Summarize this article in one sentence.
Seeds can be ordered online or from gardening supply stores and nurseries. Popular varieties of watercress include English Watercress and Broad Leaf Cress. You can also start growing from mature watercress purchased at a supermarket or farmer’s market. Cut the ends, then soak the base of the stalks in water for a few days to encourage root growth and proceed to plant them in soil as you would from seed. Choose a large container or planter with drainage holes that is at least 6 inches (15.2 cm) deep. Add a layer of landscaper’s cloth at the bottom of the container to keep the potting mix from escaping when you water. Add pieces of broken pots or small pebbles to the bottom layer of the container to allow for good drainage.  You can also use multiple small containers and place them in a larger drainage tray. Plastic containers are recommended over terra cotta ones, which can dry out too quickly for watercress. You need to water the plant often. It's soil should be wet at all times. You can put excess water in the tray or bucket to keep the plant wet. You can also place small pebbles in the drainage tray to allow water to flow freely into the growing container. Use a soilless mixture that drains well and contains peat moss and perlite or vermiculite. Leave approximately two inches (5 cm) of space to the top rim of the container and water the mix well. The ideal pH of the potting mix should be 6.5 and 7.5. Place the seeds 1/4 inch (.64 cm) deep in the potting mix, allowing three to four inches (7.6 to 10.2 cm) between each seed. Soak the potting mix deeply enough so that water fills the drainage tray below roughly halfway full, but doesn’t rise higher than the growing container. Replace the water in the drainage tray with fresh water every two to three days.  Make sure the tray never dies out. Check it daily to see if you need to add more water. To keep the soil thoroughly cover the surface with a thin, clear plastic sheet that has small holes poked in it, which will retain water and allow airflow. The sheet can be removed when the sprouts begin to appear above the soil. Mist the soil surface well with water in a spray bottle every other day. Position the watercress where it will receive roughly six hours of natural light each day, but try to avoid harsh, direct rays that can burn the young plants. You can keep the containers indoors or when the weather is regularly between 55˚F and 75˚F (13˚ and 24˚C) where you live, you can place the container outside during warmer months. Add a small amount of water-soluble, all-purpose gardening fertilizer to the water in the drainage tray at the package-recommended rate. Once the plants have grown roughly five to six inches (12.7 to 15.2 cm) in height use kitchen or gardening scissors to trim the top four inches (10.1 cm) of the plant as needed.  Avoid taking more than a third of any plant when cutting to allow the plants enough foliage to continue growing. Periodic harvesting helps encourage new growth. Rinse the watercress in cool water, dry it and use immediately or wrap it in bundles and store in the refrigerator for use with a few days.

Summary:
Purchase the watercress seeds. Prepare the container for planting. Place a larger drainage tray or bucket beneath the planting container. Fill the growing container with potting mix. Sow the watercress seeds. Water heavily. Place the container in indirect sunlight. Fertilize the watercress. Harvest the watercress. Wash the watercress.