Professor Sprout is growing Mandrakes so they can make the Mandrake Restorative Drought to reverse the petrification of students and Mrs. Norris. The Mandrakes must be harvested at exactly the right time for the drought to work properly.
Mandrakes grow one-fourth of an inch a week until they reach 3 inches tall. Then they grow three-fourths of an inch each week. When they reach 2 feet tall they have reached mature height. Two weeks after reaching mature height they are ready to harvest.