Entries must be submitted by 24 September. The weighing and unveiling of the winner will form part of the society's London Harvest Festival Show, to be held on 9-10 October in Lindley Hall, London. Runner-up prizes ranging from £500 to £200 will also be presented. Entry to the competition is free.
RHS competitions manager Georgina Barter commented: "We are expecting some really huge pumpkins to be presented at the show, making this year's Halloween spookier than ever."