Companion Planting — What the Evidence Shows

The Claims and the Evidence

Companion planting — the deliberate growing of different plants in proximity for mutual benefit — has a long history in horticulture and a devoted following among organic growers. But the evidence for many popular combinations is thinner than often claimed. Here is an honest assessment.

Combinations with Good Evidence

Alliums with carrots — well-supported. The smell of onions, leeks, or chives confuses carrot fly (Psila rosae), which locates its host by smell. Research shows mixed rows or alternating beds of carrots and alliums reduce carrot fly damage significantly.

Legumes as nitrogen fixers — excellent evidence. Peas and beans, through symbiosis with Rhizobium bacteria on their roots, fix atmospheric nitrogen. Growing them before nitrogen-demanding brassicas is sound practice with solid scientific backing.

African marigolds (Tagetes) against soil nematodes — genuine effect. Tagetes roots release alpha-terthienyl, a compound that suppresses root-knot nematodes. Particularly valuable as a preceding crop in glasshouses before tomatoes.

Nasturtiums as sacrificial plants — reasonable evidence. Aphids are attracted to nasturtiums and may colonise them in preference to crops nearby. Whether this actually protects crops or merely concentrates pests is debated, but many growers report benefit.

Claims That Need Caution

Basil improves tomato flavour — popular belief, no scientific evidence. The two make excellent companions aesthetically, and both need similar conditions. Whether volatile compounds from basil genuinely affect tomato biochemistry has not been demonstrated.

Roses and garlic — growers swear by this, evidence is limited. Garlic as a fungicide and bactericide is well-established; whether growing garlic near roses actually reduces black spot is anecdotal.

What Companion Planting Genuinely Achieves

Even where specific repellent or attraction claims are unproven, mixed planting genuinely:

• Increases biodiversity, supporting beneficial insects
• Creates a varied microclimate less amenable to pest outbreaks
• Reduces soil-borne disease pressure (monoculture concentrates problems)
• Makes better use of space and vertical growing opportunities

The most honest position is that companion planting is a valuable part of ecological growing, and the evidence for several key combinations is strong — but it should not be relied upon as the sole pest management strategy.