The pictures from Japan look fairly similar to watercress cultivation and the British grower's day-job is as a watercress farmer. But although watercress is generally cultivated in a particular way, a colleague of mine tells me he grows perfectly good watercress in a normal vegetable bed: maybe the yield isn't as good that way, but it's a lot less fuss. Japan does have a serious winter with most places experiencing snow and temperatures several degrees below zero. But probably the wasabi farms don't turn into ice-skating rinks. And it is probably the getting things warm enough for long enough that matters most.