Hi Vicki, you can grow watercress in tubs or trenches, you just need to keep the soil quite wet. You can also cheat and get the stuff from the bags in the supermarket, and if you leave it a couple of days you'll see it start to produce little roots. I'm sure if you were to plant this up in wet compost it would grow away nicely for you, not sure about this time of year though, think March is better for these kinda 'cuttings'. There was another post about watercress somewhere on the board, you could try a search and will probibly find more info. I grew landcress and watercress last year but I think the chooks got the watercress . Good luck
CC