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Tell you what: here's enough vegetable and flower seeds to stock your garden for the year (and maybe beyond). No: you don't have to pay. They're absolutely free.

Well - all right, not absolutely free. It'll cost you some packets from your stock of saved seeds: you know, those freebies from gardening magazines you didn't get around to planting, or the surplus from those beans you dried last autumn.

You've probably guessed by now I'm talking seed swaps: the bring-and-buy sales of the gardening world, in which the only currency is seeds, enthusiasm and immense goodwill between gardeners.

"Everyone's mucked in today and it's been amazing," said Sara Cundy, of the , the hugely enthusiastic organiser of the in Bradford-on-Avon (they're happening all over the country, right through into April: have a look for an ). "Everyone's just come in and brought envelopes of seeds and gone away with other bits, and sharing knowledge as well - skill, passion all bound up together."

It's all about sharing and generosity. There are no rules: you don't even have to bring any seeds along yourself, though most people do, and nobody counts what you're taking away. And, it seems, if you just trust everyone to do the right thing, nobody takes advantage: there was plenty for everyone and more.

I brought along several packets of sprouting seeds, left over from an ill-advised attempt to interest my family in beansprouts this winter. Sprouted lentils are a subject we can no longer mention politely at home - but my three-quarters full packet of seed was snapped up.

I also got rid of gave away several spare packets of tomatoes, all those free packets of rocket seeds I'll never get around to sowing, some spare herbs and packets of globe artichokes, kale and alpine strawberries I won't have room for this year.

In return I filled my bag almost full: among my haul were Clematis tangutica seeds (I'm always up for a challenge), seeds (ditto) and several packets of beans, including some I'm really excited about which are descended from those cultivated by American president and gardener . Heirloom varieties, handed down from generation to generation and unavailable in the shops, are at the heart of what seed swaps are all about.

"Most seeds have to be on these national lists and you obviously pay a lot of money, so they've got to be commercially viable," says Sara. "This is celebrating how much variety and uniqueness there is out there."

Seed swaps began 10 years ago in Brighton, where two people from the local gardening club went on holiday to Canada and came back all fired up by a seed swap they'd chanced upon there.

Little did they know what they were starting. The is now the biggest in the UK, and parent to almost 30 similar events this year taking place from Sussex to Scotland.

'It's a really good way of interesting and harnessing people's passions about growing,' says Sara. 'There's been everyone from real experienced gardeners to novices. It's sharing knowledge person-to-person.'

This was Bradford-on-Avon's first Seedy Sunday - but judging from the delighted smiles on people's faces as they left, I've a feeling it won't be the last. Meanwhile, if you'll excuse me, I've got some seeds to sow...