Companion planting is the planting of different crops in close proximity with the aim of improving nutrient uptake and fertility, controlling pests and weeds or improving pollination.

Companion planting works because one or more of the following occurs:

• one plant produces a substance or substances that help another plant in various ways;
• the growing habits of one plant compliment the growing habits of another;
• one plant attracts predators that eat the pests on the companion;
• one plant repels the pests that eat its companion;
• one plant disguises its companion so that pests do not recognize it;
• one plant creates a micro-climate for another;
• one plant attracts pollinators that are required by its companion.

It should be noted that planting some plants together can create the opposite situation whereby pests are attracted or soil fertility is affected adversely.

Planting to stimulate growth

Some 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 intersperse throughout your crops and they also make great borders for your garden, and have uses of their own.

Planting to improve soil health

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 require a lot of nitrogen.

Deep-rooted herbs like comfrey, borage and dandelion collect nutrients from deep in the soil and hold the 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.

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.

Interplanting to repel pests

There are essentially two ways in which companion planting can repel pests, by changing the way a plant looks or by changing the way in which it smells.

If you were to plant 10 cabbages all together you would almost certainly lose several to catapillars unless you carried out regular spraying of insecticides. If you however interplanted them with celery and silverbeet and some peas you would be unlikely to have few if any catapillars because of the varied smells and shapes within the planting area.

Planting to attract predators

Flower gardens tend to have fewer pests than vegetable gardens because flowers attract more predators like birds, dragon flies and wasps. Many flowering plants are also highly aromatic and less attractive to pests. Planting some flowering shrubs throughout your vegetable garden will attract predators and provide your garden with some colour.

Planting to create 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.

Staying away from bad company

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

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.

Planting to attract pollinators

Pollinators like bees, flies, wasps and butterflies are important in your garden because without polination many vegetables will not bear. To obtain the best results from your pollinator attracting plants:

• Plant flowers in clumps rather than singly or in rows.
• Select plants that are known to attract pollinators in your area.
• To attract different types of pollinators choose flowers with a variety of flower shapes and colors.

In addition to planting flowers that attract pollinators, you can take other steps to bring pollinators to your garden, such as:

• Installing bat houses and bee nesting blocks;
• Keeping a hive of native stingless bees;
• Keeping a source of fresh water;
• Leaving a dead tree or limb in wooded areas as natural nesting spots;.
• Avoiding use of pesticides in and around the house and garden.