Did you know there is thought to be more than 7,000 varieties of tomatoes?

While those usually found in supermarkets include generic varieties like Roma, field and cherry tomatoes, horticulturalist Paul Nicholson from the Sydney Royal Botanic Gardens said the best were those grown in the backyard.

"The tomatoes you often buy in the supermarket have no taste," Mr Nicholson said.

"They're watery, they have thick flesh — so grow your own tomatoes, and there's wonderful groups like diggers and heirloom seed savers."

Paul Nicholson says tomatoes grown in Sydney should be harvested by January. ( 702 ABC Sydney: Amanda Hoh )

Tomato fun facts People have been selecting and breeding tomatoes since 500BC, originally the Mayans and then the Aztecs in central America.

People have been selecting and breeding tomatoes since 500BC, originally the Mayans and then the Aztecs in central America. Tomatoes come from the western slopes of the Andes near Peru.

Tomatoes come from the western slopes of the Andes near Peru. Tomatoes are part of the nightshade family or solanaceae plants. It is one of the largest plant families with more than 90 genera and 2,000 species.

Tomatoes are part of the nightshade family or solanaceae plants. It is one of the largest plant families with more than 90 genera and 2,000 species. The tomato is in the same family as the potato, eggplant, chilli and tobacco.

The tomato is in the same family as the potato, eggplant, chilli and tobacco. Some nightshade plants produce alkaloids — chemical substances that are potentially very toxic, while others are used in medicine and modern pharmaceuticals.

Mr Nicholson, a judge at this weekend's Tomato Festival, said tomatoes grown in Sydney should be harvested by January.

Any later and the high humidity, strong sun and summer storms will cause "mashed-up tomatoes", while insect pests, particularly mites, can spread viruses to the tomatoes causing them to wilt and die.

Mr Nicholson said some of the tomatoes best grown in Sydney included the black Krim and black Russian varieties.

"They're both fairly large, beefsteak-style tomatoes, so very fleshy, quite soft skin," he said.

"Most of the supermarket tomatoes will be F1 hybrids, they'll have names that don't have the romance of black Russian or mortgage lifter."

The mortgage lifter tomato was bred in the United States and developed by a man known as "Radiator Charlie".

"He developed it during the depression and the story is he paid off his mortgage with the proceeds of this tomato," Mr Nicholson said.

How to pick the best tomato?

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 22 seconds 1 m 22 s How to pick the best tomato ( 702 ABC Sydney: Amanda Hoh )

To get the best colour and flavour, Mr Nicholson suggested tomatoes should ideally be ripened on the vine.

"The sun helps to concentrate the flavour, so sticking it in the refrigerator is a little bit like death to the tomato," he said.

"You're looking for an evenness of colour, no entry or exit holes from caterpillars, and you're looking for a size that's in proportion to what you'd expect from that variety.

"Either oversize or undersize won't be ideal ... so growing the 10-kilo tomato is not necessarily the best."

Mr Nicholson also said tomatoes should not be "spongy" or have any "soft spots".

Paul Nicholson says tomatoes are best ripened on the vine before harvesting. ( 702 ABC Sydney: Amanda Hoh )

Thin-skinned tomatoes were best for eating fresh, while the thicker-skinned varieties, like the Romas, were best for storage and for creating sauces like passata, Mr Nicholson said.

The best way to judge a tomato, though, was to eat it, he said.

"It really comes down to taste. In the end that's the clincher."