From the Super Mario Wiki
When Mario sprinkled too much of Dr. Toby's Super Plant Growth Formula onto Mama Mario's dying basil bush, it caused the plant to grow into a leafy, semi-humanoid monster that was capable of running around and holding people in place. The Plant Monster managed to grab ahold of Luigi, who tried numerous methods of freeing himself. It wasn't until Luigi tried running that he managed to escape from the Plant Monster's grasp, but the Plant Monster eventually managed to grab ahold of both brothers.
Fortunately, Mario was still able to use the phone, and called for Dr. Toby, who brought over an antidote for his formula. The Plant Monster tried to attack Dr. Toby before he could use the antidote, but Mario tackled it and physically restrained it long enough for Dr. Toby to administer the antidote, restoring the plant to an ordinary basil bush.