Answer: If the leaves are dry when you gather and tie them, the heart of the plant won't rot. If there's rainfall, or overhead watering done after you've tied the leaves, you'll need to untie and allow them to dry before retying. It's labor-intensive, but it's the best way to blanch endive. I think when you put cardboard over the plants you excluded light and perhaps trapped moisture, which is why the leaves turned brown.
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