Assertion : The ‘spring’ variety of wheat, barley, rye are normally planted in the spring and come to flower and produce grain before the end of the growing season.

Reason : Winter varieties of wheat, barley, rye, however, if planted in the spring would normally fail to flower or produce mature grain within a span of a flowering season

Assertion is correct. Spring varieties of wheat, barley, and rye are planted in the spring and are specifically adapted to grow and complete their life cycle within the shorter, warmer growing season of spring and summer. They typically have a shorter time to maturity, so they can flower and produce grain before the end of the growing season.

Reason is also correct.  Winter varieties of wheat, barley, and rye are adapted to be planted in the fall (winter planting) and undergo vernalization, a cold treatment that helps them break dormancy and initiate flowering when exposed to warmer temperatures in the spring. If winter varieties were planted in the spring, they might not have sufficient time to vernalize and develop to the flowering and grain-producing stage within the same growing season, which could lead to a failure to produce mature grain. 

Assertion and reason both are true and the reason is not correct explanation of assertion.