Posted: 27/07/2014 at 23:51
This is one of my arum lily's. Mine are all white because when they get too big for the space I dig them up, usually in spring or autumn, and divide them. Don't know if coloured varieties need different treatment or if indeed this is the correct way to deal with it, but it has worked well for my Arum's. I find it sprouts up quite easily from bits of stray root when I divide one.
Just had a look at RHS website and they say divide in spring. Would have added a link but I'm a techno-dunce, sorry. Good luck.