Tuberous begonias are tender perennials that grow from tuber-like root systems. They do not tolerate frost, but are easily dug up each fall and rooted again in spring after frost danger is past. Begonias are one of the few flowering perennials that bloom well in partly shaded areas, making the extra work of digging and replanting well worth the effort. Root tuberous begonias each year once the ground has warmed outside, or root them in a pot indoors in early spring then transplant them to the garden for even earlier blooms. During the growing season cuttings can also be rooted to produce new plants.
Start the tubers four to five weeks before you plan to transplant them outside. Fill a seed-starting tray with peat moss, available at garden centers. Water the peat moss until it is evenly moist but not soaking wet.
Plant the tuberous roots in the moist peat moss so that the indented side of the tuber faces up. Sow them so the top of the tuber is 1/2 inch beneath the surface of the peat moss and space each tuber 3 inches apart from the others.
Place the trays in a warm, 70 degree Fahrenheit area that receives indirect bright sunlight. Water the peat moss as necessary to keep the peat moss barely moist but not overly damp. New growth appears approximately four weeks after planting.
Prepare a partially shaded garden bed in an area that drains well and isn't prone to standing water. Work a 2-inch layer of compost into the soil to aid drainage and soil quality.
Remove the tubers from the peat moss and plant them in the garden bed so the top of the tuber is 1/2 inch beneath the soil surface. Space the plants 6 inches apart in all directions.
Water just enough so the soil is moist but not soggy. Fertilize every two weeks with a complete balanced fertilizer following label application directions, then water again immediately after fertilizing so the plants can access the nutrients.
Rooting Begonia Cuttings
Prepare to root the cuttings when the stems on your begonias are about 3 inches tall. Do not take the cuttings until you have the rooting pot ready.
Prepare a Forsythe (rooting) pot for your cuttings. Cork the drainage hole in one of the 3-inch clay pots tightly. Line the bottom of the plastic pot with paper toweling and fill the pot with vermiculite. Push the clay pot down into the center of the vermiculite.
Moisten the vermiculite well, until excess water begins to escape through the holes in the bottom of the plastic pot. Fill the clay pot in the center with water so it can leach further moisture into the vermiculite as needed.
Cut the stems to be rooted just above the tubers and insert the cuttings about 3/4 inch into the moist vermiculite. Avoid cutting into the tubers--wounds may become gateways for disease-causing organisms.
Put the Forsythe pot and cuttings into a clear plastic bag. Fasten the top of the bag to lock the humidity in.
Place the assembly in a warm spot with indirect light during the day.
Open the plastic bag once or twice a day for a few minutes to let in fresh air and to replenish the water in the clay pot. The clay pot acts as a reservoir, and its porous sides allow water to seep through into the vermiculite.
Open the bag a little more each day for a few days after the cuttings are well rooted, which should be in two to four weeks. This allows the plants to adjust to normal humidity.
Transplant the cuttings into pots and harden them off. This means gradually acclimating them to the great outdoors. Start by putting them outside in a partly shaded area for one hour before moving them back in. Water as needed. Increase the time outside gradually so that by the end of two weeks they are outdoors all day. At this point they can be transplanted into your garden.