Watercress is available all year round but is at its best from April until September.
Choose the best:
Go for crisp, dark green leaves, with no sign of yellowing or wilting.
Wash and shake dry just before you're about to use it. Both the leaves and stems are edible - just trim off any tough roots.
Watercress is highly perishable, so store it in a perforated bag in the fridge and eat it within a couple of days. Alternatively, treat it like a bunch of flowers and put in a glass of water in the fridge, covering the leaves with a plastic bag - it can last a little longer that way.
In a salad with rocket and orange segments; combine with potatoes in a soup; use in tarts and omelettes; use to make sandwiches or as a garnish for cooked foods such as game.
Above words and picture from here