Cow parsley is the closest wild relative of cultivated chervil, and shares the same fresh, spicy flavour.
Cow parsley is the first common umbelliferate to come into flower in the spring. This is often enough to identify it, but it resembles other species, some of which are poisonous.
The most dangerous sources of confusion are fool's parsley and hemlock, and i include a table showing important differences between them. However, this is not a substitute for a well-illustrated field guide.
Hemlock also has an offensive, mousey smell when any part of it is bruised. Fool's parsley has unmistakable drooping green bracts growing beneath the flowers, giving them a rather bearded appearance.
Cow Parsley Up to 1.2 m Stout, pale-green, furrowed, slightly hairy
Fool's Parsley Up to 0.5 m Thin, hairless, ribbed, hollow
Hemlock Up to 2.1 m Stout, smooth, purple-spotted
Pick cow parsley as soon as the stems have developed sufficiently to be identified. Later in the year it becomes rather bitter. It dries well.
Do not gather the plant from the sides of major roads as it will have been contaminated by car exhausts.
Small quantities of cow parsley make a lively addition to salads, particularly cold potato, tomato, and cucumber.
It also makes a good flavouring for hot haricot beans, and with chopped chives, tarragon and parsley, the famous 'omelettes fines herbes'.
A second crop of non-flowering leaves often appears in the autumm, remaining green throughout the winter. Those able to identify the plant from its leaves can pick some fresh for winter soups and casseroles, hot baked potatoes, and in the French country dish, cassoulet.