It has quietly become something of a green gold rush. In woods and forests across Britain, wild garlic is being harvested for soup makers, wood sorrel gathered for Michelin-starred chefs, and spruce needles picked to infuse hand-made chocolates.

Harvesting "wild food", the seasonal salad leaves, nuts, fruit and fungi that grow abundantly across the UK, has led to a new industry in professional foraging for restaurants and a sharp surge in public interest.

They are harvesting - for free - nearly 200 ingredients throughout the year: from common crops such as hazelnuts, brambles and wild strawberries, to dozens of different fungi, through to specialist crops such as elm and lime leaves, or sweet cicely. Chefs are now paying up to £50 a kilo for wood sorrel, with its sharp lemony tang, and £40 a kilo for elusive morel mushrooms, handpicked from the forest floor.

In Scotland alone, where the wild food movement is thought to be strongest, the Forestry Commission estimates that wild harvesting, including harvesting lichens and mosses for natural remedies and horticulture, is worth as much as £21m a year. Its rapid growth - by as much as 38% since 2001 - has led the commission to launch a campaign this month to promote wild foods with a code of good practice, to ensure the increasing number of foragers harvest carefully and, where needed, with the landowner's permission.

It is no longer a niche, cottage industry. The fruit and vegetables wholesaler Fresh Direct, which supplies Harrods, high street cafes, and Michelin-starred chefs, has begun extending its wild harvesting operations from Scotland into England.

The search for wild food mirrors the surge in popularity for home-grown produce, allotments and "guerrilla gardening" - where patches of vacant and under-used inner-city land are converted into al fresco fruit and vegetable patches - championed by chefs such as Hugh Fearnley-Whittingstall.

The Dorset-based food writer has promoted the marine equivalent of foraging in forests in his latest River Cottage handbook, Edible Seashore, harvesting wild foods such as cockles, sea beet and marsh samphire from the seaside and coastline. The survival expert Ray Mears has devoted television series to wild foods on land, and the small Kent-based firm Forager is about to publish a dedicated guide and recipe book on edible plants, The Forager Handbook.

To cope with the surge in demand, the charity Reforesting Scotland has set up a "wild harvesting" trade association supported by the commission and the Scottish government. "We've had a huge response from the general public on foraging," said Emma Chapman, the charity's project coordinator. "It ties into environmental concerns: you're getting a little bit of your food in a low-impact way. A lot of the salads you get at this time of year have a huge amount of energy associated with them, with refrigeration, transport, being grown abroad and under artificial conditions, and they just don't taste so good."

Roger Coppock, the Forestry Commission's head of business policy development, said one recent survey suggested that well over a million people in Scotland alone had foraged at least once in the past two years.

"That started to reveal that it wasn't just a case of cranks and back-to-the-earth type people collecting. It went right across the spectrum from the unemployed to lords," he said.

Coppock believes that much more could be taken sustainably from the commission's land. "It is nowhere near being over-harvested," he said. "There's an awful lot of potential there."

Andy Fraser, who runs Fresh Direct Local Scotland, one of the country's largest wild food suppliers and a subsidiary of Fresh Direct UK, said amateur foragers needed to be very careful about what they picked - some plants and mushrooms could be lethal.

The bestselling author of The Horse Whisperer, Nicholas Evans, was hospitalised after eating poisonous mushrooms picked on a woodland walk near Forres in Aberdeenshire last September.

Fraser believes the Forestry Commission could increase the availability of wild foods by actively sowing its plantations with mushrooms, berry bushes and wild salads.

Foraging in forests

Spring recipe: Wild garlic pesto

500g wild garlic leaves; 200ml olive oil

75g walnuts

Finely chop and then grind in a pestle and mortar, or chop and mix in a blender. Chill before serving.

Autumn recipe: Lingonberry or cowberry sauce for venison steaks

60g lingonberries; 20g of sugar;

200ml chicken or venison stock

Drain fat after cooking steaks, deglaze the pan with stock, add the berries and sugar, then reduce by half.

Source: The Forager Handbook by Miles Irving