Companion planting (Allelopathy) is defined as the planting of different crops in proximity for pest control, pollination, providing habitat for beneficial creatures, maximising use of space, and to otherwise increase crop productivity.

The use of companion planting systems in organic gardens have a number of benefits:

Hedged investment – diversity in the space mitigates the risk of one crop failing.
Increased level interaction – when crops are grown on different levels in the same space, providing ground cover, or once crop acting as the trellis for another, increasing the yield within the space.
Protection & shelter – one crop may serve as a wind break, or provide shade to another.
Pest control – many companion plants help prevent pest insects or pathogenic fungi from damaging the crop, mainly through chemical means.
Predator recruitment/positive hosting – where companion plants produce abundant nectar or pollen that encourage higher populations of beneficial predatory insects that in turn control pests. Many beneficial insects only consume pests in their larval form and nectar/pollen in their adult form.
Trap cropping – some companion plants do their job by attracting pests away from others.
Pattern disruption – in a monoculture system, pests spread easily and rapidly through the crop, while this spread can be disrupted by plants of a different type.

It can however act in the opposite direction, with certain plants becoming detrimental to another plants growth. These varieties have the opposite effect, and should be avoided.

A comprehensive table of companion and detrimental plants is provided below:

Companion & Detrimental Crop Table