The concept of certain plants sharing mutual benefits when growing together is called "Companion Planting." Traditionally this refers to how plants interact together in a way that encourages a healthy garden environment, which produces plentiful crop yields while using few pesticides and fertilizers.
Some of the reasons plants work well together are the following:
- Physical Spatial Interactions or Nurse Cropping – Pairing different growth habits, for example, a plant with shallow roots with one that has deep roots (for example, strawberries and carrots). Another combination is a pairing a taller plant to provide shade for a shorter plant that can also act as mulch (tomato plants with lettuce).
- Symbiotic Nitrogen Fixation – Pairing a plant that has a high soil nutrient requirement with a plant that enriches the soil with that nutrient (corn has high nitrogen requirement and beans may enrich the soil with nitrogen).
- Trap Cropping – Pairing a plant that attracts predatory insects with a plant that could benefit from that insect (yarrow attracts ladybeetles that eat aphids; kale is prone to aphids) or a plant that attracts pests away from other plants (nasturtiums attract aphids).
- Biochemical Pest Suppression – Pairing a plant that is reputed to repel an insect with another plant that is prone to the same insect (garlic is reputed to repel aphids; roses are prone to aphids). Plant defense chemicals are for the most part released in response to injury; the plant with repellent properties may need to be crushed (and sprayed on the target) to produce the desired effect.
Much of the folklore associated with companion planting falls into this category. Research has been done on some of these interactions, especially with plants that are allelopathic (those that release substances into the soil that inhibit the growth of others).
There are many suggested combinations of companion plants, so choosing partners can be overwhelming (see modified table below). Experimentation is a good way to find out which plants work well together under your conditions. I suggest you start slowly and begin with spatial interactions. You can move on to targeting pest problems. Keep a record of your successes and failures so you can develop favorite pairings that benefit each other.
