Tuberous begonias provide bright, continuous color in shady garden spaces throughout the summer. These plants grow from a starchy, disc-shaped structure often called a bulb, which is technically a tuber. Timing the planting of these tubers is the biggest factor influencing their success and bloom duration. Because begonias originate from tropical regions, they are intolerant of cold, and frost will damage or kill the plant.
Getting a Head Start: Starting Begonias Indoors
Gardeners in cooler climates often start begonia tubers indoors to gain an advantage and achieve earlier blooms. Begin the process approximately six to eight weeks before the average date of the last expected spring frost. This early start allows the tuber time to break dormancy and develop a robust root system and initial foliage before being moved outside.
To wake the tuber, place it in a shallow container filled with a well-draining, moist potting mix. The container should be kept in a warm environment, ideally with temperatures consistently around 60 to 70 degrees Fahrenheit. Bright, indirect light is necessary to stimulate growth once the tuber begins to sprout.
Water sparingly during this initial phase, as the dormant tuber is susceptible to rot if the soil remains saturated. Once the first sprouts appear, which may take several weeks, increase watering slightly to keep the soil evenly moist. Nurture a small, healthy plant indoors, ready for transition to the garden after the threat of cold weather has passed.
Optimal Timing for Outdoor Placement
Begonias cannot tolerate frost, so wait until all danger of a late spring freeze has ended before planting them outdoors. The primary consideration for outdoor planting is temperature, both air and soil. Even for tubers started indoors, delay the move outside until overnight temperatures are reliably above 50 degrees Fahrenheit.
The soil temperature is also a factor, as begonias will not thrive in cold ground. Aim for a soil temperature of at least 60 degrees Fahrenheit for optimal root establishment and growth. Planting into cold soil can slow growth significantly and increase the risk of the tuber rotting.
Plants started indoors require a gradual acclimatization process known as “hardening off” before permanent placement outside. This involves exposing the plants to outdoor conditions for increasingly longer periods over seven to ten days. Start in a sheltered, shady spot to prevent shock from direct sun or wind, preparing the plants for their final garden location.
Planting Mechanics: Depth, Soil, and Orientation
Proper planting technique ensures the tuber can sprout and establish roots correctly. The begonia tuber has a distinct shape, with a rounded bottom and a slightly concave top. Plant the tuber with this concave side facing upward, as this is where the growth buds will emerge.
Plant the tuber shallowly, just below the surface of the soil, covered with only one-half to one inch of potting mix or garden soil. Planting too deeply can lead to excessive moisture around the tuber, which encourages rot. For container planting, the top of the tuber is sometimes left partially visible.
Begonias perform best in a well-draining, rich, slightly acidic soil amended with organic material. They thrive in locations that receive dappled shade or morning sun, protected from intense afternoon heat. After planting, water lightly to settle the soil around the tuber, and maintain consistent moisture without allowing the area to become waterlogged.