Companion planting is planting two plants together so that both grow better.  In some cases, the second plant is bait for the pests that would otherwise plague the first, more desirable, plant.  Before chemical pesticides were available, companion planting was used as a method of pest control, to provide nitrogen for plants that needed more than the soil could give, and improve the harvest of food plants.

Attract Beneficial Insects

In addition to repelling pests, companion planting can attract beneficial insects.  Planting flowers that bees and hummingbirds are attracted to can improve the pollination rates for your vegetables.  Companion plantings also take into account plants that do not get along well, such as dill and fennel, or that cross-pollinate, such as cucumbers and cantaloupes.

Sift Folklore From Fact

I should mention that much of this information is based on folklore, not scientific research.  Scientists are only now beginning to seriously research the issue so peer-reviewed studies are hard to come by.  However, there are records of companion planting in the writing of Theophrastus (300 B.C.E.), Pliny (50 C.E.), and John Gerard (1597 C.E.).  Something that endures for that long is likely to be of some validity.

The Three Sisters

The most famous companion planting practice in North America is the planting of corn, pole beans, and squash by Native Americans.  Not only did the corn stalks provide support for the pole beans, but the corn also shaded the squash from the hottest part of the sun.  The beans fixed nitrogen and returned it to the soil where the corn could use it.  All the plants benefited and grew better when planted together.

Another classic combination is planting marigolds around your tomatoes to deter nematodes.  I have done this and noticed significantly fewer problems with my tomatoes than when I do not plant the marigolds.  The bright yellow flowers look nice, too.

Need A Manual?

Companion planting can be very complicated with endless combinations of plants.  I do not have room to go into that here.  However, there is a classic book called Carrots Love Tomatoes by Louise Riotte that discusses the ins and outs of companion planting.  

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