Answer: This is not so unusual for this tropical plant. It often goes dormant during the winter so I suspect your bougainvillea is defoliating in protest of the cool temperatures combined with the shorter days. If you trim it back hard right after blooming and allow it to rest during the winter, it should come back for you as long as it has not gotten so cold it stressed thoroughly and died. (It would be not be that severely stressed at 40 F, but perhaps you might try a min-max thermometer to make sure it is not colder than that at night.) Ideally, you would want to keep it between 50 and 65 during the winter and water it only lightly since it is not in active growth. The roses on the other hand should be very happy in the temperature range you described -- they are far more cold tolerant plants.
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