I want to know if I can grow watercress upside-down in hanging containers. Yours very truly, Don Massey
If you can keep the roots moist without water dripping from the pot, it should work. Watercress needs constant moisture to thrive.
https://www.gardeningknowhow.com/special/containers/upside-down-gardening.htm
https://www.gardeningknowhow.com/edible/herbs/watercress/growing-watercress-in-pots.htm
I'll be growing watercress this summer (already have the seeds). I have a box constructed with outer tray for water retention as the plants have to be kept wet constantly. But I'm not clear on soil composition. Any tips please?
You can use a standard potting mix. The important thing will be the environment. Once you get the plant within its comfortable parameters, it will not be as picky about the content of the soil.
For more information on the care of these plants, this article will help:
https://www.gardeningknowhow.com/edible/herbs/watercress/growing-watercress-in-gardens.htm