Triad -- Companion planting is an idea that began in nature. Plants don't naturally sprout up with things that are alike in nice neat rows. There's a wide variety. Sometimes the combinations aren't just random. The plants actually help each other out.
This is the idea behind companion gardening.
Let's start with tomatoes. You know tomatoes and basil taste good together. Did you know they taste better individually if they grow together? Plus basil repels flies and mosquitoes. According to ghorganics.com, tomatoes have other allies too. They thrive with asparagus, cucumbers, parsley and onions. They actually stunt the growth of carrots though. The carrots will still taste good, they'll just be a little smaller.
Rick Apple from A&A Plants says never plant tomatoes and potatoes together. Potatoes are the host to a disease that will kill tomatoes.
Cissie Apple from A&A Plants says to never plant tomatoes near a black walnut tree. They can get walnut wilt. Plant them at least 50 feet away. In fact, even if you cut the tree down, give it about three years before you try to plant a tomato there.
The Native Americans came up with the Three Sisters Garden. Plant corn, squash and beans together. The corn grows tall and gives the beans a support to climb on. Corn is a heavy feeder and takes a lot out of the soil. After beans die back, they release nitrogen back into the soil to replace what the corn takes out. Squash provides shade for the ground underneath- reducing evaporation.
Everyone wants a spot beside a sunflower. According to ghorganics they attract aphids, keeping them away from other plants. The sunflowers are so tough, they don't tend to be damaged by the bugs. You'll also have seeds for the birds to snack on. Plus they attract hummingbirds, which eat whiteflies. It's even possible that cucumbers taste sweeter when planted by sunflowers.
No one really wants to be planted by a sweet potato. ghorganics says their name may be sweet but they take up a lot of room and want full sun. They tend to crowd others out. They may be better off in their own pot.