General Saftey Tips
The most important thing to remember when harvesting edible plants in the wild is caution. For example, people often make mistakes in identification and accidentally poison themselves. For this reason, it's a good idea to sample only a tiny portion of a wild plant, then wait at least 45 minutes to an hour before having any more.
Another thing to consider is that although the plant itself is safe, it might have come in contact with undesirable substances. Plants near roads are often covered with exhaust residues or even sprayed with pesticides. The same is true for virtually any plant found in a watercourse bearing run-off from a farm. Also, wetland and waterway plants in particular might be covered with microscopic parasites. At a minimum, you should rinse them; better still, boil them. Finally, always avoid anything that smells of almonds; these plants are invariably poisonous in the wild.
Fruits and Berries
In temperate climates, a wide variety of fruits and berries grow in the wild. The wild blackberry is so familiar to many rural and suburban North Americans that setting out with a pail to pick them is a widespread summer tradition. Blueberries grow wild in cooler, highland climates. Mulberry trees bear fruit that looks a good deal like blackberries, and although uncommon, the persimmon tree also grows wild and bears edible fruits. There are also the elderberry and the gooseberry, although this should be approached with caution as it is the easiest to confuse with poisonous berries.
You can eat the leaves and berries of the pokeweed, but only after you boil them at least three times (with fresh water each time) to remove the toxins. Pokeweed berries are easily confused with elderberries, but you can make the plant edible with some thorough preparation.
Plenty of wild, nut-bearing trees grow in temperate climates. The acorns from the plethora of oak species are edible, as are hickory tree nuts and walnuts. There is also the Kentucky Coffee Tree, the seeds of which you can roast as a (very bitter) coffee substitute. However, even after roasting, the seeds are marginally toxic. Because coffee is not a survival necessity, the beverage from this tree should be consumed only by the curious. In desert climes, the acacia tree produces edible seeds.
Shoots, Flowers and Leaves
In the desert, you can harvest the flesh of the various cactus and the agave species for food and water, although you should take great care to avoid their spines. In temperate climates, common dandelion petals are commonly used for herbal tea and as a salad seasoning in a variety of modern products and dishes. The flowers of the day lily and the roots of the water lily also are edible. Watercress grows wild in many temperate waterways, and this is also edible, as are cattail shoots. Prickly leaf lettuce is safe to eat, although with its milky sap, it's easy to confuse with a poisonous plant. Finally, nettles have been collected and used in making soup for centuries.
Tubers and Bulbs
Onion grass, wild onion, wild carrots and wild garlic are all commonplace in temperate zones, and all are edible with little more than rinsing. The best plant of this type is the wood sorrel. This has a tuber that is basically a mini-potato. Another good tuber is the Indian cucumber, with its little white, crisp root core.
Mushrooms are one of the few broad categories in which a slight mistake can kill you rather than merely make you ill. The best bets for making solid identifications of safe mushrooms are in the morel family and Caesar's Mushroom. The three types of safe morels in North America are the half free morel, the yellow morel and the black morel. Caesar's Mushroom is safe but illustrates the dangers of mushroom collecting. It has a very poisonous close cousin that you can identify by its white speckles, but these speckles can wash off in a heavy rainstorm.