Hi Uli, glad to hear that the Alocasia hypnosa has done well for you. Mine are still growing strong but then it is still in the 90sF each day. As soon as it starts to cool down at night the leaves will yellow as the tubers prepare for dormancy. I would take your plant out of the greenhouse and let it go dormant and dry. I store the big tubers in a bucket of dry sand in a cold garage every winter (it gets down to freezing many nights but not much lower) and tubers have over-wintered fine in ground in my 8b garden. If you dig the tubers, watch for small tubers that form on the ends of underground stems. Monica Swartz in Austin, TX 8b where several Brunsvigia bosmaniae are reaching their flowering peak today!