Companion planting means growing certain plants together for mutual benefit, and some of it works while a lot of it is folklore. The best-supported ideas are flowers that draw in beneficial insects, trap crops that lure pests away, and tall plants that shelter tender ones. Many specific pairing claims, such as basil improving the flavour of tomatoes, have little evidence behind them. Treat companion planting as one helpful tool among many, not a guarantee.
Companion planting is one of those gardening ideas that everyone has heard of and few can pin down. Some of it rests on sound biology, and a good deal of it is passed-down lore that has never really been tested.
Most of the popular advice also comes from the Northern Hemisphere, where the seasons and many pests are different. So it pays to be a little sceptical, and to focus on the parts that hold up in an Australian garden.
Does companion planting actually work?
Partly, and it depends on the claim. A few mechanisms have reasonable support, while many plant-by-plant pairings are closer to tradition than science.
The ideas worth your time are the ones with a clear reason behind them. That includes flowers that feed beneficial insects and trap crops that draw pests away. It also includes legumes that enrich the soil for later crops, and tall plants that shade or support shorter ones. Each of these has a mechanism you can point to.
The weaker claims tend to be the very specific ones. Australian sources are honest about this. Sustainable Gardening Australia notes there is only a limited amount of real scientific information on companion planting. Some combinations seem to work, while others are hit and miss.
Flowers that bring in the good bugs
One of the most reliable payoffs is planting flowers that attract beneficial insects. This is the part of companion planting with the strongest support.
Small, open flowers help support hoverflies, ladybirds, lacewings and tiny parasitic wasps. Some feed on nectar and pollen, and many of their young prey on aphids and caterpillars. Sweet alyssum, calendula, cosmos, dill and coriander can all do this job well.
The trick is to keep something in flower across as much of the year as you can. A steady supply of blooms can support a steady population of helpful insects. Aim for variety and continuous flowering rather than one big flush.
Trap crops and living supports
Some companions work by sacrificing themselves or by lending a hand physically. These are simple, practical effects rather than chemistry.
Nasturtiums are a classic trap crop. Aphids and cabbage white butterfly (Pieris rapae) often go for them first, which can draw pressure away from your vegetables. The catch is that you have to remove or squash the pests on the trap plant, or you are simply raising the next generation.
Tall plants can also earn their place. Climbing beans can scramble up a sturdy stand of corn, and taller crops can shade lettuce and other leafy greens through the heat of summer.
What the old companion charts get wrong
Plenty of well-known pairings do not hold up. It is worth knowing which ones to take with a grain of salt.
"Carrots love tomatoes" comes from the title of a 1975 book, not from data, and there is little evidence either plant helps the other. The idea that basil improves the flavour of a tomato is also not supported, since plants do not absorb flavour from their neighbours.
Marigolds are a more interesting case. French marigolds can suppress root-knot nematodes (Meloidogyne species) in the soil. They only do this when grown thickly and dug in over a whole season. A few plants dotted among the tomatoes will not shift soil nematodes. They still earn a spot, because they are good at drawing in beneficial insects.
Peas, beans and the nitrogen question
Legumes such as peas and beans can add nitrogen to the soil, but not in the way many gardeners think. The benefit goes mostly to the next crop, not to the plant growing alongside.
Peas and beans host helpful bacteria in nodules on their roots. When the right bacteria are present, they draw nitrogen from the air and hold it in a form plants can use. That nitrogen becomes available as the roots break down, which is why it suits whatever you plant next.
So when a pea or bean crop finishes, cut the plants off at the base and leave the roots in the ground. One honest caveat is that the gain in a home garden is usually modest, since much of the nitrogen leaves in the pods you pick.
Companion planting across Australia's climate zones
Timing and plant choice shift a great deal across the country, so local conditions matter more than any chart. What flowers in July in Cairns is very different from what flowers in July in Hobart.
In tropical and subtropical gardens, from Darwin to Brisbane, the warm months bring heavy insect pressure, so a long flowering season helps. In temperate areas such as Sydney, Melbourne and Canberra, the four seasons give clear windows for cool and warm crops.
In Mediterranean zones, the mild wet winters and dry summers favour winter and spring flowering. This includes Perth and southern Western Australia, along with much of South Australia. In arid and cool or alpine areas, from Alice Springs to the southern tablelands, the window is shorter. Choose hardy flowers and quick-maturing varieties to suit it.
Frequently asked questions
Does companion planting really work?
Partly. Some ideas have decent support, such as flowers that attract beneficial insects, trap crops, and legumes feeding later crops. Many specific pairing claims have little evidence, and much of the lore comes from the northern hemisphere, so it transfers imperfectly to Australia.
Does basil make tomatoes taste better?
There is no reliable evidence that it does, and flavour does not transfer between living plants. Basil and tomatoes still grow happily side by side, so plant them together for convenience rather than for taste.
Which flowers attract beneficial insects?
Sweet alyssum, calendula, cosmos, dill and coriander are all good choices. Their small, open flowers feed hoverflies, ladybirds, lacewings and parasitic wasps. Aim for a mix and for continuous flowering through the seasons.
Do marigolds repel pests?
Not broadly. French marigolds can reduce root-knot nematodes in the soil, but only when grown densely and dug in over a season. A few plants will not repel most insect pests, although they do help by attracting beneficial insects.
Do peas and beans feed the plants growing next to them?
Not while they are alive. Legumes fix nitrogen in their roots, and it mostly becomes available to the following crop as the roots break down. Leave the roots in the soil when the crop finishes.
Can companion planting stop Queensland fruit fly?
No. Queensland fruit fly (Bactrocera tryoni) is a serious pest of fruiting crops, and companion plants will not control it. Use proven methods such as netting, baiting and good garden hygiene instead.
Companion planting is at its best when you treat it as gentle encouragement rather than a rulebook. Feed the soil, plant a good mix of flowers, and let the beneficial insects do their quiet work. For more on keeping your beds healthy season after season, see our guide to crop rotation in the home garden.
Related growing guides
- Companion Planting for Sustainable Gardening Production
- Nasturtiums: The Gods of the vegetable gardens!
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