Answer: You can harvest peppers anytime they reach full size. Harvesting any type of pepper before it is fully mature will usually result in greater production from the bushes, while allowing them to mature fully can give increased sugars and vitamin content (especially vitamin A).
If you wait a while you might notice the banana peppers change color from a chartreuse/yellow to deeper yellow to an orange/yellow blush. This is their mature appearance. Variety will also affect mature appearance somewhat.
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