History

• Companion planting has a varied history, depending on the region of the world. In the Americas, for instance, indigenous farmers planted corn, beans and squash together, as each compensated for the weaknesses of another. They were known as the Three Sisters. In Europe, India and China, other companions were discovered, and today the collective knowledge of these techniques are shared across borders.
  
  

Pest Control

• Effective companion planting can reduce harm to susceptible plants. Marigolds protect themselves by excreting thiopene, which is a powerful nematode repellent. Plants in its vicinity will also benefit from the thiopene, resulting in better health for your garden.
  
  

Nitrogen

• Nitrogen is an essential chemical for many plants, such as corn. Without it, the harvest will fail. However, it does not naturally replenish itself in the soil. Either fertilizers or companion plants are needed to fix nitrogen in depleted earth. Beans do so effectively, as do clover and peas.
  
  

Shading

• Some plants need copious amounts of sunlight to thrive, while others will shrivel beneath too many ultraviolet rays. By planting tall, sun-loving plants to the south of shorter shade-loving plants, you can effectively create the environments needed for both to thrive. Squash and other leafy vine plants can also provide good ground cover, protecting the soil from sunlight and maintaining high moisture levels.
  
  

Diversity

• Planting a single species in large quantities endangers it to pests. As many pest species specialize in a single plant, concentrating that plant in a single location creates an ideal breeding ground for the caterpillars, aphids and slugs that would like to eat it.