The eighth plague was the plague of locusts (10:4). Enough time had passed since the previous plague for the wheat and spelt to sprout (see 9:32).
The Egyptian god Osiris was highly honored by the people in Egypt. Egyptians believed he went down into the underworld and brought plant life back in the spring. For them, he was connected with renewal and life after death.
This second and climactic attack on plant life showed that even Osiris had no power before the Lord. God alone controls life and death.
New plants that grow in the spring do not give eternal life. True life comes from the Lord, not from nature or from any false god.