He realised how different fresh salad tasted compared to bagged, and on his return he devoted 18 months to developing a living, 'windowsill' organic salad.
After overcoming technical hurdles and 2,000 trial drillings, he got an interview in January 2004 with a buyer from Marks and Spencer and received a very enthusiastic response. But Philip Green's bid to take over the retailer left Johnson in limbo.
Waitrose took an interest, but it was when the mother of Sainsbury's CEO Justin King tasted the product at the BBC Good Food Show that Johnson started moving towards a Sainsbury's launch in April 2005.
Tesco, Asda and Booths also now stock the product, and a special line is being developed for Waitrose.
Packs sell for about £1.49 for four servings of red chard, baby pak choi, tatsoi and golden streaks, and shelf life is two weeks.
The family-based business trades under the Home Harvest Salads parent company name and is located in Hull because of its milder climate, where it crops two acres under glass. It has a seasonally variable staff of ten, plus Johnson, who is current Young Grower of the Year, and his wife Anna.
"We are still a very small operation, but we are developing the product and providing very high customer service levels through very close attention to detail on flavour and quality," said Johnson, a farmer's son who always wanted to strike out on his own.
"We are proud of it and passionate about the Living Salads brand and the health benefits that the product offers. We are aware that there are companies trying to do what we do and going to the multiples saying they are bigger and can do this better.
"But I have grown a lot of different crops and this is the most difficult. Companies with a multimillion-pound turnover don't devote the level of attention to detail that we do, and that is paramount in this industry in building a customer base."
Johnson also realises that new product development is key to keeping the multiples interested, and to this end he recently launched a rocket salad into Sainsbury's. He is also keen to develop other combinations of leaves.
Shelf life is another area of improvement, as is yield, which has doubled to about 150g since the launch.