Room for Growth
After the sprouts are an inch or two high, transplant tubers to roomier quarters. They may be well spaced in large flats, but individual four-inch pots are better because roots will not be disturbed when plants are shifted later. You may even take a short cut now and move them to the final containers.
The size of the permanent containers will depend on the size of the tubers. If they are two inches in diameter, they will need six-inch pots. Give three-inch bulbs eight-inch pots, a size well suited to large plants. Because glazed pots and wooden containers stay moist longer and do not accumulate mosses and fungi, they are ideal. In window boxes, space plants five to six inches apart, keeping taller kinds in the back and pendulous or multiflora (nana) varieties along the front.
To grow well, sprouted tubers need a special potting mixture-equal parts good garden soil, leafmold or peatmoss, old manure (or a small amount of dehydrated), plus sand for drainage. If the plants are in small pots, use this mixture when transferring them to larger pots. When frost danger is over, plants can go outdoors, but shade them for the first week.
Need for Sun and Food
Finally, wherever you place containers, be certain
plants receive filtered or light sunshine for about three
hours a day, preferably early or late in the day, when rays
are not so hot. Many gardeners misinterpret shade to
mean the dense shade of low-branching trees. Here, most plants do very poorly. Open shade on the north sides of houses, and filtered and checkered shade through a lacy network of high branching trees offers an ideal environment for these plants. In cooler climates they can tolerate more direct sunlight.
Tuberous begonias are heavy feeders. Even when you prepare the soil richly, plants will need more sustenance through the growing period. Apply liquid fertilizer, according to directions, while plants are growing actively or topdress wth dry manure about every three weeks. But also keep in mind that too much fertilizer can burn foliage and cause bud drop. To send strength into root and leaf development, cut off the first bud while it is rather small (but not if you are anxious for early flowers). After blooming starts, nip off the two small female flowers on each side of the large, showy male blossoms. This will increase their size.
Keep Moist but Not Wet
Always keep plants moist, but not wet. Usually, watering once a day is sufficient, but this will depend on the type and size of containers, their placement, and the weather. Move plants around until you find places where they grow best. Also turn them to prevent one-sided development.