Rhizomatous begonias are a type of begonia that spreads by rhizomes, or stems, that travel over the surface of the ground. New plants grow from the joints in the stems. They are commonly found in shallow soils and rock formations, but are grown in the garden and in pots for their attractive and interesting foliage, which can be dark green or have silvery markings. Rhizomatous begonias grow well in shady locations and in containers. They bloom in warm conditions during short day-length periods such as in early spring.
Select a shallow clay pot to grow the rhizomatous begonias. The potting medium and pot must be well-drained because the stems, or rhizomes, are fleshy and have constant contact with the soil. Clay pots work well for growing rhizomatous begonias because they drain easily and don't hold moisture like plastic pots. Because the roots of the plants are so near the surface, the clay pot does not need to be deep and can be as shallow as 3 inches. The clay pot must have drainage holes.
Fill the clay pot to within 1/2 inch of the top with potting soil. The potting soil must be fast draining. The technical name for fast-draining potting soil is "coarse." It will have a low peat content because peat moss holds moisture that can rot the stems or rhizomes.
Arrange the plants and rhizomes in the prepared planting mix. Bury the roots growing from the fleshy rhizomes in the soil but leave the rhizomes to grow along the top of the soil. For stability of larger plants, you can bury the rhizomes growing from the plant just under the surface of the soil, if needed.
The potting soil should stay slightly damp, but not wet, until the rhizomatous begonias are established and actively growing. After that, water when the top inch of soil is dry.
Fertilize once a week with a standard water-soluble fertilizer formulated for container plants diluted to half strength. Add the fertilizer mixture after you water the rhizomatous begonia plants thoroughly.