybster, Caithness, with £130,000 worth of drugs.