Just like people, some plants get along better together than others.
Plant chamomile next to mint, for example, and it's said to intensify the minty taste. Put basil with tomatoes and it reportedly deters aphids and whiteflies and improves the tomatoes' flavour. Chamomile may enhance basil's flavour, but rue and sage is said to inhibit its growth.
In essence companion planting helps develop a balanced eco-system, and it does this in a number of ways. The smell of volatile oils in certain plants discourages pests by masking the smell of neighbouring plants. Some plants attract beneficial insects like ladybirds and bees with their flowers and scent, aiding pollination and providing biological pest control. Nitrogen-fixing plants in the legume family supply nitrogen to other plants. Other plants simply grow well together.
But while companion planting has been around for some time, the science behind it is often anecdotal. There appears to be no research that confirms, for example, that your tomatoes will taste better with basil planted nearby. And what may work for one gardener may not work for another. Your garden's microclimate - or even your gardening practices - may influence success or failure. For example, tomatoes that are overwatered tend to taste bland. So planting basil beside your toms may do nothing for their flavour if you're drowning them in water.
But there are benefits, I believe, to companion planting and with experience you will eventually discover which plants grow well with certain other plants in your area.
French marigolds (Tagetes patula), not to be confused with calendulas, can be beneficial in all areas. Many gardeners have heard or read that they repel nematodes (tiny roundworms that attack the roots and plant tissue of certain plants, such as potatoes, onions, strawberries and bulbs, among others) in the soil by exuding a substance from their roots. But one or two plants won't do the trick. You need to plant a solid block for one entire season, which means giving up one of your vegetable beds for the season. At the end of the season, the marigolds can be dug into the soil. Your soil should then be nematode free for two or three years, maybe more.
Marigolds are also said to repel whitefly, so they could be beneficial planted around your tomatoes and brassicas. Some say that's because whiteflies dislike the smell of marigolds. Others say it's because the tomatoes take up the substance exuded by the marigolds, a substance that whiteflies dislike.
It's not all good news for marigolds though. They are supposedly not good companions to beans.
Yellow nasturtiums are a great decoy plant for whitefly and aphids (they love the colour yellow), so you could plant these near tomatoes and brassicas to steer them away from those.
Tomatoes can help deter diamondback moths, whose larvae like to munch on brassicas, and in particular cabbages. Tomatoes emit an odour that is said to repel them, and the smell from dill and rosemary masks the smell of cabbages so that that the diamondback moths and white butterflies cannot sniff them out.
Beetroot, carrots and onions are great companion plants when planted in single species rows beside one another. Onion helps to mask carrots from carrot rust fly and beetroot covers the soil to reduce weeds, as well as providing a home for beneficial insects, such as ground beetles, which eat pest insects.
"The three sisters" is an ancient practice that's still used today. North American Indians planted a combination of corn, beans and squash, a system that works extremely well. The corn (sunflowers were planted too) provides a structure for the beans to grow up, the beans fix nitrogen in the soil, improving its overall fertility, and shallow-rooted squash becomes a living mulch, shading the soil, reducing weeds and conserving soil moisture.
The Biological Husbandry Unit at Lincoln University tried a similar system, planting upright edible amaranthus, which provided a frame for snow peas (also nitrogen fixing), and underplanting with shade-tolerant shingiku (Japanese edible chrysanthemum). You could also try lettuce, which benefits from lots of nitrogen.
Every plant seems to benefit from chamomile planted nearby. It draws certain nutrients to the soil surface, including potassium, sulphur and calcium, to the benefit of other plants, just as valerian increases phosphorus in the vicinity, and comfrey accumulates good amounts of potassium.
Chamomile is also said to improve the flavour of cabbages, cucumbers, leeks and onions - and as mentioned above, basil and mint. Nothing seems to grow well around wormwood, though. This aromatic herb exudes a toxicity from its leaves and roots.
For centuries it has been used as a moth repellent and a general pesticide, and before its toxicity was realised, it was used as a worming medicine for animals and humans.
Today it's not recommended to be taken internally, but a wormwood tea can still be made to repel slugs and snails.
- The Southland Times