Onion Companion Planting
Allium cepa
Allium (Amaryllidaceae) -- cool season
Good Companions for Onion
Onion interplanting reduces cabbage root fly (Delia radicum) oviposition by 30-50% through volatile-mediated olfactory masking. The dipropyl disulfide compounds in onion disrupt host-finding cues.
Alternate rows of onion and cabbage. Onion must be actively growing (not dormant) for volatile emission.
- The effect of non-host plant volatiles on the behaviour of the cabbage root fly -- Entomologia Experimentalis et Applicata (1988)
- Intercropping cabbage with onion reduces cabbage root fly incidence -- Journal of Applied Ecology (1991)
Onion volatiles reduce cabbage root fly (Delia radicum) oviposition through olfactory masking. The same mechanism documented for cabbage-onion intercropping applies to all brassicas.
Alternate rows of onion and cauliflower for best protection.
- Intercropping cabbage with onion reduces cabbage root fly incidence -- Journal of Applied Ecology (1991)
Onion volatiles mask kohlrabi host-plant cues from cabbage root fly and flea beetles. Onion and kohlrabi occupy different root zones, reducing competition.
Interplant in alternating rows. Both are cool-season crops with similar cultural needs.
- The effect of non-host plant volatiles on the behaviour of the cabbage root fly -- Entomologia Experimentalis et Applicata (1988)
Onion volatiles mask brassica host-plant cues from cabbage root fly and flea beetles. Same allium-brassica pest suppression mechanism.
Broccoli rabe is fast-maturing (40-60 days). Interplant with onions for pest protection during the short growing window.
- The effect of non-host plant volatiles on the behaviour of the cabbage root fly -- Entomologia Experimentalis et Applicata (1988)
Onion volatiles deter flea beetles and cabbage root fly on turnip. Both occupy different root zones, reducing below-ground competition.
Interplant in alternating rows for best volatile coverage.
- The effect of non-host plant volatiles on the behaviour of the cabbage root fly -- Entomologia Experimentalis et Applicata (1988)
Onion volatile-mediated pest suppression of cabbage root fly (Delia radicum), the primary pest of rutabaga roots. Rutabaga's long growing season (90+ days) benefits from sustained allium protection.
Plant onion sets among rutabaga rows. Rutabaga needs more time than turnip, so companion protection is more valuable.
- Intercropping cabbage with onion reduces cabbage root fly incidence -- Journal of Applied Ecology (1991)
Onion volatiles (dipropyl disulfide) mask carrot host-plant odors, reducing carrot fly (Psila rosae) oviposition by 50-75% in field trials. The effect is dose-dependent -- more onion rows increase protection.
Alternate single rows of carrot and onion for best effect. Both crops need similar watering schedules.
- Effect of intercropping with onion on the behaviour of the carrot fly Psila rosae -- Entomologia Experimentalis et Applicata (1984)
- Intercropping as a strategy for pest management in carrots -- Annals of Applied Biology (1991)
- Carrot Fly Management -- Cornell Cooperative Extension (2018)
Onion and beet exploit different root depths -- onion is shallow-rooted while beet has a moderate taproot. This reduces below-ground competition. Onion volatiles provide general pest-deterrent effect. Both are cool-season crops with similar cultural requirements.
Interplant in alternating rows. Harvest onions first, giving beets more room to size up.
- Spatial arrangement effects on resource partitioning in vegetable intercropping -- European Journal of Agronomy (2008)
- Companion planting for efficient space use -- Oregon State University Extension (2016)
Onion volatiles deter carrot fly (Psila rosae), which also attacks parsnip (same Apiaceae family). Same olfactory masking mechanism documented for carrot-onion intercropping.
Parsnip is attacked by the same carrot fly that targets carrot -- allium protection applies equally.
- Effect of intercropping with onion on the behaviour of the carrot fly Psila rosae -- Entomologia Experimentalis et Applicata (1984)
Crops to Avoid Near Onion
Allium root exudates (particularly thiosulfinates) inhibit Rhizobium bacteria in the legume rhizosphere, reducing nitrogen fixation efficiency by 20-40% in field trials.
Applies to all allium-legume combinations. Keep all onion/garlic/leek plantings separate from bean and pea beds.
- Allelopathic effects of Allium species on legume Rhizobium symbiosis -- Plant and Soil (1996)
- Onion root exudates and rhizosphere microbial communities -- Applied Soil Ecology (2004)
Allium root exudates (thiosulfinates) inhibit Rhizobium bacteria needed for lima bean nitrogen fixation. Same allium-legume antagonism as other bean-onion combinations.
Keep all alliums separated from all legume plantings.
- Allelopathic effects of Allium species on legume Rhizobium symbiosis -- Plant and Soil (1996)
Onion root exudates suppress Rhizobium bacteria essential for snow pea nitrogen fixation. Same allium-legume antagonism.
Keep snow peas separated from onion plantings.
- Allelopathic effects of Allium species on legume Rhizobium symbiosis -- Plant and Soil (1996)