There’s a lot of misinformation and confusion when it comes to companion planting. That’s because it is so hard to test theories. No season is the same as another, and no garden is like any other. That makes it hard to tell if it was the pairing of two plants near each other that gave you desirable results or if it was something else entirely. However, just because it is hard to have a control group, that does not mean companion planting does not work. It just means that you will need to test things out for yourself. One of the companion planting tenants that has shown some success is inter-planting different vegetables to thwart insect pests.
Basil repels Asparagus beetle and the tomato hornworm. It’s the scent that deters the insects, so gently touching the leaves to release their oils as you walk by improves its effectiveness.
Calendula, or pot marigold, repels asparagus beetle. It also attracts beneficial insects, so this edible flower is useful throughout your garden.
Chives will repel aphids and Japanese beetles. Be sure to use your chives, because it will spread quickly if you allow it to go to seed. Even the beautiful flowers are edible.
Garlic repels aphids, cabbage moths, and Japanese beetles. Planting garlic under roses to repel Japanese beetles is a classic companion planting technique.
Beautiful, fragrant hyssop repels cabbage moths. Hyssop is an excellent companion for all sorts of cole crops since they are all attacked by cabbage moth larvae.
Onions repel aphids, carrot rust flies, and flea beetles. The combination of carrots and onions has done well in testing. To foil flea beetles on my eggplants, you can try tossing onion peelings around them, although the actual onion plants work better.
Rosemary repels cabbage moths, carrot rust flies, and Mexican bean beetles. Although rosemary is not hardy in all zones, you can usually find small plants inexpensively at the start of the season, and you can always bring them indoors for the winter, as houseplants.
Oregano repels cabbage moths. However, it can be difficult to interplant because it is a spreading perennial. You could try laying freshly cut springs near your cole crops, but they will need to be replaced frequently.
The scent of mint repels aphids, cabbage moths, and even ants. You can just lay sprigs of mint among the plants you want to protect, so that the mint plant does not spread and take over the garden, but needs to be replaced often.