What plant did Reuben find in the field that Rachel wanted?

Answer: Mandrakes

During the wheat harvest Reuben found mandrakes and brought them to Leah; Rachel begged her for some.

“Now during the wheat harvest, Reuben went out and found some mandrakes in the field. When he brought them to his mother, Rachel begged Leah, “Please give me some of your son’s mandrakes.””— Genesis 30:14 (BSB)