Although many companion plants remain untested, various types of marigold have undergone scientific field trials. Once flowering, the marigolds are cultivated back into the soil as a crop rotation. The results are impressive. Tagetes patula 'Single Gold' topped a Dutch trial with 99 per cent of nematodes controlled. Burpee Seeds in the United States have so much faith in the ability of this humble flower that they've named it 'Marigold Nema-gone'.
Most gardeners aren't in the habit of chopping up their marigolds and digging them back into the soil, but it makes sense to put in plenty around plants susceptible to nematode damage, such as brassicas, carrots and parsnips.
If you do have serious problems caused by nematodes you could follow the Dutch example and sow a crop of marigolds, then dig them in once they're flowering.
Another well-tested theory is to plant rows of carrots and onions together. This one makes complete sense. The smell of the onions disguises the smell of the carrots from the carrot rust fly. The carrot smell also disguises the onion from onion fly.
A well known triplicity is called the Three Sisters - corn, climbing beans, and pumpkin or squash. The corn provides structure for the bean to grow up and the pumpkin scrambles over the ground shading the soil from the hot sun. Each of the plants has its own niche. Native Americans developed the system because it provided a balanced diet from a single plot of land.
Some plants are bad companions. Potatoes are no good near tomatoes as they are in the same family and share diseases. They are also likely to fight over similar nutrients in the soil, such as potassium.
Other plants are okay to grow at a distance, such as along a border. Fennel exudes a chemical from its roots which is allelopathic, in other words it inhibits the growth of plants growing near it.
But the fennel flowers are good, as they attract predatory insects which will provide a volunteer army to deal to pests on your behalf.
Fennel is also good to plant near dog kennels, as it repels fleas.
Dill, coriander, carrot, angelica and parsley all have similar umbel-shaped flowers loved by beneficial predator insects, so grow these plants and allow them to set flower and seed in your garden.
Other excellent plants to attract good insects are purple-flowering phacelia, borage, clover, chives and comfrey, yellow tansy or 'Bachelor's Button', and delicate white alyssum and chrysanthemum.
As long as you have plenty of flowers in and around your garden you will be providing habitat and nectar for a diverse range of insects. Keep a diary on what has worked or not worked so you can hone your observations for future reference.
The aim is for a balanced garden which is not overburdened by pests, and where spraying is a thing of the past.
Good and ugly
* Hover flies: The young larvae of some species are voracious carnivores and will eat small soft-bodied insects such as aphids, some chewing through as many as 40 aphids a day. Hoverflies are very cool insects and fun to spot.
They look like a cross between a honeybee and a fly but unlike bees they have two wings, not four. They fly in distinct right-angled flight paths and can hover in one spot.
Trials have proved that fields of canola planted with adjacent strips of purple-flowering Phacelia increased numbers of hoverfly, which in turn greatly reduced aphid populations.
* The same goes for red ladybirds. Their babies are ugly devils, with appetites to match and will chew through aphids as if they were juicy grapes.
* Wasps: Bigger wasps will eat a caterpillar for breakfast. Smaller wasps will lay their eggs on a caterpillar and the horror show begins as the caterpillar is slowly eaten from the inside out.
The same goes for the tiny wasps which parasitise aphids and whitefly. If you see some aphids which aren't moving and look mummified, leave them on the leaf.
They are likely to be host to wasp pupae and you will have plenty more fighters to do your dirty work for you.
Resources
Plants for beneficial insects: Kings seeds sells a Beneficial Insect Blend. Visit kingsseeds.co.nz
Also visit farmerfred.com/plants_that_attract_benefi.html
In the glasshouse: Tiny parasitic wasps are available to purchase (by the 1000) from bioforce.net.nz
Home gardener's observations: The book, Companion Planting in New Zealand by Brenda Little, is worth reading. Available from touchwoodbooks.co.nz
