Start tuberous begonias indoors from February through early April. Embed the tubers hollow side up in a 3” deep tray of moist peat moss. Cover with 1/2” of peat. Keep just barely moist. When the leafy shoots grow to 1-2”, pot each tuber in a 4-6” pot that has good drainage. Use 50% potting soil and 50% peat moss. Bone meal can be added at this time. Keep plants in fairly good sun at this time so they don’t get spindly. Turn pots frequently to avoid lopsided growth.
The plants can be moved outside when there is no more danger of frost. Expose the plants gradually to outdoor conditions by putting then outside in the shade during the day and bringing them in at night for a few days. They can be planted outdoors in containers, hanging baskets, or directly in the ground. They prefer well-drained, peaty soil in partial shade and out of the wind. If planting directly in the ground or in large containers, stake the plants at time of planting.
Use high nitrogen fertilizer when the plants are young to develop big leaves and sturdy stems. Switch to high phosphorous fertilizer when flowering time nears. Feed every 2 weeks. Water at the base of the plant. Don’t allow leaves to stay wet overnight.
Lift tubers in the fall before the first frost. Dry the tubers for a few days, then shake off most of the soil. Store covered with dry peat moss or vermiculite. Check periodically and mist very lightly if the tubers start to shrivel. Overwinter at 45 degrees.
To avoid fungus diseases, dust tubers with a fungicide before planting and storage. If necessary, spray with a fungicide during the summer. Good air circulation is also important.
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