Locally-grown watermelons are now on sale for the first time at selected Asda stores in South East England. Joe Cottingham, the farmer from Dartford who’s perfecting the technique for growing them in a cooler climate, blogs about how it’s been done.

It’s taken lots of trial work to get to the point where we’ve been able to successfully harvest this crop of 1,000 watermelons as the British climate isn’t traditionally suitable for growing it here.

Last year we trialled 20 or 30 different types of watermelon, learnt how to grow them and learnt the potential challenges too. With a combination of poly tunnels and generally warmer weather we've managed to find varieties of watermelon which will produce fruit. We selected the two varieties which we felt performed best in our trials and that’s what we’ve been growing this year.

Normally I'd say it's becoming possible to grow watermelons in Britain because we're getting slightly hotter temperatures in the summer, although that's changed in the last few weeks as we've had abysmal weather! But the weather was kinder in May and June which meant we could get the crop established. They're grown in polytunnels because they still need protection early summer when you're still getting cold night-time temperatures.

They look identical to watermelons grown in warmer climes, though they probably taste slightly less sweet than Spanish or South American watermelons. Warmth and sunlight is really important for watermelons as this increases the sugar levels, so we haven't quite got the sweetness in the crop we were after due to poor August weather, though they are nice and tasty.

Because they're new they haven't been called anything yet so are just serial numbers at the moment – though we’re open to suggestions. If customers like them and we decide to grow more next year we'll be better at growing them than we've been this year. Growing is a learning process – you never really nail it, and you learn little by little.

It's always exciting when you bring new fresh produce to the market; that excitement never goes away! Produce is always changing and so are people's eating habits. Ten years ago, if you'd said to me people would be eating hundreds of thousands of tons of sweet potatoes a week I'd never have believed you.

We’re always looking at ways in which we can adopt innovative growing techniques to try and become more self-sufficient here in the UK and provide customers with more choice when it comes to the British produce they see on shelf.

The main reaction we’ve been getting from people when they discover the watermelons were grown in Kent is one of surprise. We've had them in the office and everyone's been trying them. They'll ask where they're from and when we say our farm they say "really?!". People say they taste really good. As far as I'm aware no-one else in Britain is growing watermelons, so it’s great to be at the vanguard of something new.

We’ve also grown 300 yellow watermelons which we hope to sell from next year. The taste is very similar to the standard watermelons, but the colour of the flesh is pretty striking and they smell a bit more perfumed. They were part of this year's trial and now we know we can grow them so there's the potential to grow more of them next year.

The British watermelons grown by Joe and his colleagues at Watts Farms are available for £3 each at the following stores from 12 September, subject to availability and while stocks last: Beckton, Bexleyheath, Clapham Junction, Swanley and Tunbridge Wells.