My husband mounted these to the outside garage wall in our backyard, he used anchor bolts drilled through the tires and into studs. I then added organic soil and a spray of favorite plants. The issue I have found is that it needs watered frequently in the spring and summer months, so easy idea is to take a 2 liter bottle and drill a hole in the lid and then fill with water, bury the top of the bottle in the soil and it will water slowly, just keep checking the bottle water level. If Plastic isn’t your thing, then drilling a hole in a cork and re-cork a water filled wine bottle and it’s a classier water dispenser.
Off The Shelf 
Brilliant! Why did I never think of this? Ugh, probably because I have a black thumb and could kill concrete! But this WILL be happening as I’ve had the same issue as you, and like you, they just keep getting moved from one hiding spot to the next til I nag again. And I know just where I’ll hang them! You my lady are of brilliant mind! I’m crafty, but this? Nope, never. Only because if I even look at something planted in them they will die. Except for this ornamental grass I have the magic touch with. And callas as you have in yours. Only two things that won’t shrivel on me. So Carly rose and callas it will be! Thanks for a lovely idea! Now for football season to end, and a weekend where it doesn’t rain…..my three men will have a honey do list so long they’ll never finish if I don’t get off your pages! Lol! If they get mad, I’m sending them here! Lol!
Lol if you can’t beat them… enjoy!