Companion planting aims to place particular plants with other plants that are known to have a beneficial effect on their growth.  It also aims to prevent placing together plants that have a negative relationship.

Why?

• To ensure that your plants are not competing too heavily for nutrients and water
• Some plants are natural growth stimulants for other plants
• Some plants are natural growth inhibitors for other plants
• Some plants trap nutrients in the soil and make them available for nearby plants
• So that your plants can create suitable micro-climates for each plant (eg. Shade, wind protection)
• To confuse pest insects and attract predator (beneficial) insects
• To get more veges for less effort!

Natural Growth Stimulants

Plants that are known to have a positive effect on a wide range of nearby plants include chamomile, yarrow, parsley and lemon balm.  These small plants are great to plug and bare gaps in your garden with.  They’re also great to plant around the borders of your garden.

Natural Growth Suppressants

Some plants that often have a negative effect on each other, and should not be planted next to each other include:

• Strawberries with broccoli/cauliflower/cabbage
• Beans/peas with onions/garlic/chives

SOLUTION: Beans and onions both grow well next to broccoli/cauliflower, so plant your row/s of beans, then brocollli, then onions.  In this case the broccoli is called a “barrier plant”, as it is keeping apart the plants that don’t work well together

Nutrient Providers

• Peas, beans and clover are known to collect Nitrogen from the atmosphere and convert it to a solid form on their roots, where nearby plants can access it.  It is a great idea to plant these Nitrogen-fixers next to any leafy vegetables (lettuce, silverbeet, cabbage, broccoli, Bok Choy, etc), as leafy vegetables are know to require a lot of Nitrogen. 
• Deep-rooted herbs like comfrey, borage and dandelion collect nutrients from deep in the soil and hold this huge variety of nutrients in their leaves.  The leaves of these herbs can be chopped off and lightly dug into your soil to act as a fertiliser for nearby plants.  However, Dandelion and Comfrey should be planted near, but not in the vegie growing area as they can become invasive. They’re great to plant under fruit trees!
• As well as preventing erosion and acting as a living mulch, ground-cover plants are great at trapping soil particles, nutrients and water.  Edible ground-covers include nasturtiums and native spinach.  Native Nitrogen fixing ground covers include Running Postman (Kennedia spp.) and Hardenbergia.  Other colourful, effective, native ground covers include myoporum and convovulus.

Preventing Competition for Nutrients

• Shallow rooted vegetables include lettuce and Bok Choy.  
• Deep rooted vegetables include carrots, beetroot, and potatoes
• Medium root depths include tomatoes, corn, pumpkins, broccoli, etc

Therefore, if we plant a row of lettuce, then a row of carrots, then a row or tomatoes we are reducing the competition between these plants because their roots are accessing nutrients from different areas.  These will be healthier plants.

Creating Micro-climates

We can use our plants to create shade for each other.  For example, lettuce and celery do not last for as long if they receive too much Summer sunlight.  In contrast, tomatoes love full sun light.  Therefore, if we plant the lettuce among the tomatoes, both will be receiving their preferred amount of sun, and will grow better.