When Capulet learns of his daughter's death, he is grief stricken. He compares death to a frost that lies on Juliet, the flower.
These are his exact words:
"Death lies on her like an untimely frostUpon the sweetest flower of all the field."
He understands that she is dead, and that it is all because of his family and the Montagues, because if they didn't fight, both Romeo and Juliet would still be alive.