Onions (Allium cepa) and garlic (Allium sativum) are generally compatible and can be successfully planted near one another in the garden. Both are members of the Allium family, meaning they share many cultivation needs and offer mutual benefits, making them a classic example of successful companion planting. Achieving a good harvest from both requires understanding and managing their shared requirements and potential drawbacks.
Shared Growth Requirements
Both onions and garlic thrive under nearly identical environmental conditions, simplifying the garden planning process. They require full sun exposure, ideally receiving six or more hours of direct sunlight daily for optimal bulb development.
These Alliums also share a preference for a specific soil composition to prevent bulb rot. The ideal medium is a fertile, well-drained, loose, loamy soil with a slightly acidic to neutral pH, typically ranging between 6.0 and 7.0. Consistent soil moisture is important during the phase of bulb enlargement, but the soil must never become waterlogged. Because their fundamental needs for light, soil type, and water are the same, they integrate well in the garden.
Mutual Pest Deterrence
The primary benefit of planting onions and garlic together is their powerful ability to deter common garden pests. Both plants naturally release sulfur compounds, which are responsible for their characteristic pungent aromas. These volatile organic compounds, like allicin in garlic, act as natural insect repellents by confusing or masking the scent of nearby plants.
This chemical defense mechanism helps to create a protective zone against a wide range of insects. Pests such as aphids, onion thrips, carrot root fly, and cabbage worms are often repelled by the strong Allium odor. Studies have shown that when planted as an intercrop, onions and garlic can significantly reduce pest prevalence and damage on neighboring crops.
Managing Nutrient Competition
While they share many benefits, the main challenge when growing onions and garlic together is managing their aggressive demand for nutrients. Both are considered “heavy feeders” because they rapidly extract significant amounts of nutrients from the soil, especially nitrogen and potassium, to support bulb and top growth. Their shallow, fibrous root systems mean they will compete fiercely for available resources if planted too closely.
To mitigate this competition, proper spacing is essential to allow the bulbs to swell fully without root overlap. A minimum distance of 4 to 6 inches between individual plants is generally necessary, though 6 to 8 inches is suggested for larger varieties. Before planting, incorporating high levels of organic matter, such as aged manure or compost, into the soil is necessary to build a rich nutrient base. Supplemental feeding throughout the growing season is often required, as these crops remove 85% to 90% of their major nutrients within the first two to three months after planting.