I would plant that tuberous begonia as is because you are asking this question in the month of June so warmer nights will help the plant to branch out and make blooms instead of leaves. You may want to add a branch to prop up the heavy stem if it has grown so tall that it threatens to topple over. Once you see at least two side branches you can pinch out the tallest branch in the middle of the begonia is you prefer a more compact growth form. Next year, start your tuberous begonias in March but grow them in cooler conditions to slow down the upward growth. This may mean moving them into an unheated but frost free garage at night. Now look forward to some supersized begonias this summer. Keep growing, Marianne Binetti