I have the blackest thumbs! I've tried to propagate watercress a few times now, each time I manage to kill it off, always when repotting.

I've included a couple of photos of when it was doing really well, though there was a bit of yellowing on a couple of the leaves, everything seemed good. Several months passed and it grew and grew, though slowly. Eventually it needed to be repotted. The roots didn't seem too stable, or thick but the body of the plant seemed very healthy. I was very gentle when transplanting it into it's new home. Immediately I noticed it was looking frail, within a few hours the whole plant died.

I've propagated from shop bought watercress a few times, or attempted to I should say. I remove the lower leaves, place it in water and wait for it to grow some healthy roots. It seems to grow very slowly once in a pot, but does fine, until I place it in a larger pot when required. I grow it indoors, in a sunny but not direct sunlight location.

If anyone has any advice for the black-thumbed killer, that'd be great. Watercress is meant to be one of the easiest to propagate and grow..not for this clown! Also, does anyone know why the leaves sometimes turn yellow?