Question:  How can I keep squirrels out of my planters?
Answer:  Ah yes, a burning question that anyone who has ever done any gardening in-ground or out has had to grapple with at some point.  Squirrels are cunning little foes indeed, and you will no doubt have to try different tactics to see what works and then change those tactics often to thwart these little devils with tails. So with that in mind, here are some of my favorites:

Plastic owl on a stick:  Can't do any better than this one at least for the short-term, and they are abundantly available at most garden centers or big box department stores.  You get a garden ornament and a squirrel's worst nightmare all for 10 bucks.  The only downside is that squirrels are fairly sharp little beasts and they do communicate with each other.  Sooner or later word will spread that the big boy in the pot hasn't moved in a long time and they will get suspicious.

Plastic snake:  This one might actually work for a while if you curl him up among the plants in the pot.  Arrange him so his head sticks out for added effect.  No squirrel in his right mind is going to come up and poke his nose in there, even if he suspects he's being hoodwinked. 

Assorted motion sensors:  There are a lot of really clever squirrel deterrents out there. A bird feeder pole that senses when a squirrel is trying to climb it and starts moving, a motion sensor that emits a startling ultrasonic sound when approached by a squirrel, or my personal favorite - a motion sensor that squirts a 5 second spray of water when it sees him coming.  This stuff is not just deterrent - it's squirrel training.  You will never have to change tactics if using one of these little gadgets and  you won't even have to use it anymore after the word is spread through the population that there is a scary surprise over there by that pot.

There are many many home-spun squirrel deterrents that you could certainly try.  Moth balls, hot sauce, rocks, sticks, and numerous store-bought concoctions like fox pee and other lovely scents can be purchased reasonably, but let's face it - squirrels are here to stay and so are we so we might as well have some fun with it.   

In that vein, below is my top three wish list for things I want to see manufactured as squirrel deterrents.

Jack in the box planter:  Nice sized planter for plenty of petunias with a circle in the middle that hides a scary head with lighted eyes and jaws that clap up and down - it pops up and goes to work the second it detects an intruder and the next day the problem will be solved other than cleaning up the squirrel poop. 

Car horn planter:  Forget the ultrasonic stuff.  Let's really scare that little mother. Again, you might have to clean some squirrel poop, and it's possible that an irate neighbor might present himself too.  

Squirrel catapult:  Spring-loaded device that fits various standard pot sizes.  You set your flowers in the middle and anytime a squirrel dares to enter that space, he is catapulted back out into the yard to wonder what the hell happened. This is one of those gee whiz inventions that will entertain your family and friends for years to come.

Hope this helps!