Tuberous begonias are loved for their large green leaves and brightly coloured flowers that resemble those of a camellia. These plants do not grow well in hot, wet and humid conditions but prosper when temperatures are cool to warm and humidity is not oppressive. Growing them in winter requires no frost, very bright light, less frequent watering and long day length. Overwintering plants first grown outdoors in summer and fall is easier than starting new tuberous begonia plants in the depths of winter.
Bring containers of growing and blooming tuberous begonias inside when a frost is expected. Anytime autumn nighttime temperatures hover between -1.11 and 2.22 degrees Celsius is time to bring the begonias indoors.
Place begonias in a very brightly lit room, near windows, but out of hot, direct sun penetrating the glass. A room that is comfortably warm in the day and cool at night is ideal.
Reduce the amount and frequency of watering on the tuberous begonia plants. Slowly the lower green leaves will yellow and drop, which is normal during this transition to winter.
Pinch back, with your fingernail tips, any long stems on the plants. Promptly remove fading flowers, a process known as deadheading.
Increase the light intensity as the day lengths shorten, edging the begonia containers closer to the window. Direct sunlight on the plants is best in the early morning and late afternoon. Pull containers back from the sunlit windows if leaves scorch or brown quickly.
Expect flowering to become less pronounced in the middle of winter, or completely cease. Keep the soil slightly dry. Do not over water this time of year, as tuber and stem rot risks are heightened.
Add artificial lighting--such as fluorescent light panels over the containerised plants-- if plants deteriorate. Short day length naturally causes tuberous begonias to wither and forces the tubers into a dormancy until spring. Consult with a local greenhouse professional for recommendations on number of artificial light fixtures and bulb types to use.
Note the increased vigour and return of flowering in the latter half of winter as day length naturally increases and sunlight energy intensifies. Monitor plants and move them further out of direct sunlight as needed to retain perfect foliage.
Increase watering very slowly as the spring equinox approaches. Allow soil to become dry to the touch before watering.
Fertilise with an all-purpose houseplant fertiliser, making the dose at one-half the strength as specified on the product label. Do this fertilisation as a watering the week of the spring equinox.
Return plants outdoors in their containers once the danger of frost as passed and springtime night temperatures are above 7.22 degrees Celsius. Or, plants may remain indoors as houseplants.
Wait until the latter half of winter to plant tubers, early February or later. If indoor artificial lighting fixtures are available, plant the tubers at anytime as long as they are subjected to at least 11 hours of light.
Plant tubers in small pots of peat moss or vermiculite-based potting media, as recommended by Jill MacKenzie of the University of Minnesota Cooperative Extension. Soil must not be soggy but barely moist as tuber rot risks are high until the plants have substantial growth in their pots.
Make sure the tuber is planted right-side-up, with the depression or "hollow" no more than 1/4 inch below the potting media surface.
Refrain from getting water in the hollow of the tuber.
Monitor for sprouts of root and leaves from the general area of the hollow just below the soil surface.
Transplant fully sprouted tuberous begonias that are 1 to 2 inches tall into a 5- to 6-inch pot with houseplant potting media. Water as needed but lightly, never getting the soil soggy and allowing it to become nearly dry to the touch before the next watering.
Follow , Steps 7-11 above in "Overwintering Plants Indoors".
It is far easier to allow tuberous begonias to go dormant in late autumn and store the tubers in a cool, dry place. Replanting the tubers in late winter when sunlight is intensifying and day length increases requires less resources.
Tuberous begonias naturally go dormant during the winter months, regardless of location. Cooler soil temperatures, less rainfall and shorter day lengths coax the plants to slow growth and often fully die back to their tubers underground. Do not expect lush begonia growth and flowering in winter without considerable work or keen understanding of growing needs.