Companion planting involves growing different crops near each other to achieve mutual benefits, such as improved nutrient uptake, better pest management, or increased overall yield. This method encourages a diverse ecosystem that mimics natural environments. Gardeners frequently debate the pairing of potatoes, a deep-rooting tuber, with onions, a shallow-rooted bulb, due to their distinct growth habits and needs. The question of whether these two common vegetables should share garden space involves weighing the aromatic pest-deterring qualities of the onion family against potential resource competition and shared disease vulnerabilities.
The Compatibility Verdict
While potatoes and onions are not strictly incompatible, they are considered a neutral pairing rather than an ideal one. The general consensus is that they can coexist, but the arrangement requires careful management due to certain trade-offs. Both plants have different primary root depths, which allows them to draw nutrients and water from varied soil layers, reducing direct competition. The onion’s shallow root system draws from the upper soil, while the potato’s tubers and roots develop deeper, typically six to eight inches below the surface. This spatial separation is the main reason the pairing is not actively discouraged, provided the gardener is aware of the potential risks.
Potential Conflicts and Shared Vulnerabilities
One significant argument against planting potatoes and onions in close proximity involves the risk of disease spread. Potatoes are highly susceptible to late blight, caused by the oomycete Phytophthora infestans, which rapidly spreads in warm, wet conditions, causing rot on the foliage and decay in the tubers. Although onions are not hosts for late blight, the dense potato canopy traps humidity and reduces air circulation. This creates an environment highly favorable for the proliferation of fungal pathogens.
The plants also present logistical conflicts concerning soil disturbance and water requirements. Potatoes require hilling, where soil is regularly mounded around the stems to protect developing tubers from light and increase yield. This necessary hilling process can easily damage the delicate, shallow-growing onion bulbs and their root systems. Furthermore, while both crops require consistent moisture, onions need drier soil near harvest to cure properly, whereas potatoes need steady moisture throughout their growth cycle to prevent misshapen tubers.
Specific Pest Deterrence Benefits
The primary benefit of pairing onions with potatoes lies in the natural chemical defenses of the Allium family. Onions, along with relatives like chives and garlic, emit a pungent, sulfur-containing aroma that acts as a natural pest deterrent. This strong scent can effectively mask the presence of the potato plant from insects that rely on smell to locate their host. Onions can help to confuse or repel key potato pests, including the Colorado Potato Beetle and various species of leafhoppers.
The Colorado Potato Beetle is a major target for this olfactory confusion. By intercropping onions within the potato patch, the strong odor interferes with the beetle’s ability to sense the potato foliage, discouraging it from landing and laying eggs. Onions have also been noted to reduce populations of aphids and spider mites, which can transmit viruses and weaken the potato plant through sap-feeding. The strategic placement of onions serves as a chemical camouflage, offering protection against several common garden threats.
Maximizing Yield with Alternative Companions
Gardeners seeking to avoid the conflicts of the onion-potato pairing can choose alternative companion plants that offer more definitive benefits without the inherent risks. For potatoes, deep-rooted legumes like bush beans and peas are highly recommended because they fix atmospheric nitrogen into the soil, a nutrient heavy-feeding potatoes require. Other good companions include brassicas like cabbage and broccoli, which possess shallow roots that do not compete with the developing potato tubers.
Onions benefit greatly from being planted near carrots, as the two plants repel each other’s primary pests: the onion fly and the carrot rust fly. Lettuce and spinach are also excellent companions for onions because their shallow roots and short growing season allow them to fill the space between onion rows without competing for nutrients. Planting onions near tomatoes or peppers is also favorable, as the onion’s aroma can help repel thrips and spider mites that plague those plants.