Diana Mattea advised that Companion Planting is an organic planting method for plant diversity, increasing plant health and reducing disease. This is achieved by the selection of plants by type and colour, creation of a healthy growing environment to attract beneficial insects, improving soil structure and elimination of toxic chemicals.
To avoid using chemicals we should observe and correctly identify the pest or disease and determine an appropriate control method. EG. Cabbage white butterfly and other insects may be deterred by planting certain herbs or using essential oils sparingly, 1-2 drops diluted in 500ml water.
Reference books recommended by Diana –
Garden Pests, Diseases & Good Bugs by Dennis Crawford.
Companion Gardening by Jackie French