Companion planting is a management strategy of planting crops together. The idea is that each of the crops will benefit one another. Perhaps one repeals insects that attack another. Maybe one of the companions provides nutrients that the other plant can use. One plant could keep sun from the soil and impart weed management.
In general, we hope that a companion planting will: manage insect pests, attract pollinators, add nitrogen to the soil, block out weeds, provide trellis, harbor insect predators, and other benefits. There is not a perfect companion plant out there that can address all of these needs nor does the main crop need all of these needs met. One of the oldest companion planting methods is the Three Sisters method. In this method, corn, beans, and squash are all grown together. The corn provides trellis for the bean plant. The bean plant provides nitrogen for the corn. The squash shades out soil to prevent weed growth. All three have potentially different pollinators and pests which in turn might mean different insect predators attracted to them. One of the problems I see with the Three Sisters method may be harvesting as the plants are growing very close together. I also see it as a way of teaching children and others the benefits to growing your own food and with companion plants.
Herbs have been commonly used as companion plants. Their aromatic properties have shown in some cases to be useful in deterring insect pests. The same can be said about certain flowers such as marigold. Basil, Lavender, Mint, Nasturtium, and Peppermint have all been commonly recommended as companion plants for vegetables.
Much of the recommended companions that we see are not always tested out in a research study. They may be more anecdotal in that this person grew these together and felt like they were companions. There is not anything wrong with that notion but it may not be telling the full story. For instance, your neighbor might be growing the exact same plant as you but have better yield, no pests, and other issues compared to what you are seeing. Obviously, there is no harm in growing companion plants with others. If I plant basil to deter insects and it doesn't work, I still have basil. You may find Louise Riotte's "Carrots Love Tomatoes" book useful as it goes into a lot of different companions out there.
Your two main tasks for companion planting are determining the right match and the right setup. Some companions need to be planted in their own row. Other times the setup needs to be all grown together. Planting correctly will ensure that you are getting the benefits of the companions.
One study that I like to use is from Iowa State/Companion Planting. This study determines the best companion for tomato, zucchini, broccoli, cabbage, and lettuce when using a number of different herbs and targeting specific insects. What is important to take away from this study is that you may need many more of the companion plant for the main plant to have a lasting effect. The study also concludes that any type of herb/companion showed assistance for pest management compared to the control.
I hope this has got you thinking more about your garden setup. If you have already planted, there is still time to add companions to your garden and around your main plants. Happy gardening and enjoy your weekend!