What is Companion Planting and How Does it Work?

What is Companion Planting?

Companion planting is the practice of planting two or more types of plants close together for some kind of benefit, such as the control of pests, increased health and vigour, resistance to disease, or higher yields. These are termed “good companions”.

Companion planting is also concerned with plants are detrimental to each other and must therefore be grown apart. These are termed “bad companions”.

How Does Companion Planting Work?

There are several means by which companion planting works:

1. Pests Repellent and Disease Suppressing Properties

Some plants exude chemicals from their roots, leaves or flowers that suppress diseases or repel pests to protect neighbouring plants.

• French marigolds exude substances which repel nematodes from their roots, and horseradish exudes substances which protect potatoes from root diseases.
  
• Tansy and lemon-scented pelargoniums exude pest-repellent substances
  
• Clove pinks (dianthus) exude pest repellent substances from their flowers

2. Nitrogen Fixing

Plants and trees of the Legume family are nitrogen-fixers, they have root nodules which provide a home for Rhizobium bacteria. These bacteria can take nitrogen from the air and “fix” it into a form that the plant can use as fertilizer.

The bacteria exchange the nitrogen fertilizer they produce with the legume for some of the sugars it produces from photosynthesis. This is a symbiotic relationship, as both the legume and the bacteria are benefited by this teamwork. The nitrogen that is fixed by legumes also benefits neighbouring plants.

Useful nitrogen-fixing companion plants and trees include:

• Peas
• Beans
• Clover
• Cowpeas
• Fenugreek
• Lablab
• Lucerne (alfalfa)
• Lupins
• Tagasaste (tree lucerne)
• Trefoil
• Vetch
• Acacias (wattle trees)
• Austral indigo

3. Pest Decoys (Trap Cropping)

A plant that is more attractive to pests can be planted nearby as a decoy. This creates a diversion to draw pests away from the main plants you are trying to protect.

Trap crops act as sacrificial plants, they can sustain pest damage and still manage to keep growing the following year if perennial, or set seed and produce new seedlings if annual.

Dead-end trap crops attract the pest insects, but then go a step further and kill the pest insects that eat them.

• Nettles attract aphids away from other plants, and also attract ladybirds who eat aphids
  
• Land Cress (Barbarea vulgaris) and American Upland Cress (Barbarea verna) are used as dead end trap crops for controlling Diamondback moth and Cabbage moth. These plants attract moths and butterflies to lay their eggs there, and then kill the young caterpillars that eat the leaves.

4. Scent-Masking

Some pests identify their food sources through scent, by following the scent trail carried by the wind back to the plants they eat.

Companion plants with strong or repellent aromas are planted upwind to release scents which mask those of neighbouring plants, and confuse pests.

Plants used for this purpose include:

• Wormwood
• Southernwood
• Rosemary
• Lavender

5. Visual Masking (Camouflage)

Some pests identify their food sources through their physical outline (shape) of the plant.

• Pest-repellent companion plants can also be interplanted amongst the crop plant to mask their shape, making them harder to locate, so that pests miss them altogether.
  
• Mixed planting of crop plants with other plants, such as culinary plants or beneficial insect attracting flowers can make it harder for pests to identify them.

6. Stacking

Companion plants can also benefit neighbouring plants by creating microclimates or protective environments that support plant growth and protect from sun, wind or cold.

In the permaculture principle of stacking, many layers of plants are used, with taller growing plants that need more sun providing supportive cover for lower growing understorey plants that need more shade. These in turn can create a sheltered ground level for more delicate ground cover plants, which results in all the plants receiving the conditions that they need to grow optimally.

The net effect of plant stacking is that more plants are growing in a given space, resulting in higher yields per area.

7. Nurse Cropping

In forestry, a nurse crop generally is a crop of trees or shrubs whose height or dense-canopy protects more vulnerable plants during their development from frost, sun or wind by providing shade or acting as a wind break.

In agriculture, a nurse crop generally is a crop of annual plants used to assist in establishment of a perennial crop. Nurse crops reduce the growth of weeds, prevent soil erosion, and protect tender seedlings from excessive harsh sunlight.

Oats are commonly used as a nurse crop in agriculture to protect legumes such as clover and lucerne as they establish. The nurse crop serve a dual purpose, as they can usually be harvested for grain, straw, hay, or pasture.

8. Habitat for Beneficial Insects and Other Fauna

Another way to control pests is to attract beneficial insects, arthropods and birds into the garden to eat the pests.

Beneficials which control pests in the garden include:

• pollinators, such as bees
• insects that are predators of pests, such as ladybirds, lacewings, hover flies, praying mantids
• arthropods that are predators of pests, such as spiders and predatory mites
• parasites of pests, such as wasps

Beneficial insects need companion plants which provide their basic needs:

1. Nectary plants, which provide nectar as an alternative food source
   
2. Insectary plants, which provide a permanent habitat for them to live in, and overwinter

For example,a corn field, which contains nothing but corn, is an ideal place for pests that eat corn to live and feed, but provides nothing to support the “good bugs” that eat these pests. There is nowhere for these beneficial insects to live, and no nectar sources!

Plants with shallow flowers, such as those from the daisy (Asteraceae) family such as calendula, the parsley, carrot, dill (Apiaceae) family and others such as Sweet Alyssum are used for this purpose.

Perennial plants are required to provide homes for beneficial insects.

Trees, shrubs and plants which attract birds that eat pests are a useful addition to gardens for natural pest control.

9. Biodiversity

Having a mix and variety of plants together creates a more resilient ecosystem if pests or adverse weather conditions weaken or wipe out a particular variety, or type, of plant.

This biodiversity provides a form of security that ensures that the whole ecosystem does not collapse because one type of plant is attacked or fails.

For more companion planting information, what plants you should and shouldn’t grow together, please see the listing of all the good and bad companion plants in our Companion Planting Table