Begonias are popular ornamental plants, prized for their vibrant flowers and striking foliage. Originating primarily from tropical and subtropical regions, the vast majority of common garden begonias are naturally tender perennials. They are not equipped to survive freezing temperatures and are typically cultivated as annuals in most temperate climates. Outside of USDA Hardiness Zones 9 through 11, these plants require specific intervention to endure the winter months.
Categorizing Begonias by Cold Tolerance
The level of cold a begonia can handle depends heavily on its root structure, which defines the three major cultivation groups. Tuberous begonias, known for their large, showy blooms, grow from a fleshy underground storage organ called a tuber. These plants are strictly tender and must be lifted from the ground before the first frost. The tuber will rot or freeze if left exposed to cold, wet soil.
Fibrous-rooted begonias, commonly known as wax begonias (Begonia semperflorens), possess a standard fibrous root ball and are the least tolerant of cold conditions. They lack any specialized structure to survive a drop in temperature, which is why they are treated as disposable annual bedding plants.
Cane-type and rhizomatous begonias, including Angel Wing and Rex varieties, are often grown as houseplants. They share a similar sensitivity to low temperatures. These varieties rely on their stems and surface-level rhizomes for survival, making indoor protection necessary for perennial growth.
Identifying Critical Temperature Limits and Frost Damage
The most immediate threat to any begonia is a temperature drop to the freezing point of water, 32°F (0°C). At this threshold, water within the plant’s cell structure crystallizes, and the ice punctures the cell walls, leading to irreversible cellular damage. This destruction is visible as foliage and stems that turn black, translucent, and mushy soon after a hard frost.
Even sustained exposure to temperatures in the “chilling zone,” between 32°F and 50°F (0°C and 10°C), can severely harm begonias. In this range, the plant’s metabolism slows significantly, and chilling injury can occur. Signs of this stress include stunted growth, premature leaf drop, and necrotic brown spots along the leaf margins. For optimal growth, most varieties prefer a temperature range of 60°F to 75°F (15°C to 24°C). Night temperatures consistently above 50°F are necessary to maintain vigorous growth.
Protecting Begonias During Cold Weather
To preserve tuberous begonias, allow the plant to naturally enter dormancy after the first light frost causes the top growth to yellow. Cut the foliage back to a few inches above the soil line before carefully digging up the tubers. After cleaning off excess soil, the tubers must be allowed to air-dry or “cure” for several days. This process seals minor wounds and prevents fungal rot during storage.
The dried tubers should be stored in a breathable medium, such as dry peat moss, vermiculite, or sawdust. Keep them in a cool, dry, and dark location. Ideal storage temperatures hover between 40°F and 50°F (4°C and 10°C) to prevent freezing and premature sprouting. This ensures the dormant tuber remains viable until it can be replanted in the spring after all danger of frost has passed.
Fibrous-rooted and cane-type begonias grown in containers can be transitioned indoors before outdoor temperatures fall below 55°F. Before moving them inside, inspect the plants for pests and consider a light pruning to manage size. These plants do not enter full dormancy and require bright, indirect light near a window to continue growing as houseplants. During their indoor stay, significantly reduce watering, allowing the top inch of soil to dry out between waterings. This prevents root rot in the lower-light conditions.