I have a few "sinks" in which I grow alpines, these seem to have been overtaken by liverworts in the last months. I have removed the slate chippings which were there as they too were covered in liverwort growths. I have added a lot more sand and grit to all the sinks and left them open to weather. I read someone had some success ridding these growths with iron sulphate. Any advice or suggestions would be welcome.