Traditionally, watercress would be grown in running water, but that is not strictly necessary. Watercress is a native, frost tender plant. While you can collect wild watercress from streams, this can be dangerous because of the possibility of being contaminated with liver fluke, if cows and sheep use the water upstream.
Health Benefits. Weight for weight, watercress contains more vitamin C than oranges, more calcium than whole milk, and more iron than spinach.
Watercress is easy to grow in an ornamental pond. Using a plastic flowerpot, cover the drainage holes with porous ground cover fabric and then fill the pot with poor soil (too rich a soil will cause an algae bloom in the pond). Cover the surface of the pot with another piece of porous ground cover fabric, or a layer of sand, before anchoring several watercress shoots on top of the fabric with pebbles, to stop them floating away. You can use the watercress shoots from shop-bought watercress.
Alternatively, sow seeds on the surface of the soil and germinate them before placing the pot into the pond.
Place the plastic pot on a ledge or bricks so that the top of the pot is about 5 cm below the pond surface. The watercress shoots will grow roots and find the nutrients in the pot and also in the water.
It is also possible, if you do not have a pond, to grow the watercress in an old washing up bowl filled with waterlogged soil, or even in very damp or boggy ground. I find that watercress now seeds itself around my plot in odd shady patches of damp ground, and is particularly good in the Autumn.
Harvesting. The top shoots of the Watercress are best picked just before use.