Root crops have just as much foliage as any other plant. They need the foliage (leaves) to do photosynthesis. Without enough foliage they can't do the photosynthesis needed to produce the the chemical changes needed to develop the root crops. Regardless if it's a root crop, or above ground fruit crops, they both need enough foliage to do the photosynthesis needed to produce the fruit (above ground or below it).
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