"We're coming up on the best apple trees in Boulder," said Welty, a geographer and PhD student specialising in glaciers in the University of Colorado's Environmental Studies programme. He was approaching a front-yard grove of trees.
Last summer and fall, Welty said, he never went to a supermarket for fruit. He had two apple trees in his own yard, but began looking for more produce when he bought a cider press. Once he started paying attention, he was astonished at the bounty, and determined it should be shared. Now, it can be, thanks to a website Welty started with a fellow student with shared interests in computers and urban foraging.
And the sharing goes well beyond Boulder.
Want to find walnuts free for the picking in Iowa City? Locate loquats in New Orleans? Discover where a mulberry tree grows in Brooklyn? Check out fallingfruit.org, the site Welty and Caleb Phillips launched in March. They have gathered information mapped by amateur enthusiasts across the country, and delved into inventories many cities and towns keep of trees on public spaces.
"I go around now with my head up in the canopy, looking for new things," Welty said.
When he's not cycling or walking the streets of Boulder looking up, his head is bowed over his laptop, searching for new sources or opening emails with offers of maps. As word spread, fallingfruit.org went international, with information coming from Australia, Britain, India, Israel and elsewhere.
Welty was even sent a map of breadfruit trees on the Indian Ocean island of Mauritius.
"Clearly, there are some people out there who think both these things are cool," Phillips said of the nexus of computer and food savvy. "Apparently there are enough people who think both of these things are cool that we have 600 000 sources."
Welty and Phillips had each created small, local urban edibles maps before they happened upon each other at a meeting of Boulder Food Rescue, a volunteer organisation Phillips had helped found to ferry to the needy the kind of perishable food that often gets tossed out by restaurants and grocery stores.
Phillips, a Portland, Oregon native, was headed to the San Francisco area, where he now is based as a telecommuting CU adjunct professor of computer science. The day he met Welty, he had time to do little more than hand him his card.
"I didn't expect anything to come of it," Phillips said in a telephone interview. "Then, over the next two months, it was like, every day, I was working late at night on this."
Two months later, in March, their site was launched.
Users first click on a hot spot on the site's large-scale map. Then, with a series of clicks to more and more detailed maps, home in on, say, a myrtle in Tallahassee, Florida.
A spotter has added a note about its guava-like fruit - "a bunch, and super tasty."
Links from there lead to more information from the Department of Agriculture or Wikipedia about a particular plant. Welty and Phillips welcome additions and updates.
They see the site as a way to bring together people who share their interests all over the world, online. They also envision real-world connections: Site users are advised to ask first before picking fruit on private property, which could lead to neighbours taking time to chat.
Many people with fruit trees in their yards can be overwhelmed by produce, and are more than happy for help harvesting and consuming, Welty and Phillips have found. Some of the information they have mapped comes from property owners who want the world to know about their trees.
The site also lists organisations that can get surplus produce to the hungry.
"It's a gateway activity - it certainly was for me - to some of the larger issues with food," said Welty, who grew up in Seattle and France, and says he learned an appreciation for local food in the latter.
"If Caleb and I really get to dreaming, and we do, then it's much more about invigorating an awareness of the potential for urban agriculture," Welty said.
The variety of food that can be found growing in cities is striking. Not just dates or apples, but lindens, whose leaves can be used for tea, or Brazilian pepper trees in San Francisco whose fruit can be dried, ground and used like black pepper. Welty said a friend of his has even tapped one of the sugar maples lining Boulder's main streets.
One of his own favourites is an apricot tree on the edge of a neighbourhood with grand homes dating to the 19th century. The tree, whose fruit Welty has used to make beer and jams, looks as old as the nearby homes.
Its sturdy, gnarled trunk invited climbing on a day spring flowers adorned its branches.