Watercress, to me, tastes of the clear chalk streams of my childhood. When I eat watercress now I am nowhere near this Eden. Instead, I am surrounded by sirens, and the nearest streams are strewn with shopping trolleys.
But I've learned that you don't need the River Test to grow delicious watercress (don't go foraging for wild watercress until you've read up on liver flukes: patient.info/doctor/Fasciola-Hepatica.htm). You just need a shady corner and a pot, or in my case an old tin baby's bath, and with a little care you can pick watercress all year.
In order to thrive, watercress must be kept permanently wet. It can grow submerged in water (as it does in a stream), but will do just as well in damp soil. The simplest way to achieve this is to sit your container in a deep saucer filled with water. Periodically flush the container with fresh water to keep the pot from becoming stagnant. (In hot weather, you'll have to do this more frequently.) By early summer, apply a liquid feed such as nettle tea.
The soil outside now is too cold to sow watercress seed, but by March or April it will have warmed up enough to sow direct into a pot. The seed germinates freely, so sow thinly. Don't cover the seed with compost: it will germinate happily enough on the surface. This usually takes seven to 14 days at around 8-15C.
Even if you don't have outdoor space, you can still grow watercress as a windowsill microgreen, harvesting tiny leaves of peppery goodness once 5cm or so high. Watercress can be sown year-round as a windowsill green because it needs only a little heat to get going. It's best to sow into regular seed trays (or takeaway trays with holes punched in the bottom). Fill the tray with compost and sit it on a plate of water until the soil is saturated. Scatter seed across the surface, cover the tray with a clear plastic bag (or a clear shower cap) and place it on a windowsill. Once the seeds have germinated, you can start watering; sit the tray inside another tray so that it is kept damp. You'll get one, maybe two cuts per tray.
If this sounds like a little too much work, cheat. Go to the supermarket and hunt out a packet of watercress with visible white roots around the base of the stem. Place the stem in a glass of water, removing any lower leaves that will rot underwater, and watch the roots grow. Change the water regularly to stop it going slimy and pot into compost once you have clear signs of growth. A single stem will happily grow in a 15cm pot on a windowsill.