In the Armageddon movies, as glaciers roll over Manhattan and supermarkets vaporize from lethal microwaves, you never see a character like Russ Cohen, author of Plants I Have Known . . . And Eaten, leading groups of refugees through fields of sheep’s sorrel and chuckleberry to snack on nutritious greens.

But should we find ourselves in the midst of a disaster of similar magnitude, the bearded expert on wild food foraging, clad in cap, jeans and rubber Wellington boots, is just the guy you’d want as a buddy.

Of course, we hardly need a full-blown crisis to enjoy munching on what Lennie in Of Mice And Men called the “fattathelan.”

The more than 30 people who turned out last Saturday at Polly Hill Arboretum for Mr. Cohen’s three-hour lecture and romp in the fields were happy to enjoy May’s sweet breezes, dappled sunlight, and nibbles of such delicacies as chunks of Japanese knot weed and the leaves of lamb’s quarters which, while they resemble spinach, actually taste better and provide more nutrients.

The naturalist’s preferred snacking concentrates on invasive species since no one worries about ever running out of them. In the official list of Massachusetts Invasive Species, out of the 66 enumerated, 22 are edible. Mr. Cohen is also adamant that plentiful nutrition exists for both humans and wildlife.

“There’s no 100 per cent overlap between what people eat and what animals eat,” he points out. For instance, poison ivy berries and leaves are delicacies for birds and deer.

When we do feed from the same source, he says, chances are we target different parts of the plant or tree: “The purple-reddish berries of the shad bush — the taste is a cross between cherries and almonds — is something we harvest from the lower branches, while the birds feed on those at the top.”

The key to foraging is the use of common sense.

“Don’t pick anything along a heavily traveled roadway,” he advises. “And stay away from any plant life that looks stressed.”

Mr. Cohen, from the Boston area, maintains the best sites for foraging are in wildlife-managed areas where pollution is forbidden. He also recommends organic farms where chemicals are restricted and good tillage yields heaps of weeds growing happily along with the cultivars.

He suggests approaching the owners of properties such as these and asking, “Is it okay if I pick your weeds?”

Who could refuse so advantageous an offer?

On the patio behind Polly Hill’s wood-and-glass atrium, the plant expert has spread a blue tarp topped by dozens of tasty weeds. He holds up a specimen of black garlic that’s so invasive it’s on lots of gardener’s hate lists. Mr. Cohen explains that if you boil it first, its florets taste like broccoli rabe.

“You should dispose of it in the trash rather than composting it,” he adds. “Otherwise it spreads.”

This reminds Mr. Cohen of a favorite cookbook for edible wildlife food fans, From Pest To Pesto, put out by the Kalamazoo, Mich., Nature Center, a compendium of chefs’ ways with garlic mustard.

Mr. Cohen shows us a stalk of winter cress which he says grows all over the Island, immediately recognizable by its yellow flowers at the top of spindly shoots. It’s best eaten before the flowers bloom, and then you boil it first for another hit of the broccoli rabe experience.

In the same family, wild mustard, also an Island irregular, bears similar yellow flowers that are actually delicious, with a flavor similar to a raddish.

“They’re astronomically high in vitamins,” Mr. Cohen says in the tone of a proud papa.

Another surprising find are the leaves of the wild daisies that flourish at the edges of our gardens. You’ll need to recognize them pre-flowering because once the telltale blossoms arrive, the leaves lose their sheer deliciousness. Mr. Cohen warns of a dangerous look-alike with furry leaves that tastes bitter enough that you know you’ve picked the daisy petal that concludes, “She loves you not.”

After an hour or so spent on tarp goodies, we begin our serious exploration of the grounds.

Mr. Cohen points out that foraging makes us more aware of the seasons than shopping in stores, since today’s supermarkets, with their produce flown in from Rio and other points distant, have virtually banished the seasons as a factor in dining. His book, in fact, contains a spread sheet that can point us to the nearest vacant lot and tell us what can be gleaned from it, edible this minute of the year.

Some of this seasonal sampling can be planned in advance. Beach-plum pickers at this very time of year are on the lookout for the fuzzy white flowers that signal the existence of the otherwise nondescript bush. Mr. Cohen tells us to make a note of these blossoms in our own vicinity so we can jump on them for their berries come September.

Another Island celebrity is the rosa rugosa, whose frilly white and pink tutu buds will be everywhere apparent any minute now. Mr. Cohen says that all roses are edible, the petals a fine accent in salads, but the rugosa has the added benefit of the mid-to-late summer berries. “When you de-seed them, what you have left you can eat or make into a jam or jelly. These berries are packed with Vitamin C,” he adds.

As the afternoon wears on, we gorge ourselves on cat briars, and learn to identify sassafras for tea, the mulberry tree for berries later in the season (“You can just stand and stuff your face with them,” gloats Mr. Cohen), wine berries and wild asparagus, to name but a few of the field cornucopia we cover.

The great thing about this crash course in natural eating is that you start looking for it back in your real life. This reporter, the morning following the Polly Hill walking symposium, stopped beside a vacant lot on Tuckernuck avenue, and plucked some wild mustard leaves to eat in situ. Yum.

Wild Plants I Have Known . . . And Eaten (Greenbelt Press, $15) is available at Island bookstores, along with Linsey Lee’s book with a local slant, Edible Wild Plants (Vineyard Conservation Society, $10.95).