My experiences with overwintering mini roses indoors have not gone well. They tend to succumb to invisible pests, I think because is drier in the house, and they are weaker because they haven't had as much light. They need about 16 hours of light per day, so they are also better off if you have a light system.
How cold does it get in Latvia? Could you insulate them by burying them in a larger container outdoors? I think you could put them in a dark dry place if it were also cold enough that they go into dormancy. It's my understanding that roses need some dormancy in order to set buds properly the following year.
I'm in zone 3 and minis are often not hardy here. The last couple of years I've dug a hole close to the house, taken the roses out of the containers once dormant, shaken the dirt off, and buried them in peat moss, leaves, dirt and a pot turned over top. Then I dig them up in spring and start them in pots under lights and with fresh soil indoors. It helps if I soak them first. They go outside as soon as it is warm enough to do so. In the past I've done this in February or March, once the ground has thawed (this spot next to the house often thaws early during our Chinooks) and I think I should do it a month or so later because they don't tend to do well if they have to stay indoors for too long.
I'm pondering whether I should dig up my minis that are in the ground and treat them this way, because the survival rate is much better, and growth is faster than the ones I leave in the ground.
