I found this helpful pdf from Utah State, in case anyone else is interested. I'll probably pick up a watercress bunch at the grocery store....root it in water, and plant in a pot nested inside another pot/or saucer filled with water. One more thing...does anyone have experience growing watercress in shade, or does it require full sun?

Here is a link that might be useful: Watercress in the garden

Not in a container but here is my experience growing watercress in the garden, Star Stuff:

The neighbor has a 4" irrigation pipe along the fence line. A few years ago, his main irrigation valve had a leak and the water traveled about 100 yard were it leaked out of the irrigation pipe along the fence line. It was actually leaking thru several years.

I talked to him about how I was going to take advantage of that: put in a 2" pvc pipe and ran that slowly leaking water to a little pool in my garden about 6 feet away from the fence. The pool was about 2' by 4' and 6" deep.

The soil here is very rocky so drainage isn't a problem. I just replaced some of the top soil in the bottom of the pool after I'd dug down about 10".

A trip to the grocery store provided me with the watercress starts. I remember that the pool ran dry once or twice for half a day or so. That didn't seem to make any difference to the plants. They were lush and beautiful.

The next year, the neighbor replaced his irrigation valve and my little pool was filled in and went back to being a bed in the garden. I think I embarrassed him . . .

digitSteve