I think you'll want the roots to dangle in the water to filter.
Unless you have other plants that will take care of the filtering...
JMO

Nedra

"btinternet" wrote in message
...
im thinking of growing watercress as a vegi filter. I have a header pool
on
my main pond which is 10'x3' and 14" deep. I am thinking of growing
watercress in seed trays floating on the surface. What would you use as a
planting medium? I was thinking of mixing 2 parts gravel to 1 part clay
cat
litter. Is this a good idea? My main pond holds 3500 gallons and is home
to
11 koi and 4 goldfish.
thanks
Leigh