Answer: Bougainvilleas are tropical in nature and will not winter over outdoors in your climate no matter how much protection you try to provide. So, keep your plant in a pot rather than planting it in the ground.
There are two ways to get sun and heat-loving plants through the winter. They can be kept actively growing (which means providing plenty of light, warmth and humidity), or they can be coaxed into dormancy and kept cool and asleep until early spring.
If you have a sunny spot and some extra room, most tender exotics can be kept growing and possibly even blooming right through the winter. Bougainvillea, jasmine, citrus trees and geraniums will bloom in a sunny window or on a glassed-in porch that doesn't drop below 40 degrees. A grow light that provides 12 hours of light each day will also work well.
It's important to keep these plants well watered and to fertilize regularly with a water-soluble fertilizer when they're in active growth. A good choice is Plant Health Care. Avoid crowding (a small fan improves air circulation) and keep the humidity level between 30 and 45 percent by misting or leaving pans of water to evaporate in among the plants. In cold climates keep the humidity just over 30 percent, any higher will cause condensation on your windows.
Keep in mind that the ideal winter environment for most tropical plants would be a cool greenhouse: 50 degrees at night and 65 degrees during the day. Warmer air temperatures lead to weak, leggy growth and bug problems. Whiteflies, spidermites and scale are the most common indoor pests. As long as you keep on top of the situation, they can usually be controlled quite easily with several doses of insecticidal soap. For serious infestations, you can dunk the foliage in a dishpan filled with water and a teaspoon of liquid dish detergent.
Most plants that go through the winter in fairly active growth should be pruned back at least once or twice. This is good for the plant and gives you the opportunity to root some new cuttings. I find it keeps the plants bushy and reduces pest problems.
If you don't have the sun or the space to keep your tender beauties in active growth, you can put them to sleep. Bring them into a cool, dark place and they'll get the message. Their leaves will gradually yellow and drop. Woody tropical shrubs such as bougainvillea should not be cut back until early spring (unless you need to do so in order to fit them into the house!). Keep these plants in a cool (40 to 45 degrees F), dark (or very low light) place such as the basement. Water sparingly. Revive them with water, sun and fertilizer in early spring, allowing for about a month of indoor growing time before the weather is warm and settled.
Enjoy your bougainvillea!
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