What is watercress?

Watercress (Nasturtium officinale) grows with its roots in running water. You know it by the hollow, juicy stems and the rounded leaves, almost like coins, arranged in pairs along the stem. It is the more peppery of the two.

Garden cress (Lepidium sativum) grows in soil like any other leafy vegetable, germinates in days and has much smaller, more divided leaves. It is the one sold as a microgreen and the one children grow on damp cotton wool.

For cooking, the practical difference is texture: watercress brings a crisp, juicy stem, garden cress is all leaf and wilts at once.

A sprig of watercress puts out roots in a glass of water within days, with no soil and no pot. It is worth not overdoing it there: in plain water it roots and holds on for a while, but it is living off its own reserves. To keep producing leaves it needs nutrients: move it to a pot with soil that never dries out — standing in a saucer of water — or give it a very diluted liquid feed. Without that, within weeks the leaves come out smaller and paler and the stem stretches.

Anyone who grows it gets a surprise: the leaves on their plant do not look like the ones in the bunch. It is not a different species, it is the same one in a different phase.

Lying flat in running water, cool and well fed, watercress grows as a mat and makes simple, round leaves like coins. That is the form you see in a stream, in a hydroponic tray and in the bunch at the greengrocer: tender, plentiful leaf — which is exactly the part you eat.

In a pot, with less water, more warmth or longer days, the stem stretches upwards and the leaf turns pinnate: one large leaflet at the tip and pairs of smaller ones along the sides. That does not mean it stopped growing — it means it started preparing to flower.

How to bring it back: cut the stretched stems back to 5 to 8 cm, keep it in half shade with water always in the saucer underneath, and pinch out the flower buds as soon as they appear. The cuttings you take root in water within days, so you get new plants out of it as well.

Watercress: small, rounded leaves, thick juicy stem, peppery flavour.

Chicory greens: long, toothed leaves, a duller green, bitter flavour. It is chicory, related to radicchio and endive, not to watercress.

Rocket (arugula): also a brassica and therefore the closest in flavour, but the leaf is long and lobed and the taste leans nutty rather than peppery.

Short rule: if the leaf is round and it bites, it is watercress; if it is long, toothed and bitter, it is chicory.

In 2014 a study published by the CDC ranked 41 fruits and vegetables by nutrient density — how many nutrients each provides relative to its calories — and watercress came first, with the maximum score.

The small print matters and almost nobody mentions it: the ranking is per calorie, and watercress is almost all water. It wins largely because it weighs so little in calories, not because a plate of watercress delivers more nutrients than a plate of spinach. A whole bunch cooks down to nothing.

Concretely, it is very high in vitamin K and provides vitamin C, beta-carotene and a fair amount of calcium for a leaf. Since it is mostly eaten raw, that arrives intact: a short blanch loses little, but a long boil takes some of the vitamin C into the water.

The heat in watercress appears when the leaf is cut or chewed: only then are the mustard compounds released. That is why it is intense raw and turns mild, almost sweet, when cooked.

From that comes a useful rule for substituting it in any cooked-greens recipe: a short blanch — 30 to 60 seconds in boiling water, then squeezed well dry — takes the edge off without turning it into a rag.

Overcooking it is a waste: you lose the crisp stem, which is half the point.

Wild watercress gathered from streams or irrigation ditches can carry parasites from livestock grazing near the water, which is why wild watercress is not eaten raw. It is a known and documented risk in the Andean region — and the same warning applies to wild watercress in Europe and North America, where liver fluke is the usual concern.

The greengrocer kind, cultivated or hydroponic, does not have that problem: wash it like any other leafy green and eat it raw without a second thought.