JWBH wrote:
Have been given a piece of growing japanese green mustard root (Wasabi).
its a bit like a tiny brownish green carrot, with hair roots at the lower
end and a few small green leaves at the top end.

At the moment its in the garden pond. Is that the best place for it?

Probably not. Most people grow it in loose, very well draining soil,
in shade or indirect light. In the wild in Japan, it is often found
along stream banks (according to what I read, not from personal
experience).

If left to its own devices it just sinks, so should I wire it to the pond side
so that the green leaves are above the water and in the air? [snip]

Commerical operations in Japan are said to grow it in sand above
flowing water but it doesn't sound like you can replicate that.

Here are two webpages with relevant info, one an Aussie gov site and
the other a nursery that sells the plant.

http://www.richters.com/newdisplay.c...ets/d6720.html
http://www.rirdc.gov.au/pub/handbook/wasabi.pdf