The most common question that many super villains ask me is: "How can I turn my geranium into an unspeakable horror?" It's a valid question, and something I've found many a would-be mad scientist inquires about. There is, of course, a right way of growing your own chlorophyll-powered abomination and there is a wrong way.
Firstly, you should choose a plant of at least moderate intelligence. I suggest a chrysanthemum or dandelion. Weeds not only have a great deal of intellect, but have a deep thirst for vengeance. Many feel that a Venus Fly Trap would be nice, but be wary. Venus Fly Traps are undisciplined and may well turn on you, not to mention very cliché in villainous horticultural circles.
The second step is to acquire your plant. I suggest getting a plant from the wild, since florist-bought plants are already placated and domesticated. Once you have your plant, feed them high concentrations of nitrogen and uranium-232. Fresh seaweed is an excellent source of nitrogen. You'll need to replant them into bigger pots almost every week if you keep it well fed.
The third step is weaning the plant off water and onto human blood. You would think that blood being 95% water would make this task easier, but it is actually a very delicate procedure. Keep in mind that most plants are vegetarian in nature, so you don't want to shock them too much with their new diet. The fresher the blood, the easier they'll take to it, and for God's sake don't feed them animal blood. We want a man-eater, not a dog-eater!
The fourth step is to incite a hatred of mankind. The easiest ways I have found to accomplish this is either one of two ways. You can either have the same episode of Seinfeld playing over and over. The other fashion is to take your plant to a Tony Robbins seminar. There is a third option, by which having the plant join Amway, but I found that was just too cruel to the plant.
Once the plant has reached the desired height and temperament, it is time to send it off to terrorize the locals. One last bit of wisdom, if you accompany your plant, be sure to wear a fully sealed suit with a breathing apparatus. The herbicide they will use to keep your plant-monster at bay really stings the lungs.