You can put it in a suitcase or carry on. Reduce the root ball and clip the oldest leaves if they are oversized. Put the plant in a baggie with the rootball wrapped in damp newspaper or paper towels to keep dust to a minimum. You can also take the cut leaves in a plastic bag if you wish to start more. Convention attendees bring back dozens of begonias in suitcases both in carry on and in checked bags without too many problems (some spilled soil, some broken leaves, maybe a decapitated stem but these can always be used to start more).

Most plants like some humidity but rexes should do fine with normal air. If you burn wood then you may want to add some moisture (more for your own comfort than the plant's). Good light helps but not all that necessary either. My daughter's rex survived and thrived in a dim area of her dining area for a couple of years. (Go figure) Let the plant tell you what it likes and doesn't. I keep most of mine under shop lights in the basement but have also had them on a baker's rack in the kitchen right next to a south facing window. If I feel like it is too hot then I turn the blinds up to deflect some of the sun but I don't think it hurts the plants either. Do be aware that rexes often go into a semi-dormancy in winter so don't worry if they drop all their leaves - DO NOT keep the soil wet if this happens. Don't let it dry out completely either (they don't go into full dormancy like a bulb). Rexes also are susceptible to mildew in cooler temps. Spray the leaves with some kind of anti-fungal treatment if you see small spots appear. Mildew shouldn't kill the plant but can make it very unsightly.

Thanks! The winters are usually very mild, but we may have occasional cold spells with temp. dropping to almost zero. Frost is very rare. The soil in my garden is fast-draining due to a lot of rocks. I have tested several Begonias outdoors. Some do very well, others perish. Recently I have planted B. hatacoa and B. venosa. Time will tell if they live through the winter...

I'm pretty sure it is a humidity issue too. Hot and dry. It gets dry enough to deal with static electricity, especially in the winter. In the winter it is hot because of the heat and location (upper level floor). In the summer it is also hot. The air conditioning unit can only be fit in a wall space. The wall space isn't the plant room. Anyways I took most of the begonias outside. We will see how they do. There are still a couple in my apartment. One is in a ziplock bag and the other in a clear plastic bin. Those two seem to be okay so far. But they are also outgrowing their covered space, so what to do when they get too big to fit their container? Now what? --- Re tap water: Most of the time straight from the sink. Sometimes directly watering from the sink or shower. The shower has a detachable head. If I have any water siting around it is because I was either lazy after filling it, or filled it before having to go some place. Watering can is empty most of the time, even after filling. I have wall planters with some vines that take up a lot of water. If I have any water left over from my drinking cups, I use that too. I have about 2 watering cans I use. One regular sized and one large one. Hard water is a huge issue. There is also a some environmental political issue about there being too much nitrates in the cities (or nearby cities) water supply.. The water tastes terrible too. Kind of a fertilizer/ fishy/ algae/ chorine like flavor. Yuck. It comes from the Mississippi River. I finally resorted to using a filtered pitcher to drink from, but I need it more than the plants. I'll see about eventually getting a 2nd one to use for plants. It is just a less priority than staying hydrated myself.