If the carts are black I would worry. Black absorbs sunlight and heats up. White and silver reflect light so they will not burn your plant like a black frame will. But silver will catch the eye and thats not good.

Whenever you are looking at security you gotta think like a thief. Where to enter? How to do the job fast. And how to get out of there fast. Where will they park? And most importantly what you going to do if you catch them?

I find a pack of dogs in a fully fenced yard with Beware of Dogs all over it works best. With rip offs it is not a question of if but when. And if your dogs maul the first rip, word will spread that you have dogs and others will not want to happen to them , what happened to the first guy.

Just some random thoughts.