Shoppers will have to pay up to 15% more for pumpkins this Hallowe'en - and the wet summer means they have not turned their usual orange colour.

The problem has led to some growers heating the harvested green pumpkins to encourage them to take on the orange tinge that retailers demand.

"The crop is not looking good - it's green and scrappy - the worst I've seen in 35 years of growing pumpkins," said David Bowman, Europe's biggest grower.

"At the moment, we're having to force heat the green pumpkins to 25°C in order to make them go orange."

The energy and extra handling required to heat the pumpkins is one factor pushing up costs. But poor growing conditions during the summer have also taken their toll on production volumes.

Bowman estimated he was 10%-15% short of his usual crop of two million pumpkins. He is also having to negotiate with retail customers on relaxing the quality and size specifications of the pumpkins. There are a fewer large pumpkins of 6-7kgs and lots of much smaller ones, up to 1.5kg, but not much in between, he said.

"We're raising prices up at least 10%, although the supermarkets are dragging their feet. It will have to be reflected in retail prices."

Major growers such as David Bowman and Barfoots of Botley are still negotiating with retail customers over pumpkin prices. But smaller producers in England said they had secured prices up to 15% higher than last year from local retailers.

"Those who don't heed the laws of supply and demand will find there are no pumpkins available," warned Mike Smales, a Salisbury-based grower.

And with shortages across Europe, there was little opportunity to source cheaper pumpkin from outside the UK, he added.

"The supermarkets are hypocritical to be shouting about carbon emissions then demanding growers heat their pumpkins."