From the Super Mario Wiki
When Mario sprinkles too much of Dr. Toby's Super Plant Growth Formula onto Mama Mario's dying basil bush, it causes the plant to grow into a leafy, semi-humanoid monster that is capable of running around and holding people in place. The Plant Monster manages to grab ahold of Luigi, who tries numerous methods of freeing himself. It is not until Luigi tries running that he manages to escape from the Plant Monster's grasp, but the Plant Monster eventually manages to grab ahold of both brothers.
Fortunately, Mario is still able to use the phone, and calls for Dr. Toby, who brings over an antidote for his formula. The Plant Monster tries to attack Dr. Toby before he can use the antidote, but Mario tackles it and physically restrains it long enough for Dr. Toby to administer the antidote, restoring the plant to an ordinary basil bush.