The 70-year-old tomato seeds planted by Canberra gardener Jim Cleaver have finally grown fruit, ensuring the survival of the unique species to at least one more generation.

Mr Cleaver discovered the seeds last year while looking for photos to contribute to Canberra's centenary celebrations. In a box of black and white pictures, he found an old envelope labelled "Bullheart Tomatoes 1944", with nine seeds inside.

He planted them and when one plant flourished, growing up to 20 centimetres tall by February, interest in the unique heirloom plant exploded. But there was only one problem - winter was coming.

"When I realised the tomato was thriving I had to then make an insulated hothouse to keep out the winter chill," Mr Cleaver said.

"It was thermostatically controlled under floor to keep the temperatures of the tomato rather warm because tomatoes like warm soil and warm roots."

The hothouse keeps the special tomato plant at a regular 21 degrees, which Mr Cleaver regulates during the day by opening the door occasionally.

Mr Cleaver said he'd already had numerous requests for seeds from his remarkable tomato plant, with the Canberra Organic Growers Society having shown great interest as well.

"It's just a pure old tomato," he said. "It hasn't been crossed or hybridised or genetically engineered like a lot of them are. It's just about as pure bred as you can get."

When the tomato seeds were first sprouting in February, The Digger's Club seed manager Yvette Jungwirth said she was excited about the tomato seeds' possibilities.

She said as it was from before the era of genetic hybrids and supermarkets, it might offer a superior taste and quality.

"[Hybrids] were bred for convenience, whereas heirlooms were bred for fantastic flavour and beautiful colours," she said.