What if everyone around you was exactly like you?  That sounds to me like some kind of perfect hell.  Well the first and foremost rule of companion planting is diversity.  There are a lot of books written on this subject, filled with long lists, many of which contradict the other.  We say don't get too carried away by matching each plant according to someone's list.  There are however a some simple rules to grow by.  We'll expand on each below:

• Use plants with different nutrient requirements and roots at different depths as they compete much less.
• Some plants are considered natural growth stimulants for other plants
• Some plants are certainly natural growth inhibitors for other plants
• Some plants capture nutrients from deep in the soil profile and make them available for nearby plants
• You can create suitable micro-climates for one plant by using another (eg. shade and wind protection)
• Confuse pest insects using strong smelling plants, and a diversity of shapes and colours
• Invite predator birds and insects and other wildlife into your garden with appropriate plants

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

If all the plants are the same, they are competing for exactly the same nutrients at exactly the same root depths.  Planting plants that have different root-depths next to each other ensures that the plants are not searching for nutrients in the same areas.

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.  If you don't know how deep the plant roots are (Robert Kourik's Roots Demystified has many diagrams) don't worry!  Just get a diversity of plants together and it's better than a monoculture.

Natural Growth Stimulants

Plants that are considered 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, and have uses of their own.

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 broccoli, 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

Strawberries are perennial plants (they live for more than one year) whereas the other vegetables mentioned are all annuals (they live for less than a year).  In general you should keep your perennials and annuals separate because you don't want to disturb the soil around your perennials, and each group prefers and establishes different soil biology.

Eucalypts and some other plants are allelopathic, meaning that they produce toxins suppressing nearby plant growth, so you need to keep your vegies protected from their root systems.

Nutrient Providers

• Peas, beans and clover collect nitrogen from the atmosphere and convert it to a bioavailable form inside root nodules.  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.  Be careful with comfrey in the vegie growing area as whenever you cut a root, a new comfrey plant springs up so it can become invasive. It's 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 warrigal greens.  Native nitrogen-fixing ground covers include running postman (Kennedia spp.) and hardenbergia.  Other colourful, effective, native ground covers include myoporums and convovulus species.

Confusing Pests

A single monoculture of one type of plant is like a giant landing strip for pest insects. Non-flying pests can walk from one plant to the next. Pest insects find your plants by both sight and smell. Creating a diversity of plants confuses them.  Mix your plantings up so plants of the same or similar type aren't next to each other, and plant strong smelling plants like nasturtiums, wormwood, rosemary and lavender nearby.

Inviting Predators

Certain plants attract predator insects.  These include plants from the daisy family, the Umbelliferae family (think parsley, carrots, fennel, celery etc) when flowering, yarrow and alyssum.  Dense prickly habitat shrubs for small birds nearby helps too, as does a pond which brings in beneficial insects, frogs, lizards and more birds.

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 afternoon sun.  In contrast, eggplant and capsicum love full sun light.  Therefore, if we plant the lettuce among or to the south east of the eggplant and capsicum, both will be receiving their preferred amount of sun, and will grow better.

Now think of all your little plants out there growing harmoniously in companionship!  If only we could learn from them the world would be alright.