The folks at the Petaluma Seed Bank dreamed up the first annual National Heirloom Exposition as a simple produce exhibition, with prizes and ribbons.

But like a pumpkin ripening in a sunny field, it just kept on growing.

"We started the event thinking wouldn't it be great to showcase all the produce out there," said Paul Wallace, manager of the Petaluma Seed Bank. "Then, it was, &‘Why don't we have a few speakers?'"

The three-day event, to be held Sept. 13-15 at the Sonoma County Fairgrounds, quickly mushroomed into a veritable world's fair of food featuring 60 speakers, including keynote speakers Dr. Vandana Shiva of Navdanya International in India, Alice Waters of Chez Panisse in Berkeley and Jeffrey Smith of the Institute of Responsible Technology.

Gathering experts from around the globe, the National Heirloom Exposition will serve as a fertile forum for some of the country's most pressing sustainability issues, from seed-saving and food politics to organic farming and renewable energy.

After booking the fairgrounds, the organizers decided to add an exhibit of heritage breeds of poultry and livestock, an art exhibit themed to food and farms and a series of chef's demonstrations showing how to use all that fresh produce.

To round out the old-fashioned fun, they booked some old-timey music by singing cowboy Sourdough Slim and the local yokels Blue and Lonesome of Petaluma.

Trade vendors have signed up for booths in Grace Pavilion, hand-selected food vendors plan to serve up healthy fare outside Finley Hall and movies such as "The Future of Food" will be screened throughout the event in the Garden Annex, behind the Hall of Flowers building.

To round out the farm fun, there will be demonstrations of drop-spinning with wool, milking with Jersey cows and shearing sheep.

Meanwhile, schoolkids will arrive by the busloads Sept. 14 to participate in sack races, treasure hunts, butter-making, seed planting and more.

"Several school groups will be bused in from Hayward and north up to Willits," Wallace said. "There will be local school groups as well."

All of the proceeds from the not-for-profit event will be donated back to local organizations, such as school gardens and food programs.

Tickets are $10 each day for adults, or $25 for three days. Kids 17 and under are free. To reserve and find a complete schedule of speakers and films, go to theheirloomexpo.com.

"It's great to have a pure food event, because people want to get back to basics and know more about their food system," Wallace said. "People from the East Coast are calling, because they want to be part of it."