In order to cut a sample of this weed for the potion, you must give the mouse something to distract it. If you didn't solve the refrigerator puzzle yet, return to the cottage and do so. Once the puzzle is solved, you'll receive the inventory item needed to get rid of the mouse. Warning: Don't try to pick up the mouse. If you do, you'll have to travel to the doctor's office to retrieve that first aid kit.
"Where is the fountain that throws up these flowers in a ceaseless outbreak of ecstasy?"