A S BOSS of North Devon's biggest garden centre, David Oliver never experienced the joy of growing and picking his own tomatoes in a modest greenhouse.

It was always a case of looking after 10,000 plants a year in huge glasshouses for other people — his customers — along with some 10,000 freesias, 100,000 chrysanthemums and so many cucumbers he can't pinpoint the exact quantity.

Now mostly retired from his pivotal role at St John's Garden Centre in Barnstaple, David has hit on an ingenious way of growing these luscious redskins in a cold greenhouse - and boasting nine trusses of fruit or flowers by the first week of July.

He did it with a load of grass mowings. To these he added three bags of shredded redundant Christmas trees and a few handfuls of Garotta, the compost activator.

Down the centre of the 8ft by 6ft greenhouse David dug a trench, 10in deep and 5ft long, which he filled with the magic mix.

Within a few moments the trench began to warm up as the carbon dioxide was given off — a sort of global warming in miniature as the CO2 was harnessed to distinct advantage leading to a rich harvest.

By the end of April David planted out 13 Shirley tomato plants in 10in pots, each with holes in the sides as well as at the bottom to prompt the roots to drill down as well as wrapping themselves round and round.

Every time David cut his lawn at his home that overlooks St John's, he would top up the trench with extra mowings to maintain the heat.

For feed, he applied a high potash solution, doses of Miracle Gro and one rich in nitrogen, he grew clumps of marigolds nearby to drive out the whitefly and he removes some of the lower leaves to deter mould-based diseases which can strike if the air becomes stagnant.

David, who turned 80 on June 30, is delighted with his rewards and says: "You can feel the warmth being given off, especially by night.

"Here we have a method of producing superb tomatoes that's virtually free, that produces CO2 naturally and anyone can do it."

To back up his beliefs, David is counting every tom he picks and is weighing each crop so that, at the end of the season, he'll have an accurate measure of his success.

"I'd like to find out exactly how much this method of growing is beneficial," he says. "Already I can see there will be a huge crop."

Growers should heed David's handful of warnings. Avoid walking on the compost— tread on the edges instead. With this in mind, he is planning a trench "upgrade" by placing wooden pallets across the framework for easier access.

More crucially, he says, is the need to avoid any form of weed killer if lawn mowings are to be used in the heating process.

"One part in a million of hormone weed killer can kill the plants," he stresses.

David, who first opened St John's in a muddy field in 1958, has now largely handed over the day-to-day running to sons Nick, Tom and Simon. Yet retirement is still a foreign word.

"I never think about it," he says and adds amid his characteristic boom-boom laugh: "How can I retire when I've never been out to work?

"I've only been out to play!"

The garden centre now employs around 80 staff and all were invited to a party to celebrate David reaching 80 not out. And with David and his wife Patsy dancing the night away to Victor Silvester — or did they indulge in the Oliver twist? — it was clearly roses, roses all the way.