To me, nettles have always been a little like acorns--something I'd vaguely known was edible, yet never bothered to pick and eat. The first time I ate what my friend Josh calls "electric grass" was after I moved to California. I harvested them in mid- January, when they were about 8 inches high--at their prime. Nettles don't come out until March in most other places.
Liking wet places and dappled shade, nettles are easy to spot. They grow in large patches straight up, and they have large, thin leaves that look a little like lemon balm or mint, only covered with fine, stinging hairs. Never grab them by hand, or you will be stung. Wear a glove or use a thick bag as a shield.
To defeat the plants' stinging defenses, you must blanch them briefly in salty, boiling water, then shock the greens in a bowl of ice water to set and brighten the color. Simply served with olive oil and a little garlic, they are just okay--pretty to look at but bland. You really need to do something else with them. Italians use nettles a lot, especially for pesto and in pastas, mostly as a filling for ravioli. Greeks add them to their wild greens pies, like spanakopita.
The most common wild mustard is black mustard (Brassica nigra), which, if you had the patience to gather the seeds and grind them, would make one fierce mustard. Collect the seeds in late summer or early autumn, when the pods are dry. I admit I've never done this, though. Instead, I wait for the first flush of flowers, then gather "wild broccoli"--a tight cluster of flower buds of another brassica, such as mustards, broccoli, cabbages, Brussels sprouts, or collards. We've bred broccoli to have that big flower head, but wild mustard does something similar, except, like broccoli raab or rapini, it has smaller, looser heads with a definite mustard bite. They are delicious blanched for 1 to 2 minutes in salty water, then sauteed in olive oil with lots of garlic. Pick them before the weather gets hot, or the flower buds will be impossibly bitter.