Patch perfect: How a hitherto unknown horticulturalist has become Britain's greatest allotment authority

Here is a story to silence all those people who complain that the world is full of ignorant celebrities selling fat, illustrated books off the back of a TV series: the number-one position in the gardening-book sales chart on Amazon for months now has belonged to an unknown horticulturalist.

John Harrison is a down-to-earth northerner, and his Vegetable Growing Month by Month has trumped heavyweights from DG Hessayon to Alan Titchmarsh, as well as the sexier wiles of Carol Klein and Andi Clevely. And that's without any posh PR. Published by Constable & Robinson, a strong point is its £5.99 cover price ("No colour pictures," laughs the author).

Harrison's life has been surprisingly exciting: a 20-year career in business raising venture capital all over the world, with a brief moment as a multimillionaire in Spain. ("On paper, thanks to the dotcom boom," he grins.) He now runs one of the world's most popular horticultural websites, Surfing another big trend, I point out. "It wasn't intentional!" he laughs. "I just like growing my own food.

"I always loved gardening, but I'm not a flower person," he adds. "My grandad was a vegetable grower, and I learnt from him, and also from my wife's father. In fact, when the book came out, it was my wife's dad whose opinion I was worried about most."

No doubt his father-in-law is impressed: Harrison's book is crammed with useful information, unencumbered by any trendy graphics. "Newbie allotment holders don't need a huge, beautiful picture of a swede and just three sentences on how to grow it," he laughs. Instead, it's perfect for all those gardeners who just want a book to tell them exactly what to do, and when.

And, for a more advanced query, you can try Its success comes from the fact that it combines Harrison's passion with good old-fashioned advice from an astonishing 5,800 regular posters on the forums. It's a model of what a website can be – people chatting over a virtual garden fence, posting photos of their chickens... Even the money-making bits are coupon-friendly, offering special deals to website users on brands from Harrod Horticultural to Mantis composters.

But if you're any good at growing veg, there's just one real question to ask: where on earth do you put it all once you've grown it? Harrison starts laughing. "Freezers," he chortles. "I've got a tall upright in the garage, a fridge-freezer in the kitchen, a small upright in the back bedroom..."

Plot prompts: John Harrison's top tips

1. Don't get too caught up in the organic

"I don't want to use pesticides," says Harrison. "But if you have a pest problem, sometimes you have to. I also occasionally use herbicide, especially for one-time clearing of an area. For fertiliser, I prefer blood, fish and bone anyway. But if something does badly, I use prilled urea, which isn't organic. Do what's sensible."

2. Do what you're good at

"Sweetcorn I'm really good at, and I just love it. Yet I find it tricky to grow brassicas. Mine always get caterpillars."

3. Get off to a good start

"Get your site cleared. Give it a spray. You can go organic once you've sprayed it; I've seen people just give up trying to dig it over, taking weeds out by hand. Then incorporate organic matter and get the soil in good condition. And read the back of the seed packet!"