cassia alata question

hello everyone,
well... winter's here. i have a cassia alata among other things (well, and two plumerias, and possibly a hibiscus mutabilis).
the problem is... in spring/summer i planted them out in the flower garden, they grew, now they are in individual pots... in the house.
I have a question... under what conditions will they survive? - in a cool room, do they need warmth? what about water needs?
i haven't done this. would love to be able to plant them outside once again next spring.
and... how long can i still do this?
does it harm these trees? i bet once they are too big... they'll just be... too big to handle like this twice a year...

anyway... any info is appreciated... i live in a very cold area, getting freezing degrees for a few months.
starts getting warmer in march, even april.

thanks....
Calin

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