What is Companion Planting and How Does it Work?

hoverfly feeding on nectar osteospermum daisy flower
hoverfly feeding on nectar osteospermum daisy flower

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:

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.

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, artropods 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

12 Comments

  1. Perry says:

    What would be a good resource for matching plants together?

    1. Perry says:

      never mind found it right in front of my eyes.

      1. Blackthorn says:

        No worries, the companion planting table on this site shows you which plants work well with each other and which ones don’t. Enjoy!

  2. Perry says:

    One more question. When planting companion plants, say for example tomatoes, parsley, and garlic, what should the spacing be on these.

    1. Blackthorn says:

      Hi Perry,

      If you want to do it by measurement, just use the same spacing as you normally use for the plants. If the spacing for garlic is to plant them 10cm apart, then plant them this far from each other. If the spacing is 70cm for tomato plants, then each plant would stretch out 35cm in every direction to make a 70 cm circle. If you plant the gardlic about 10cm from where this imaginary 70 cm circles edge is, that’s fine. That’s the ‘by the book’ approach if you’re using planting tables or the instructions on the seed packets or seedling labels.

      In reality. you just use common sense to estimate how big plants get (assuming you already have grown these plants before and know how they grow), and plant them accordingly, ensuring they won’t shade each other out. Smaller plants can fit in the gaps between bigger ones (as long as they get enough light of course!) and it all comes together in a harmonious mix. I must point out, gardening gets easier with practise as you get to know the plants really well and understand their requirements and exactly how they behave.

      For example, Tomato plants need good air space between them to avoid fungal diseases, so spacing is important, while parsley can be packed quite densely as you see in garden parsley patches, with no trouble at all. Garlic has long thin leaves which don’t shade out plants and can be situated close to other plants without any issues.

      Regards

      1. perry says:

        Thank you!

        I’m starting a small 4 x 4 sq ft garden to start out and I’m very interested in your ideas, as well as permaculture, as a whole. I plan on placing tomatoes in that box, with some companion plants to help out. Plus it will all be shaded by my giant stand of Southern Pecan trees!

  3. David says:

    Hi Blackthorn, for an easy to use companion planting database check out http://www.plantingpals.com I don’t think it’s as expansive as your table but it fills in some missing plants. Thanks!

  4. Jana says:

    I would like to try table planting next spring, table is 12’X 5′ X 1’deep. What are the main concerns with table planting?

    1. Angelo (admin) says:

      A planting table is a shallow raised bed, fine for most annual vegetables as they’re shallow rooted. The biggest concern is water availability, make sure you keep up with the watering during summer as that volume of soil won’t hold much water and the plants will use it up faster than if they were planted in a deeper garden bed. You would want to mulch the soil mix so you don’t lose water from evaporation from the surface.

      Companion planting works exactly the same way as in the ground, only difference is that you can’t use some of the larger growing companion plants that need more space and root depth.

  5. ron renny says:

    I grew fave beans as a cover crop in all my boxes, staggering the sowing so they are at various degrees of maturity now. I am sowing carrots and beets now and your companion guide says tall beans are bad companions for beets. Does this still apply after the fava beans are cut down? In this case what does bad companion mean? Note I’m in Northern California and this is the season for what I’m planting. Thanks.

    1. Angelo (admin) says:

      Tall beans are only a bad companion to beets because of their height and their potential to shade out the beets. If your broad beans (fava beans) have been cut down, then they aren’t a problem. If you leave the fava bean roots in the soil, they release any nitrogen they may contain back into the soil, helping the rebuilding of soil fertility.

      1. Ron Rzesniowiecki says:

        Thanks, that is what I was hoping to be the case. Here’s an idea for your consideration, since there are several reasons for being a bad companion perhaps numerical footnotes could be added to identify which reason applies to a particular one. Now on to sowing beets, and thanks again.

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