In many fridges, “salad” used to mean flavourless iceberg lettuce and a tomato or two, but a quick scan of the supermarket shelves will now reveal as many as 25 different bagged salads on offer.

The main UK supermarkets sell a staggering 334 million bags of salad per year. So what turned salad from a cottage industry into a £400m a year business?

Steve Rothwell, a farmer and self-declared “leaf boffin” with a PhD in Watercress from Bath University, has watched demand for leaves explode since the 1970s, when supermarkets first began to sell lettuce in bags.

“In the 1970s, supermarket produce counters were just a jumble of tired-looking whole-head lettuces,” he said. “But in 1976 Britain had a long, hot summer, and when watercress was launched in bags it flew off the shelves. I think that started what became a massive trend.”

During the 1980s and 1990s consumers began to pay more attention to flavour, buying into new types of leaf such as baby spinach or rocket, which was championed by Delia Smith.

Today, there is another concern which motivates shoppers’ choices, and that is the impact their food has on the environment.

Steve’s Leaves (stevesleaves.co.uk) is the only salad producer compliant with the Conservation Grade scheme, whose stringent protocols require farmers to set aside 10 per cent of their land for conservation purposes. While this is impressive, Steve admits he has yet to solve the main environmental problem raised by bagged salads – the bags can’t usually be recycled. Though he believes a compostable salad-bag could be just around the corner.

Salad inevitably sells better in hot weather, and with forecasters predicting above-average temperatures this year, bags could be set to fly off the shelves the way they did in 1976.

So, how should you distinguish between the huge array of options? It depends whether you’re prioritising taste, novelty or health benefits. Steve hopes to cover each of these bases with his own range of salads, recently launched as '”Steve's Leaves”.

The wasabi rocket – branded as Super Hot Rocket – was bred for its powerful flavour, and will appeal to those who like their salad with a spicy kick. For those who want a taste of the next big thing, his “Shoots and Leaves” mix contains pea-shoots, the latest leaf to grace London restaurants. And for the health posse he offers “Nose-Tinglingly Peppery Baby Watercress”, which contains more iron than spinach, more vitamin C than an orange, and more calcium than whole milk.

All week we'll be posting recipes that make the most of these leafy treats, but you can substitute your own salad leaves if you prefer. Come back tomorrow for the first!