Contamination
Beware of contaminated soils. Roadsides, railroads, power lines, conventional farms, golf courses, mining sites, superfund sites, and other areas may contain dangerous herbicides, pesticides, and heavy metals.
Allergies
When consuming wild species that are new to you, start by a small portion and then wait a day. This should be enough time to allow food allergies to appear. This practice is especially important for people with other food allergies or sensitivities.
Contraindications
Herbal medicine is not one size fits all! Some species should be avoided by people with certain illnesses or conditions. Others may have contraindications with prescription drugs. It is best to check with a healthcare provider before beginning herbal regimens. If that is not accessible to you, research the contraindications online.
Poisonous Species
Depending on where you live, there may be potentially deadly plants and mushrooms growing in your area. The best way to avoid eating these is to learn where they live, what they look like, and what edible/medicinal species might be look alikes.
Identification
Practice positively identifying the foragables in your area. This requires the use of a trustworthy field guide and learning some scientific terms and principles. Identifying species by their scientific names is crucial; common names overlap frequently and can lead to confusion. (Scientific names are written as Genus species; for example Urtica dioica is the genus and species for stinging nettle.) Also, plants, fungi, and animals are classed into completely different kingdoms (the broadest of the taxonomic classifications) and therefore require completely different identification methods. Don't assume that you can consistently identify mushrooms just because you can do so with plants. And never assume you can identify something just by comparing it to a photograph! Many species require close attention to detail. You may need the help of an experienced forager, especially if book-learning doesn’t come naturally to you.
Timing
To obtain the freshest flavors and most tender textures, harvest the part of the plant that's currently growing. Generally, gather greens in spring, berries and flowers in summer, nuts in fall, and roots in late fall, winter, and early spring. Leaves should be light green, soft to the touch, and smaller than their full size. They may be partially furled or have creases from recently unfolding. When the flower stalk has started growing, it's usually past prime for leaves. The stalk is best before flower buds or blossoms form. Flowers are best harvested when nearly or completely open and not wilted. Fruits, nuts, and seeds should be completely ripe; determining ripeness depends on the species. When harvesting roots, the top portion of the plant should be dying back, completely dead, or just coming up, for fall, winter, and spring respectively.
Try and Try Again
When foraging your own food, it’s important to taste test plants as you’re picking them. (Assuming, of course, that they are safe to consume that way.) The same species can taste vastly different depending on various factors, such as the habitat, weather conditions, the microclimate, and the subspecies or cultivar. If you try one plant and don’t like it, don’t assume you won’t ever like it! You may just need to find it growing in a different area, or try a different part of the plant, or prepare it a different way. Some plants will never taste great in the field, and need to be processed to remove bitterness, astringency, or poisonous compounds. However, the same principal applies; the same species will taste different when picked from different areas.
Sustainability
Sustainability is a tricky thing to generalize because it varies so much between species. For native or naturalized plants, harvest only as much as you can use, and only when bountiful. Pick a minority of the leaves, flowers, or whole plants to leave enough for other animals to enjoy and for the plant to propagate itself. “Bountiful" and “minority" are relative terms; some plants can sustain major harvests while others are extremely sensitive. For example, wild leeks Allium tricoccum often cover acres of woodland, but this seeming bounty is deceptive. This native wildflower is harvested in massive quantities for restaurants and specialty stores, but is very slow to replace itself. As a result, it is on the dramatic decline in many areas. This is just one reason why it's imperative to research before you reap!
When harvesting invasive species, do so carefully to prevent spreading them to new areas. Even better, do what you can to manage their populations. The specific method will depend on the species.
Mushrooms have a very different life cycle from plants. The mushroom is actually the fruiting body of the mycelium, which is an underground (or underwood) network of cells analogous to the stems and leaves of a plant. Mushrooms reproduce by sending out millions (or even billions!) of tiny spores from their gills, teeth, or pores. Once this is accomplished, the mushrooms decompose, but the mycelial network remains. It is therefore difficult to harvest mushrooms in an unsustainable manner, as it is akin to picking apples from a tree. There is no evidence that digging up mushrooms harms the mycelial network, and in fact, picking the entire mushroom can be important to identification. Collecting mushrooms in a mesh bag or woven basket can help distribute spores; but usually by the time you pick a mushroom, it has already released millions of spores on its own. Carrying mushrooms in breathable containers is important to help prevent the mushroom from spoiling, however.