What Are the Best Companion Plants for Onions and Garlic?

Onions and garlic are among the most useful companion plants in the whole garden — their strong sulphur compounds deter a wide range of pests and have mild antifungal properties when concentrated. But they also have their own companions that help them grow better, and a list of plants they strongly inhibit. Understanding both sides of this relationship lets you get more from your allium bed and use alliums more strategically throughout the garden.

Carrots — the Ideal Allium Partner

Alternating rows of onions and carrots is one of the most widely recommended and genuinely effective companion combinations. Carrot fly navigates by detecting the volatile scent of carrot foliage, particularly when it is disturbed or bruised. Onion fly similarly homes in on the scent of onion leaves. When both crops are interplanted, the mixed scent cloud from the two rows confuses both pests. The effect is strongest when the rows alternate every 30 cm and both crops are equally dense. Leeks planted with carrots produce a similar effect.

Brassicas Benefit from Allium Interplanting

Garlic and chives interplanted in or around brassica beds provide two benefits. First, the sulphur compounds in their foliage deter cabbage white butterflies from landing on the plants to lay eggs — the butterflies identify brassicas by scent and the allium odour disrupts this process. Second, sulphur compounds have mild antifungal properties that may reduce the incidence of blight and mildew on brassica leaves. Plant garlic sets in autumn between brassica transplant positions so they are established by the time the spring butterfly flight season begins.

Roses and Strawberries Need Garlic Nearby

Both roses and strawberries are frequently plagued by aphids and fungal diseases, and both benefit from garlic planted at their base. For roses, garlic planted densely around each bush — three to five cloves per rose — provides a season-long aromatic deterrent. For strawberries, garlic and chives interplanted in the bed help reduce botrytis grey mould incidence. Garlic also makes productive use of soil space in a strawberry bed during winter and spring when the strawberries are dormant.

Summer Savoury as an Allium Companion

While alliums benefit many other plants, they themselves benefit from summer savory as a companion. Summer savory interplanted with onions appears to improve onion bulb development and may help deter onion fly. It is also harvested for the kitchen and takes up very little space. Chamomile in small quantities is another traditional companion for onions and is said to improve their flavour — though the evidence for this is largely anecdotal.

What Alliums Inhibit

Beans and peas are the most important plants to keep away from alliums. The root exudates of alliums inhibit the symbiotic bacteria that allow legumes to fix nitrogen — which defeats the main agricultural purpose of growing legumes. Keep all beans and peas in a separate bed from onions and garlic. Asparagus should not be grown near onions or garlic either, as the allium root chemicals appear to inhibit asparagus growth. Beyond these exceptions, alliums are among the most beneficial plants you can distribute throughout your garden.