Tasmanian farmer Roger Bignell is no stranger to growing extremely large vegetables.

In 1973, at the age of 16, he grew the world's largest carrot.

Now, the 61-year-old has accidentally grown what looks to be the world's largest turnip at his Waterhouse property, 20 kilometres east of Bridport.

Mr Bignell planted turnips in a paddock last year for stock grazing.

He told Sarah Gillman on ABC Radio Hobart that the turnips did the job and he left them be.

Mr Bignell says the secret to growing large vegetables is the area's sandy soils. ( Supplied: Roger Bignall )

But some of the vegetables went to seed.

"Only last week I went down to the paddock because I was going to spray it off, and I noticed these huge, great turnips there," he said.

"I'd never seen anything like it."

Mr Bignell loaded some of the turnips onto his motorbike and took them back to his house and weighed them.

A couple of them were on par with the record, but one stood out.

"It weighed more than the standing world record."

Scott and Mardie Robb from the United States held the record since 2004 with a 17.7-kilogram turnip.

Mr Bignell's heaviest turnip tipped the scales at 18.36kg.

"Often those large vegetables are misshapen," he said.

"This one wasn't grossly different, but it was just huge."

Mr Bignell's grandson Connor Jones has his work cut out for him moving the turnips. ( Supplied: Roger Bignall )

What's the secret to growing giant turnips?

There are gardeners around the world who dedicate huge amounts of time to growing large vegetables.

They are carefully tended to daily in the hope of winning competitions.

Mr Bignell first made the headlines in 1973. ( The Mercury Newspaper )

Mr Bignell sees the humour in his record, because the paddock he grew the turnips in hadn't even been fertilised for 12 months.

He puts the success down to the sandy soils of Tasmania's East Coast.

"These coastal sands lend themselves to growing turnips," he said.

"It's wonderful turnip-growing country."

The turnip title can now be added to his 1973 accolades for the world's biggest carrot.

It weighed 3.45kg and was reported in the Mercury newspaper.

The turnip has been officially weighed and witnessed, with official documents signed.

How it will be used is yet to be decided.

"Perhaps turnip is on the menu for a while," Mr Bignell said.

The results have been sent to the Guinness World Records.