What is companion planting?

by Kate McKee July 11, 2016

Companion planting is a method of growing particular plants together on the basis that they will benefit each other in some way. Grouping plants together is a form of permaculture, meaning that it is a consciously made agricultural ecosystem that is both environmentally sustainable and self-sufficient. Simply put, companion planting is a way to work with nature rather than against it.

What is companion planting?

A good illustration of what companion planting is all about is the “Three Sisters” system, used by Native Americans for many generations. The Three Sisters are vegetables — corn, squash and climbing beans — that when planted together provide specific benefits to one other. The beans climb up the corn stalks, reducing the need for beanpoles, and provide the soil with nitrogen that boosts crop fertility. The creeping ground squash keeps out weeds and retains soil moisture, while its prickly stem works to dissuade pests. Now, that’s teamwork!

The phrase “companion planting” in itself, although widely recognised, is considered too vague for many plant scientists and professionals. “Intercropping” or “plant associations” are actually more accurate terms but “companion planting” seems to have stuck.

There are a number of documented benefits from grouping certain plants together, though admittedly these benefits can be contradictory. There are myths and conflicting advice about companion planting that can be confusing to say the least. Some of the misunderstanding is down to the fact that Australia has a different environment from many other parts of the world and what works for gardeners in Europe won’t necessarily be successful here.

It’s also important to note that companion planting is not a cure-all for your Garden maladies but rather a complementary method to be used in conjunction with other topical applications and soil health.

Disclaimers aside, the good news is that companion planting does work and can help improve the health and visual appeal of your garden. Grouping certain plants together can deliver physical, chemical and biological changes that can improve the formation and survival of certain plant species as well as cut the time you spend of getting rid of those pesky garden pests.

Improve soil health naturally

Peas, beans and other members of the legume family actually have the ability to capture nitrogen from the atmosphere. This means plants that grow in the vicinity of peas and beans profit from the nitrogen legumes add to the soil.

“Practise crop rotation and don’t plant the same family of plants in the same spot in the garden each season,” suggests James Quealy, organic gardener and founder of Organic Food Gardens, which specialises in permaculture methods and creating home vegetable gardens.

“Each plant family will use certain elements from the soil. Lettuce and spinach, being leaf vegetables, use lots of nitrogen from the soil so, after a season of harvest, replace the leaf family with peas and beans.” Another combination to try is grouping silverbeet with beans and onion.

Confuse pests with decoy tactics

Planting a number of different species in the garden can actually confuse garden pests. Flowering plants and herbs release perfumes that can work to mask the smell of fruits and vegetables.

“By planting herbs with vegetables, certain herbs can provide a variety of smells that confuse garden pests,” says Quealy. Plants such as thyme, lavender and geranium produce strong oils that help mask the scent of other plants you don’t want to draw attention to. When planted around your growing garden edibles, decoys such as calendula will attract slugs and snails. Planting onion, beetroot, lavender and silverbeet can also work well to confuse pests.

Different shapes and an assortment of plant heights can throw pests off their game. Make things even harder for pests by planting in scattered groups rather than in straight lines that give predators an easy target to work through.

Simply put, companion planting is a way to work with nature rather than against it.

When it comes to growing and maintaining plant life, Chelsea van Rijn, horticulturist and manager of the Trevallan Lifestyle Centre in Ipswich, Queensland, believes strongly in encouraging plant diversity as well: “Don’t just have edibles; have flowers, too. Every plant has a purpose. Mix it up and make sure something is always in flower.”

Mutual benefits

Grow plants with deep roots alongside those that have shallow roots, as they won’t be competing for moisture or nutrients due to different root lengths. Shallow-rooted plants such as lettuce and onions won’t compete for nutrients when planted among deep-rooted plants such as carrots.

Taller or hardy plants can also work well to protect delicate plants or seedlings from too much sun or wind. Some good sun/shade combinations include planting celery with tomatoes, or sweet corn with potatoes, dill and mint. On the flip side, plants in the same family may not grow well together as they tend to attract the same pests or compete for the same nutrients in the soil.

In addition to being brilliant for biodiversity, companion planting can also add lots of visual interest to your garden — and help it smell heavenly as well. Add a strawberry patch in the vicinity of lettuce, carrots, pumpkins, corn and capsicum for a vibrant and festive garden display.

Scented plants such as lavender, jasmine and basil can make your garden smell as beautiful as it looks. Having something always in bloom in your garden means beneficial insects will be drawn towards it to pollinate or attack garden nuisances. Try planting mint, thyme, sage, rosemary, coriander, oregano and dill together with cabbages for a sweetly scented culinary garden.

You can also efficiently save on garden space by training climbing plants to grow up taller species of plant (like corn stalks) instead of a trellis.

Plants that don’t play well together

One thing we do know for certain is that plants must either adapt to their environment or alter it to survive, which brings us to allelopathy. This is where a plant releases chemicals that can either inhibit or benefit other plants.

The release of certain chemicals by some plants can greatly affect the growth of others in either a good way or a bad way by leaching chemicals into the soil or by shedding leaves and bark on to the ground below. An example is the black walnut tree, which is known to release an allelopathic substance called juglone through its roots, bark and leaves. Juglone can cause damage to tomatoes, eggplants and corn.

Allelopathic plants are commonly weeds but also include different landscape and crop plants such as barley, pine trees and Australian eucalypts. Allelopathy can be a desirable addition to a harvest when used in mulch to inhibit weeds in vegetable beds, but this can be a tricky business to experiment with and relevant studies are still in the early stages.

Encourage garden friends, discourage garden foes

Certain insects can help with natural pest control and so do some of your garden work for you. Flowering and brightly coloured plants attract “good” insects such as native wasps and bees, which help your garden grow while simultaneously eating the insects that are feasting on your salad greens.

Sometimes the most unlikely garden bedfellows can make for spectacular results.

“Some of the greatest companion plants are those that have nothing to do with edibles,” says van Rijn. “If the garden is never without blossom, it is therefore never without the array of animals that come with that, such as birds, pollinating insects, reptiles, beetles and all sorts of helpful garden good guys.”

Yellow-flowered plants such calendula, sunflower, paper daisy and chamomile or the brightly coloured marigolds are ideal for attracting insects that will contribute to your garden’s health. In addition to adding a burst of colour to your garden, French marigolds are also said to inhibit nematodes in the soil through root secretions.

Handy garden friends include bees as they help to enable pollination and like to snack on small insect pests. Ladybeetles may look like they couldn’t hurt a fly but are actually natural predators that feed on large amounts of aphids, whitefly, thrips and alfalfa weevils, as well as many other soft-bodied insects and eggs. Consider a spider’s web in your garden to be a good thing and not something to be disposed of with the garden rake. Spiders can catch flying insects such as aphids and flies in their webs and get rid of them for you. Other beneficial garden predators include lacewings and the praying mantis.

Sometimes the most unlikely garden bedfellows can make for spectacular results. For example, who would have thought that planting garlic and roses together would work so well? The garlic repels aphids from the roses and makes the roses more sweetly scented and more resistant to disease.

“By encouraging this assortment of good guys and keeping the plants healthy with organic fertilisers, you can combat the bad guys without the need for harmful chemical warfare,” says van Rijn, in reference to noxious pesticides that are far from selective when meeting a target.

“Chemical sprays kill both good and bad bugs,” she add. “We need bad bugs in limited quantity to feed good bugs. We need good bugs to help pollinate and distribute seeds. By having an organic garden you are helping nature do what it does best.”

Repellent plants

Some plants do actually repel garden pests, but only to a certain extent due to the numbers you actually need in order for them to be successful.

Grow plants with deep roots alongside those that have shallow roots, as they won’t be competing for moisture or nutrients.

“Basil can deter insects but you need at least 10 large basils per vegetable plant to make even a small difference,” points out van Rijn. “Backhousia citriodora has a lemony citronella scent to its leaves and is said to repel mosquitoes, but once again you need a lot of them and the leaves need to be disturbed to release the scent, so either you need to rub the leaves or it has to be a windy day.”

According to Quealy, the wormwood shrub acts as a great repellent and garlic chives are handy in warding off thrips. In addition to wormwood, cotton lavender, tansy and santolina act as repellents as they give off a scent or taste that pests dislike. Tansy repels moths, flies and ants and works well if planted beneath peach trees. Catnip can repel fleas, ants and rodents and most insects seem to dislike members of the onion family such as garlic, chives and onions.

A healthy garden is your best defence

The healthier plants are below the surface, the better they look above it. The basics of a healthy garden, says van Rijn, are “great soil and regular use of a certified-organic slow-release fertiliser and liquid fertiliser. The healthier the soil, the healthier the plants, the fewer problems you have.”

Natural compost is a great way to dispose of your garbage and encourage garden health. If your soil is in need of potassium, banana skins can help with that. Eggs shells can add calcium (and deter snails and slugs) and coffee beans can lower the pH of your soil.

Healthy plants are also less likely to come under attack from pests. “Keeping your plants healthy is your first and best defence,” says van Rijn. “Studies have proven that insects are more likely to attack sick plants or weak new growth over healthy plants. Using certified-organic fertilisers regularly is the best defence. If you do need to use sprays, there are certified-organic sprays on the market — and always check to make sure the product has a certified organic label.” Alternatively, you can make your own sprays with ingredients from your garden such as garlic and chilli.

As every home green thumb knows, there is no exact science as to what will work in your own garden and what will not. A lot of this has to do with your local environment as well as with garden health and plant position. Sometimes, however, you can follow instructions to the letter and still have lacklustre results. As in many areas of life, companion planting is as much about trial and error as about knowledge and know-how.