Sweet potatoes are great baked or mashed, not to mention used as the basis for pie or gnocchi. But, asks CookingGirl, how do you avoid the stringy-textured ones? One solution is to respect their season. KitchenCo says that yams [ed. note: The word yams is often used interchangeably to mean sweet potatoes, presumably the intention here] are in season from August to October. At other times of the year, the vegetables you’re buying have been cured and held in storage long past their prime. KitchenCo has also heard that stringiness develops when they are grown during a particularly rainy year, and finds that it helps to select plump, round-shaped ones rather than long, skinny ones.
erica suggests using the variety of pale sweet potato often found in Latin or Asian markets. The flesh of these is a creamy white, rather than the bright orange of the more commonly encountered sweet potato, and they have none of the orange sweet potato’s stringiness, even out of season.
Board Link: How to pick non-stringy sweet potatoes