There are only a few main points to cover regarding winter maintenance. Here’s what I’ve done and what I do:
Bougainvillea Care In Winter
Tucson: USDA Plant Hardiness Zones 9A/9B
Santa Barbara:: USDA Plant Hardiness Zones 10A/10B
In Tucson I water mine once a week, via drip for 1 1/2 hours, in periods of no rain. Currently my drip is off & has been off for 3 weeks. We haven’t had rain for a few weeks now so I’ll see what’s happens weather wise and will most likely turn it back on soon.
In Santa Barbara I didn’t supplementally water mine at all. My bougainvilleas there were very well established. The climate is much more temperate along the coast of California than here in the high valley desert. I lived 7 blocks from the beach so my bougies received moisture via the maritime layer as well as the winter rains.
If your bougainvilleas are young, newly planted to 3-4 years old, you’ll want to water them in long, dry spells. Depending on the temps, water them deeply every 3-4 weeks. Too much water can lead to root rot or too much green growth and not as much flowering in the spring.