Companion Planting 
I often get asked about “companion planting”. It’s a wide subject with some really non-scientific “believers” in the magic of plants hating each other, or loving each other!I’ve talked before about the Nematodes and marigolds and the belief that the marigolds will repell the nematodes in the soil. It's not as simple as that, though! “Companion planting” has been studied and proven in just a few occasions. Rows of onions and alternating rows of carrots appear to have beneficial effects on the populations of onion flies and carrot rust flies respectively. But that’s about it!
There are other forms of companion planting that make a lot more scientific sense:
Tomato fruitworm (Heliothis armiger) is the same caterpillar known as “corn earworm”. This caterpillar tunnels into vine-ripened tomatoes. The same caterpillars love to get into developing corn cobs as well. If you grow corn in amongst tomatoes, you’ll find that the caterpillars will prefer to get into the corn. Now I don’t care much for corn, really, so I used to grow “sacrificial corn” in amongst my tomatoes in Auckland, just to keep the caterpillars out of my tomatoes!I noticed a similar trick on Great Barrier Island, whereby growers grow flax and other flowering plants that attract masses of tui. These tui are grumpy, territorial little attack missiles that chase the kaka away from fruit trees!
There are lots of examples whereby plants “help” each other in what's known as 'cultural assistance':  
* I tether my tall tree dahlias to flaxes and other sturdy plants, so wind doesn’t blow them over.* Beans climbing up old corn stalks or the skeleton stalks of artichokes.* Some native seedlings germinating on the lee-ward side of established flax plants.* Squash shades the ground, which suppresses weeds, helping the corn plants and climbing beans.* Here on the Port Hills with often strong NW winds or cold NE winds, it pays to have some shelter. So I have some flowering native shrubs that will assist with that shelter.
So for me, companion planting is basically all about increasing the biodiversity in the garden and planting the right plants to attract our Natural little helpers too!YOU CAN LISTEN TO RUUD'S INTERVIEW WITH JACK ABOVE