Germinate the watercress seeds if you prefer to start the plant from seed. Wet a paper towel and spread the watercress seeds across it. Moisten the paper towel daily. If desired, put the paper towel in a sealed bag to keep it moist.
Wait for the watercress to germinate; this normally takes about 10 days. Once the watercress has germinated, fill 3-inch pots with potting soil.
Sprinkle several germinated watercress seeds on the soil in each pot. Barely cover the watercress seeds with soil. Keep the soil very moist. In three weeks, the seedlings will be large enough to move to the water.
Fill a bucket halfway with cool, clean water. Carefully remove the watercress plants from the soil and place them in the water. The watercress will continue to form roots in the water.
Replace the water in the bucket every day. Watercress requires clean water to thrive.