Unfortunately there's never just one ant
image by Jay: commons.wikimedia.org
If you see an ant on one of your plants outside, you may not even look twice. The problem with this is that there is seldom ever just one ant; look closer. If upon further inspection you come to realize it's just that one ant plus every ant in his entire colony, there's a problem. Fortunately this is the type of problem that can be solved in several different ways. For the sake of your plants and your sanity, there's a solution that can help.
Several Simple Suggestions
Line a planter that you want to keep ants out of with double-sided tape. Ants won't be able to climb up to the plant and will become stuck.
Coat the base of the stem of the plant with Vaseline; this makes it very difficult for ants to climb.
Mix boric acid powder and sweetened condensed milk into a thick consistency and roll it into balls. Place the balls around the plants and the ants will take it back to their nest and kill them.
Sprinkle catnip or set out peppermint or cinnamon near the plant. Ants don't care for the smell and will stay away.
Sprinkle cornmeal or dry grits around the plants. The ants will take home what they think is dinner and won't come back for seconds. This is a good way to attack the ants at the source, even if you don't know where it is.