Joel 2:13 calls the people to rend what, and not their garments?

Answer: Their hearts

"Rend your hearts and not your garments, and return to the LORD," for He is gracious, compassionate and slow to anger.

““Yet even now,” declares the LORD, “return to Me with all your heart, with fasting, weeping, and mourning.” So rend your hearts and not your garments, and return to the LORD your God. For He is gracious and compassionate, slow to anger, abounding in loving devotion. And He relents from sending disaster.”— Joel 2:12-13 (BSB)