Thanks for your post! Yes, Caladiums do have to go dormant. However, in your case the growing season is shorter than what they require before they go dormant. For example, down here in Florida, USA, my Caladiums started coming up in March and will be up till October (at least) before fading to dormancy. If I had taken them out of the pots last Fall, dried them, and kept them in an area where temps didn't go below 60 degrees F, I might have kept them down for an additional month, but that still gives them a growing season of 7 months! You can "encourage" dormancy by switching to a high-phosphorus fertilization regime about 6 weeks before you want them to go down. In any event, you will have to protect them from frost and freezing in the beginning of their growth period as well as at the end.
Do you have a greenhouse or a well-lit area in your house where plants can be kept? My suggestion is that you plant the tubers in a pot that has a lot of holes (like a Superoots Air-Pot) so the roots can emerge but the tuber and pups can't escape. When you "plant" the pot, leave about 1/2 inch of the rim sticking up so you can see where it is when it is time to take them up for the winter. This way you won't have to dig and try finding all the tubers or worse, risk cutting right into tubers. After you pull them up by the pot, you can keep them growing, if you can, until they go dormant on their own, or you can just let them dry out and force dormancy.
Contrary to what some information online indicates, you do need to fertilize Caladiums if you want them to prosper and multiply. I use 18-6-8 Nutricote incorporated into the soil media before planting, then several times a week I may apply a weak (1/4 strength) solution of regular Miracle Gro soluble. This regimen makes the tubers get large enough so that I get blooms and large leaves. Blooms are essential to my hybridization efforts.
