Begonia grandis is generally considered hardy in USDA hardiness zones 6 to 9. The lowest survivable temperatures for this plant can vary depending on the specific conditions and the duration of cold temperatures. As a general guideline, Begonia grandis can tolerate temperatures as low as -10°C (14°F) for short periods.

However, it’s important to note that prolonged exposure to freezing temperatures or sudden temperature drops can damage or kill the plant, especially if it hasn’t had time to acclimate. Providing protection, such as mulching or moving the plant indoors, can help increase its chances of survival in colder climates.

If you live in an area with colder winters or outside the recommended hardiness zones, it’s advisable to take extra precautions or consider growing Begonia grandis as an annual or container plant that can be brought indoors during the winter months.