Companion Planting

How does it work?

Pesticides and chemicals
With over 250,000 new chemicals produced each year, we still have no idea of their side effects.

 Tests have shown some pesticides to be so toxic that they have had to be withdrawn. Yet there are still 3 million acute pesticide poisonings each year worldwide.

Health authorities document the problems arising from herbal medicines and supplements yet few real clinical trials are carried out to determine the safety of over 100,000 foreign chemicals that are released into the environment annually.

 

Companion planting
Companion planting can help reduce our reliance on spraying with chemicals when things go wrong in the garden, and how it works can be seen in three stages:


Herb resistance
Herbs are quite resistant to attack by insects and disease.

Due to their particular active constituents we use herbs medicinally and to bring zest to food.

Often these active components are specifically produced by the herbs for their own defence.

Different families of herbs evolved with their own main active defence constituents:

The Compositae or daisy family mainly use bitter-tasting sequiterpene lactones to deter insect attack
Many Umbelliferae or parsley family members contain phototoxic furnanocoumarins that can be vulnerable to sunlight
The Labaitae or mint family notably have large amounts of monoterpene oils in their leaves and some of these can deter or prevent insect or slug attack


Specialist exploiters
These family defence systems work against a wide range of attackers, but a small number of attacking species overcome the plant’s defence to become specialists that can exploit the plant with little competition.


Companion planting
The stage is now set for the concept of companion planting:

Some herbs have roots which produce protective secretions

Others have leaves containing large amounts of volatile oils which will evaporate on warm days. These volatile oils form a mantle around a herb in still air that can prove a barrier to some attackers, eg, the thujone in sage and wormwood can deter insects and caterpillars

This protective vapour mantle can give protection to adjacent plants

Specialist insects that selectively exploit one family of plants, may be deterred by an adjacent companion plant family’s defence system