Why are my bell peppers only getting as big as walnuts?

If your bell peppers are staying small, the most common causes are heat stress, inconsistent watering, poor pollination, or too many fruits on the plant. Temps above 90°F (day) or 70°F (night) can disrupt pollination and fruit growth, while uneven moisture can also stunt development. In extreme heat or humidity, pollination often suffers as well. Another problem is the plant may be trying to grow more fruit than it can handle. Too many peppers on one plant can prevent any from growing large. Thinning some fruit can help the plant focus its energy and produce larger peppers.

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Related Resources:
Pepper
Peppers
Home Gardening Series: Peppers