Petunias are always the best place to begin for all new gardens. They benefit so many vegetables (especially Beans!)
The Petunias act as a magnet for many harmful insects, like aphids and the dreaded Mexican Bean Beetle. Although they may deter other harmful insects.
The trick when companion planting with trap crops, such as the Petunias, is to monitor closely. Trap crops do not deter insects out of your garden. They attract the harmful insects away from your food crops. In this case the beans.
Many gardeners have said that the Petunias will actually deter many harmful insects. This may be true as well, but from what I've experienced over the years you would do well to monitor them as a trap crop.
I have watched as Butterflies, Bees and Ladybugs are attracted to the Petunias. This is good! The ladybugs will get to work on the aphids, while the butterflies and bees stay busy pollinating and putting forth a lovely show.
In the photo above you may notice Cilantro blooming on the end on the Petunias. These blooms beckon the ladybugs into this area of the garden.
There are many other types of plants and flowers that will beckon the ladybug. You might want to read more about this on a previous post I've written "How To Attract Ladybugs To Your Garden."
Luckily I've never had to many problems with Mexican Bean Beetles. They can really cause some destruction. So as you monitor your Petunias, keep an eye out for the populations of both the aphids and Mexican Bean Beetles.
The beetles can be crushed with fingers (gloves on) of course. Although lots of gardeners don't care if they squish with bare hands. This method is quite effective, but if they swarm or you have a ladybug deficiency, by all means spray them with a mild soapy solution. (Never Use Chemicals)
The happy news is that your beans are safe and guarded by their love, The Petunia.