Reiger Begonias don't like to be overwatered...once a week is usually enough if indoors and maybe twice a week if outdoors. If possible, water from the bottom by soaking the pot in water because they are prone to powdery mildew if the leaves get wet. If this isn't possible, be careful not to wet the leaves when you water overhead. You say there is nothing in the middle, so that's where I would water them.

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