Companion planting can be a fuzzy and confusing topic. Books often seem to parrot other books rather than either data or personal experience. Some purported companions, like tomatoes and carrots, are pretty much unworkable. (Don't miss Carol Deppe's hilarious attempt to make those two work in her book The Tao of Vegetable Gardening.)
But successful companion planting is something we see every time we walk in the woods or look at a grassy field.
Native prairies combine grasses with legumes and flowering plants in a community that fertilizes and defends itself. Traditional pastures do too. Farmers in many parts of the world grew crops mingled together, some by tolerating chance associations and some by design. A few groupings have been recognized and become standards.
A classic grouping is the Three Sisters of North and Central America--corn, pole beans, and squash (more on this one in the following post.) Traditional African farmers combined millet or sorghum with cowpeas and yams; again, there is a grain, a legume, and a broadleafed groundcover. In the best groupings, each offers a "service" to the others, such as shade, root exudates, nitrogen fixation, pest control, pollinator attractant, smothering weeds, or other benefits. But to be honest, we don't know many of the ways plants interact. Sometimes all you can do is what gardeners have always done--just try things out.
Plant pairings allow you to grow more in a given space mostly mostly by staying out of one another's way. Here is a checklist for making your own pairings:
Finding vegetables to match can be difficult. Herbs and flowers are easier to use as companions for vegetables than other vegetables are.
They demand less fertility from the soil (often less water, too) and provide the nectar for beneficial insects. In wild systems, the sheer diversity of plants protects them all from pest and disease outbreaks. In gardens, there are fewer kinds of plants, and those are less able to defend themselves. The compounds that help plants resist pests often make them inedible. By growing succulent leaves fruits or seeds for us, plants leave themselves more open to attack. So it is especially important to add lots of nectar-producing flowers for insects like lacewings, minute pirate bugs, hoverflies, parasitic wasps, and other predators to protect vulnerable vegetables. You can find seeds for often-used and effective herb and flower companions HERE
Here are some of the services non-vegetable companions can offer: