About half is sold as whole plants, with the rest of the plants broken up, washed in sterile water, dried in a washing machine spin cycle and bagged.
It all started when Mr Cadman was given a hydroponic kitset as a present after selling his hotel in Tauranga. He'd always been interested in plants and found the kitset fascinating.
"It was a great hobby. It got really interesting."
He was supposed to be retiring, but decided to grow a few things to sell.
Twenty-two years ago he got serious about it and bought the Waverley property. He was originally from Opunake, and for him it was a return home.
Waverley was an ideal place for the business, he said. Day and night temperatures are similar, which is great for lettuce. It's also one and a half hours from Palmerston North and New Plymouth, and he likes living in a small town.
South Taranaki has elite growing soil - but he doesn't use that for the lettuces. They're all grown above ground, first as seedlings in potting mix bought from Bark & Boulders Landscape Supplies in Wanganui, and then in plastic spouting, with water and nutrients constantly flowing past their roots.
Mr Cadman started by growing iceberg lettuce for McDonald's, and said lettuce was an ideal crop because it needed little labour and grew year-round without added heat.
Each plant takes 21 to 50 days to mature - longer when sunlight hours are fewer and longer for red varieties.
He grows six lettuce varieties, and trials new ones. The most popular is the ultra-frilly Green Coral.
Seed is imported from The Netherlands. It's coated, to ensure even germination. Seeds are hand planted into small containers of potting mix, and moved to the spouting when they're big enough.
The Hydroponic Fresh plastic houses are supplied with mixed water and nutrients by three pump systems. The water is mainly rainwater, collected off roofs on the property and stored in tanks. If that runs out Mr Cadman can use Waverley's town water. But he said rainwater was better, because it was pure.
He followed the news of the lettuce that gave about 100 people a stomach bug recently. He said those plants were grown in the ground in just one area.
His pumps have to run constantly, and he lives on the property to keep an eye on them. A nutrient mix suitable for lettuces is supplied from barrels, with the rate controlled by computers.
One of the biggest costs is electricity, to run the pumps. After the March 2012 storm took out power he had to rush to Hawera and buy generators. The only other loss was the roof of one plastic house.
"My bank manager told me I was the luckiest man in Waverley," he said.
Running trucks was another big cost for him, along with wages for seven staff.
"A job here is quite sought after. It's a good clean job. There's no bending over."
Plants grown hydroponically are just as good as those grown in soil, he believes.
"If it's not better it's equally as good as ground-grown produce, because they're only fed exactly what they want."
Mr Cadman has silverbeet, tomatoes and strawberries growing hydroponically in one of his plastic houses.
His daughter Corinna will take over if he ever has to retire. But he doesn't want to stop growing.
"I'm very lucky. There wouldn't be many people around who have their job as a hobby.
"I just love it," he said.
