A subtropical plant, bougainvillea grows in areas that seldom receive winter frost. Even a light frost can damage or kill the plant. Young bougainvillea plants are at the most danger from freezes, but even mature plants are susceptible to damage. The bougainvillea may appear healthy immediately following a frost, but within a few days, the leaves fall off and the plant begins to decline. Protecting outdoor bougainvillea at the first threat of frost may allow you to save the plants.
Water the bougainvillea thoroughly the day before the expected frost. Moisten the top 8 inches of soil, but avoid over-watering that leads to standing water or soggy soil.
Spread a 4-inch layer of mulch around the base of the bougainvillea. The mulch helps insulate the soil and protects the crown of the plant.
Place a sheet of plastic or a blanket over the bougainvillea before the temperatures drop below freezing. Arrange the blanket so it doesn't touch the plant, using stakes to hold it away from the foliage if necessary.
Place a string of incandescent holiday lights under the blanket and around the bougainvillea. The warmth from the lights helps raise the temperature under the covering.
Use rocks or soil to anchor the bottom of the blanket to the ground. The blanket traps the warmth rising from the ground, protecting the bougainvillea.
If your area is prone to prolonged freezes, plant bougainvillea in pots and bring them indoors to overwinter.