I planted my first tuberous begonias right out in my flower border where they were at the mercy of rains and dampness. In the end, they rotted.
The next season, I decided to plant them in pots so I could control the moisture. I designed these myself but any clay pot about 10” inches wide at the top, 6” inches at the base, and 4 1/2″ inches deep would do as well if it has a large drain hole.
To Start Bulbs
I leave them in a warm, dark place without soil and keep them dry. When shoots are one-half to three-quarters of an inch high, I pot the bulbs in discarded cottage cheese containers, first punching some holes in each base with an ice pick and covering the bottom with a layer of coarse mica. Then I fill the box halfway with African violet potting soil.
I set one sprouted bulb on the soil in each container, filled the soil to an inch from the top, and then added water.
As the plants grow, I hill soil at the center, and this hill supports the plant and keeps it dry when the soil is watered at the outer, lower edge. You can leave plants in the cheese boxes until they are 10” to 12” inches tall.
Some bulbs produce more than one shoot. Let one grow and cut off the others unless you want additional plants. For rooting, wait until the extra shoots are 2 inches high, then cut them off and slip them in the sand.
Later transplant these to a cheese box in regular potting soil. They should bloom in about two months. Dust young plants with sulfur every ten days to control mildew.
Preparing Garden Pot
To prepare a pot for the garden, cover the drain hole with a large piece of the broken clay pot and then a handful of coarse mica. For potting soil, I mix equal parts sand, leaf mold, and compost, use it to fill the box halfway, then transfer the plant from the cheese box. I fill in the soil to an inch from the rim and hill it up at the center as before.
The Plants Need Some Sun Outdoors
They can stand more than most people think. If pots are set under a tree or on the north side of the house, put them on flagstone or brick so they will have good drainage and ventilation.
Water them (with rainwater if you can) whenever the edge of the soil is dry, and add a soluble fertilizer to the water every 10 days or so. If buds fall off before opening, it may indicate too much water.
In late fall, remove plants from the pots with their bulbs, wash the bulbs with their stems still attached, and place them in the sun for a few days until the stems fall off or until they can be broken off without damaging the bulbs.
44659 by J. H. Mura