Chinese purple long beans are available year-round. A cousin to the cowpea or black-eye pea, Chinese purple long bean is much more of a trailing, climbing variety, and often reaches heights of 9-12 feet. The pods grow to 12 to 30 inches long. Pick the pods at maximum length but when they are smooth, before the seeds mature or expand. At this young stage, they can be snapped and cooked in various ways: stewed with tomato concasse sauce; boiled and drained, then seasoned with lemon juice and oil; or simmered in butter or oil and garlic. The dark purple bean is meatier and sweeter than the dark green bean, which has a less delicate taste. 

USDA storage recommendations are 40° to 45°F (4° to 7°C) at 90 to 95 percent relative humidity, with an approximate storage life of 7 to 10 days.

Here is an informative site about Chinese Long Beans and other ingredients from the Far East:

http://wegmans.com/kitchen/ingredients/orient/veg.html#Chinese Long Beans