"Sounds like WW2 is back in the UK!"
The dream of self-sufficiency popularised in the 1970s comedy is not beyond the reach of people living in Britain's five million flats, the organisation says.
Lettuce, tomatoes, beetroot and a variety of herbs can all thrive on windox sills – allowing those without gardens and allotments to join the "grow your own" trend.
Kingston Lacy, a country mansion in Dorset, is one of the Trust properties where windox boxes have been installed. At other properties, hanging baskets have been converted to grow vegetables.
The National Trust has calculated that the nation's windowsills could provide the equivalent of 600 acres of farming land, producing tonnes of cheap, locally-sourced food a year.
The "vertical vegetable garden"campaign is part of the trust's drive to encourage more people to get involved with the growing and preparation of food. It has already given over land on its properties across the country to be create 1,000 allotments.