Companion planting is a horticultural practice where different plant species are grown near one another to provide mutual benefits, such as enhanced growth, increased yields, or natural pest control. Onions and garlic, both members of the Allium family, are frequently utilized in this strategy due to their distinct biological properties. Their strong presence in a garden bed contributes to maximizing space and maintaining the health of neighboring crops through strategic pairing. Understanding the specific actions of these pungent vegetables allows gardeners to make informed decisions about which plants will thrive when placed in close proximity.
Allium Function in Pest Deterrence and Soil Health
The primary mechanism by which onions and garlic influence their environment is through the release of volatile sulfur compounds, such as allicin and various thiosulfinates. These compounds function as a natural defense system against various threats. The strong scent acts as a mask, confusing common insect pests that rely on olfactory cues to locate their preferred host plants. This scent-masking capability is effective at deterring insects that target specific host plants.
Beyond pest deterrence, the sulfur compounds also possess fungicidal and antibacterial properties, contributing to the overall health of the garden soil. When intercropped, the presence of these compounds can help suppress certain pathogens. Alliums are characterized by a relatively shallow and non-aggressive root system. This morphology allows them to be planted densely alongside crops that develop deeper taproots without intense competition for nutrients and water, maximizing the use of garden space.
Companion Plants That Benefit from Alliums
The pest-deterring qualities of alliums make them beneficial partners for a wide variety of garden crops, particularly those susceptible to insect infestations. Root vegetables gain significant protection when interplanted with onions and garlic. Carrots and parsnips are often targeted by the carrot rust fly, but the alliums’ aroma confuses this pest, disrupting its ability to lay eggs near the root crops. Beets also respond positively, as garlic can improve their growth and help protect them from certain fungal infections.
Leafy greens, such as lettuce, spinach, and kale, thrive near alliums. The strong odor provides a scent barrier that helps shield young kale and spinach plants from common brassica pests like flea beetles and cabbage loopers. For lettuce, interplanting alliums helps deter cutworms and aphids. The shallow-rooted nature of both the greens and the alliums ensures minimal competition for resources in the upper soil layers.
Certain herbs benefit from the presence of alliums through pest deterrence or enhanced growth. Dill, for example, is protected from aphids and spider mites when planted near onions. Chamomile is a valuable companion, as it enhances the flavor and general health of nearby onions and attracts beneficial insects like hoverflies. Mint acts as a deterrent for aphids and flea beetles, but its aggressive spreading habit means it is best grown in a contained pot near the alliums.
Fruiting plants, including tomatoes and peppers, gain protection from allium companions. Onions and garlic deter a range of pests that target these nightshades, such as the green peach aphid and Japanese beetles. For tomatoes, planting alliums nearby promotes resistance to certain soil-borne diseases, including Fusarium wilt, by stimulating anti-fungal activity. Peppers benefit specifically from the alliums’ ability to repel green peach aphids, which are vectors for plant viruses.
Incompatible Plants to Keep Away
While onions and garlic are beneficial companions for many plants, they are detrimental to a few common garden crops, primarily due to chemical interactions and resource competition. Incompatibility exists with legumes, which include all varieties of beans and peas. The sulfur compounds released by alliums inhibit the growth of these plants and disrupt the nitrogen-fixing bacteria residing in their root nodules. Since legumes depend on these bacteria to convert atmospheric nitrogen into a usable form, this chemical interference stunts their development and yield.
Asparagus should be kept separate from the Allium family. Asparagus is a perennial crop that requires significant space and nutrients to establish its deep root crown. Both alliums and asparagus are heavy feeders, leading to intense competition for essential soil nutrients if planted too closely. Planting alliums near the asparagus bed can interfere with the sensitive, shallow roots of the asparagus crown, compromising the perennial’s long-term health and productivity.
Certain aromatic herbs, such as sage, are not recommended for pairing with alliums. Both plants have contrasting needs regarding soil moisture, and alliums can chemically inhibit sage’s growth. Although brassicas like cabbage and broccoli benefit from the pest-deterring scent of alliums, they are heavy feeders. Placing them in a small bed with onions or garlic can lead to nutrient competition that negatively affects both crops.