Corn is affected very little by hail damage until some time after emergence because the growing point is below the soil surface and in the leaf whorl. Plant damage due to hail at these early stages rarely results in any significant stand or yield loss. Approximately three weeks after emergence, all nodes and internodes have developed, and the growing point is elevated above the soil surface due to internode elongation. For the next four to five weeks, the plant grows rapidly and becomes increasingly vulnerable to hail damage up through the tasseling stage, which is the most critical period. Once past tasseling, hail would cause progressively less yield loss as the plant approaches maturity.
* Source: Purdue University Corn & Soybean Field Guide 2008 Edition