Answer: I think you are leaving them on the plant too long. They turn tough and fibery and the outer color changes when they are past the fresh eating stage. (Left later they can be used as you would southern peas.) Depending on the variety, they are usually picked between 65 to 80 days to harvest. Although they can potentially grow up to three feet long, it is really unusual to see them get that big. And for fresh beans, most are harvested at about a foot long and definitely before they begin to swell -- meaning start to show the bean seeds forming inside. A rule of thumb for tenderness is pick at a diameter of maybe 3/8 inch rather than going by length. Lack of water can also cause them to have thin, skinny tips while the first beans inside swell, and make them tough, but I don't think that matches your description. Picking them promptly should help them continue producing but again to some extent yield will depend on the specific variety you planted.
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