The technique has been practised for centuries, perhaps most vividly in the traditional British cottage garden where flowers, vegetables and a whole range of plants jostle as neighbours.

There are many examples of companion planting, most with unproven benefits (does garlic really make the rose smell sweeter?) We explore what really works, using plants to:

Attract pest predators
Deter and trap garden pests
Improve your soil
Create shelter
Manage weeds

Attracting predators

Insects such as ladybirds, lacewings and hoverflies are predators of pests such as aphids and caterpillars. Providing a food source and home for these predators increases their chances of survival.

To do this, make sure you have a range of flowering plants grown together to create a succession of pollen and nectar throughout the growing season.

Top Tips:

Mix coriander or fennel flowers, the Californian poppy (Eschscholtzia californica), poached-egg plant (Limnanthes douglasii) and calendula (Calendula officinalis) within your vegetables.
White deadnettle (Lamium album) produces nectar over a long season making it valuable to early bees and insects. This not only helps control pest populations, but also, vitally, increases pollination.
The cool, shady conditions created under the leaves of weeds such as fat hen (Chenopodium album) provide good habitats for predators such as ground beetles.

Pest deterrent

Some plants can deter pests from their neighbours. It was thought that odour was the key factor, but the evidence for this is conflicting. For instance, growing onions with carrots were thought to confuse the carrot root fly – but trials have shown inconsistent results. (Covering with mesh or fleece is more likely to achieve consistent control.)

We now know that visual signals are more important in muddling the pest than the scent of the plant.

Top Tip: grow brightly coloured marigolds in the greenhouse with tomatoes. The bright orange/yellow petals attract the white fly away from the tomato plants.

Some plants attract pests away from other more susceptible neighbours.

Top Tips:

Nasturtiums will pull blackfly away from beans, and Chinese cabbage will attract cabbage white butterflies away from neighbouring brassicas.
Why not sow an extra row of lettuce and spinach to divert slugs from more mature plants such as corncobs and squashes.

Soil improvement

Healthy soil with lots of micro-life and a good structure is at the heart of organic gardening. One way to achieve this is by growing the right mix of plants.

For instance, beans and clover fix nitrogen in the soil creating extra fertiliser for other plants. They are known as green manures.

Top Tips:

Try growing strips of clover in the spaces between fruit trees or bushes. If mown regularly, the cuttings can be thrown under the bushes, where the nitrogen-rich leaves break down quickly to supply nutrients to the fruit trees. (There is often a misconception that nitrogen-fixing beans will supply other surrounding plants with nitrogen, but this has been shown to be not true. Unless you cut down the bean plant before it flowers and seeds, it will take up all the valuable nitrogen it has fixed!)
Grow a combination of plants that root to different levels within the soil. For example, deep-rooting carrots and shallow rooting onions can be grown side-by-side without causing too much competition.
Add mineral rich leaves of certain plants to the compost heap. For example, yarrow (Achillea millefolium) accumulates phosphorous, calcium and silica; and comfrey (Symphytum species), which accumulates nutrients from deep in the soil from its deep tap root.

Plant shelters

One of the most obvious benefits that plants can provide one another is shelter from the wind and sun. Windbreaks and hedges are important garden structures, minimising extremes in weather and reducing wind speed, which in turn reduces water loss (transpiration) from plants.

In the vegetable garden, lettuce can be planted around cucumbers, carrots, radishes and strawberries, all of which benefit from the humid environment this creates. In ornamental areas, lavender can be grown to support and shelter the lower stems of lilies.

Weed management

Growing plants close together reduces light levels and therefore stops weed seeds germinating. (However, care must be taken not to crowd plants too closely as competition may become a problem. This is especially important when growing vegetables.)

Some plants secrete chemicals from their roots. Dandelions give off ethylene gas which inhibits the germination of seeds around it. Mexican marigolds (Tagetes minuta) secrete chemicals that can suppress weeds like horsetail and bindweed. These secretions inhibit weeds and herbaceous plants but have little effect on woody species. Lupins also suppress weeds, especially fat hen.

Try growing strips of clover in the spaces between fruit trees or bushes. If mown regularly, the cuttings can be thrown under the bushes, where the nitrogen-rich leaves break down quickly to supply nutrients to the fruit trees. (There is often a misconception that nitrogen-fixing beans will supply other surrounding plants with nitrogen, but this has been shown to be not true. Unless you cut down the bean plant before it flowers and seeds, it will take up all the valuable nitrogen it has fixed!)
Grow a combination of plants that root to different levels within the soil. For example, deep-rooting carrots and shallow rooting onions can be grown side-by-side without causing too much competition.
Add mineral rich leaves of certain plants to the compost heap. For example, yarrow (Achillea millefolium) accumulates phosphorous, calcium and silica; and comfrey (Symphytum species), which accumulates nutrients from deep in the soil from its deep tap root.