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How to use companion plants

Companion planting is a method of growing different plants adjacent to one another for the benefit of one or both of the companions. Some plants are thought to confuse or act as a decoy to potential pests, while a few provide food for the pest's natural predators. Many of the specific plant combinations are based on garden folklore rather than scientific evidence, although many organic (and non-organic) gardeners swear by them. In my garden I do mix up flowers and herbs with the vegetables, not only because they look good, but also because if there is any chance of confusing the enemy I’m up for it! What’s more, there is growing scientific evidence to support certain widely used plant combinations.

Confuse the enemy.

Many pests locate their food by sense of smell, so if you can mix up the plants enough to disorientate or confuse, far fewer of your plants will be attacked. For example, the cabbage white butterfly can be fooled into passing by a crop of brassicas simply by planting French marigolds in between the plants, while parsley planted among carrots repels carrot flies. Other plants that produce confusing scents include: rosemary, thyme, sage, lavender, chives, wormwood and garlic.

Use plant decoys.

A few plants are particularly attractive to pests and can be used near other susceptible crops to draw potential pests away. For example, nasturtiums are so loved by aphids that they will be attacked in preference to other plants. Similarly, there is scientific evidence to show that Tagetes sinuata can be used to keep potato crops free of eelworm by attracting the pest to its own irresistible roots.

Feed natural predators.

Some plants produce flowers rich in nectar that are loved by common pest predators such as lacewings and hoverflies. For example, if you want to grow nasturtiums as aphid-free as possible, plant marigolds nearby. The marigolds are loved by hoverflies, which are voracious aphid predators. Other plants that produce irresistible flowers include: echinacea, coreopsis, asters – don't bother with double flowered hybrids as they have no nectar or pollen.

Increased vigour.

Companions can also give added health to their neighbours. Parsley gives increased vigor to tomatoes and asparagus, horseradish planted near potatoes make them stronger and more disease resistant. But beware! Horseradish should be dug up every year to prevent it spreading too much. It’s a thug!

Use resistant varieties.

Some varieties of plants are more resistant to attack by pests and disease than others. This is particularly important when looking at growing fruit and vegetables. For example, the apple varieties 'Discovery', 'Ellison's Orange', 'Sunset' are resistant to apple scab, while 'Bramley's Seedling', 'Laxton's Superb' are resistant to canker disease.

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