Companion planting reduces the work of the gardener and the need for manmade chemicals by enabling plants to manage their own environment. It's like a buddy system in the wild jungle that is the planting bed, and can be as simple as two plants that trade off something to help each other. Here are some of the ways companion plants help each other:
Trap Cropping: Planting 'sacrificial plants' can help to save your food crops. One example of this is in using nasturtiums to attract aphids and other insects away from your vegetables.
The advantage of diversity: Variety ensures that even if one plant is under attack, others may either be able to help or will at least be somewhat untouched. Even just planting different cultivars of the same plant can increase resistance to pests and diseases.
Insect refuge: In an outdoor environment you may want to attract certain bugs or good predators. Certain flowers will easily attract butterflies and other good predators to the garden.
Diversifying the canopy and 'nurse cropping': Planting tall sun-loving plants and shorter shade-loving plants will create an environment that maximizes space and puts the plants' height to work. In addition to providing shade, taller plants can also help reduce wind for more delicate plants. In return, low-lying ground cover types of plants can reduce water evaporation from the soil and suppress weeds.
Biochemical benefits: Some plants actually emit chemicals that are natural insect repellants! Planting chives, for example, will repel some harmful insects, yet will still attract bees which are necessary for pollenation. African marigolds are another well-known example; they have a chemical in their leaves and roots that will suppress harmful nematodes.
Nitrogen fixation: Some plants love consuming nitrogen, which is used for growing healthy leaves and stems. Legumes will fix nitrogen into the soil, making it available to these plants.
But not everyone gets along: You should always check to make sure that the plants you have chosen are not known to actually cause problems for each other.
So after all of that, one then has to figure out where to start. There are many combinations listed on the internet, and Louise Riotte's books, Roses Love Garlic and Carrots Love Tomatos are very thorough books on the subject.
Although you can find many print and online resources, these are really a jumping off point. Most combinations have been discovered solely by the tireless experimentation and careful observation of gardeners. Your own planting and observing will be your best method.
Matt will be writing a weekly column on plants, flowers and gardening. Feel free to e-mail questions to firstname.lastname@example.org