| Tuberous begonias tend to do that as summer comes to a close. They take a rest over the winter and resume growing in spring. If you have it out in the garden you can leave it there over the winter. I usually gently weed around mine, remove the old stalks as they become loose, and keep a watch for the new shoots coming through in spring. When they do you can sprinkle some slow release beads around the plant and lightly mix them into the soil, and add some light, fluffy compost. They tolerate drenching amounts of rain over the winter provided the soil they're in is free-draining. They'll also take a few degrees of frost while dormant - but won't be happy if the frost comes when they've started into growth. If your plant is in a container - stop watering. Stop feeding. Put it into a dry and airy place to let the old mix dry out. The stems will fall off and you'll be left with this lumpy-looking flattened doughnut form with a dip on one side where the stems were this last season. Shake off as much mix as you can and keep the tuber in a cool dry place protected from mousies and insects. Check on it in late February, early March. On the side with the dip you will begin to see little pink bumps (or greenish) which is the start of the new season's growth. You can plant it into your first pot for the coming season. Remember that roots can grow from the top of the tuber as well as the bottom - and ensure the mix gives enough cover for those higher roots to form. They're a lot like sprouting potatoes: enough light to grow sturdy stems rather than leggy shoots. Wait until the leaves are well open before moving out into full-strength sunlight - and do it little by little to prevent sunburn on the leaves. | 
