Since the plants are outside, nature will find a balance. It's only really in an indoor environment that aphids get crazy. Not to say that the aphids aren't still little bastards that will suck the life out of your plants and eat alive you and everybody you've ever loved, just that after they do so a ladybug will probably eat them.
You have time before the aphidpocalypse though. I probably wouldn't jump straight to the super-secret assassin poisons yet. Releasing ladybugs on your plants is a good, safe, natural way to go. You can also go with some dishsoap/water in a spray bottle as an easier/cheaper option. The dishsoap will kill any aphids it comes in contact with, but probably doesn't do the beneficial bugs any favours either. I'd recommend using it to kill the established infestation, then lay off of it so that the natural predators can do their work.
Also, good call on the human centipede.
