Any inventive ideas on how to stop people stealing my plants?

I don't have a garden but I've created several dozen on my street and around my neighbourhood around tree beds. I live in an inner city location so there's a lot of traffic and the plants are often damaged or stolen. I don't mind people taking cuttings, etc.. but it is upsetting when whole plants are uprooted and taken. If they like it so much, it's right there and they can come visit it anytime!! I've had ideas about tying the roots down, getting some sort of ink spray that will leave footprints when they take off with it, dog poo? Seriously I'm ready for any interesting suggestions.. Preferably ones where people don't get hurt but just get put off.. Thanks