It can definitely be a bit tricky to get your Bougainvillea to survive the winter in climates where it can't grow outside. There are, however, some tips and steps you can follow to give your plant the best possible conditions:
Check the soil Before moving your plant from where it has been during spring / summer, make sure the soil is dry to avoid mold growth.
Check for pests When your plant is moved to a new location and proceeds into the winter months, it might get a bit weak. Its normal resilience against pests is therefore lower. Make sure to check that your plant is healthy and free of pests before moving it to its winter location.
Pruning It might be a good idea to cut back your plant severely before bringing it inside. This will help with handling and space issues, and you'll also be rewarded with more dense growth and colorful flowers the following year.
Lower the temperature Place your Bougainvillea in a cool room. For example, a garage, shed, basement, attic, summerhouse or a cool stairway if you are living in an apartment. You can also place it on a sheltered balcony if you keep control of the temperature (so it doesn't get too cold). Recommended temperature for this plant:
Add extra grow light Pick a full-spectrum light bulb and place it 20-30 cm / 8-12 inches above the plant to prevent leaf burn. Keep the light on for 14-16 hours / day.
Reduce watering Cutting back on watering during winter is key to help your Bougainvillea to survive. The amount of water it will need will depend on the amount of light it will get and what temperature it's placed in. But in general: it's better to water it a bit too little during winter than a bit too much.
“Never ever feed your Bougainvillea during winter. You can also flush the pot during the last waterings before it's time to winterize it to rinse away any leftover fertilizer." — PLANTA TIPS