Answer: Broccoli is a cool season crop and prefers coolness and moisture. It will bolt if temperatures turn warm or if not picked early enough. Next time you may wish to plant earlier (it is very cold tolerant) and keep an eye on the variety you are growing. The seed packet or label should give you an indication of the days to harvest. Most varieties make nice side stems so you can harvest the center "floret" early and then continue to harvest side "florets" off the plant to extend your harvest season. In any case, you need to harvest while the buds are still very tight, because once they begin to open they dry out.
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