From our perspective, we Bougainvilleas are tropical and subtropical beings, thriving in warmth and abundant sunlight. Our cells are not designed to withstand freezing temperatures. When the mercury dips below 40°F (4°C), our growth slows to a halt. A hard freeze, anything below 32°F (0°C), is catastrophic. Ice crystals form within our tissues, rupturing cell walls—a damage from which we cannot recover. Our vibrant bracts and leaves are the first to suffer, turning black and withering. Prolonged cold, even if not freezing, weakens our entire system, making us susceptible to diseases and root rot. Our vascular system, which moves water and nutrients, simply shuts down in the cold.
Your assistance before the cold arrives is crucial for our survival. As autumn days shorten, please cease fertilizing us. High nitrogen levels encourage tender new growth that is exceptionally vulnerable to frost damage. Instead, allow us to harden off; our stems need to mature and become woodier to offer even minimal resistance to the chill. Ensure we are in a state of good health—well-hydrated leading into the cold snap and free from pest infestations. A stressed plant is far less likely to survive a challenging winter. If we are in the ground, a deep watering before a predicted freeze can actually help, as the water in the soil releases heat slowly, offering a slight buffer to our roots.
For those of us planted in the landscape in marginally hardy zones (typically USDA zones 9-10), we require significant shielding. The most critical part of us is the root crown and main lower stems. Please mound a thick, loose layer (at least 12 inches) of mulch, straw, or shredded leaves over our base. This layer acts like an insulating blanket, protecting our lifeblood—the roots—from freezing solid. For our upper structure, you must create a protective barrier. Drape breathable fabric row covers or frost cloth over us, ensuring it extends to the ground to trap the earth's residual heat. Avoid using plastic sheeting directly on our foliage, as it can cause severe damage where it touches. For added protection, you can frame us with stakes and then drape the fabric, creating a mini-greenhouse effect.
For us Bougainvilleas living in containers, our needs are simpler but more urgent. We cannot survive freezing temperatures in our pots. You must move us indoors well before the first frost. Do not place us in a dark garage or shed; we require light even in dormancy. An unheated but bright space like a greenhouse, sunroom, or a cool garage with a window is ideal. The goal is to keep us in a dormant state where temperatures are maintained between 40-50°F (4-10°C). In this environment, we will likely drop our leaves, which is a normal survival response. Please water us very sparingly during this period, just enough to prevent our roots from completely desiccating. Our metabolism is nearly halted, and we require very little moisture.
As the danger of frost passes and daylight increases, you can begin our reacclimatization. For in-ground plants, gradually remove the protective layers. Do not be too quick to prune any damaged growth, as it continues to protect the inner stems from further cold. Wait until active new growth emerges in spring, then prune back to live wood. For container plants, gradually introduce us to warmer temperatures and brighter light. Increase watering slowly as we show signs of new life. A gentle application of fertilizer will help us rebound and produce the spectacular display of bracts you cherish so much.
