If you're looking for a natural way to get rid of aphids on your outdoor plants without using any pesticides, consider trying ladybugs. You can buy them at gardening centers and they will eat the aphids on your plants.
Once you buy them, put the container in the refridgerator for 15 minutes right before you take them outside. Only place them outside at night when it is cooler (otherwise, they'll all fly away and won't rid you of your aphid problem!). Place them at the bottom of your plant (this will encourage them to climb and eat the aphids they find along the way).
You can also purchase a Ladybug House which will entice them to stay even longer. Give it a try and let us know how it worked for you!