Tuberous Begonias are like succulents and can't take too much water. This sounds like your problem if they are turning to *mush*. Dig the plant up and look at the roots. They should be white. Brown root means too mush water. They prefer shade too. Unless the plants you bought were *cuts* from a larger plant that hadn't rooted as yet, I'd say over watering is the problem. when in a pot, I myself, water from the bottom and empty any leftover in about a half an hour. In ground, there shouldn't a problem unless the soil is too heavy (clay)? They need well draining soil.

Hope this helps.

Lainey