Companion planting arranges different crops in close proximity to encourage beneficial interactions. These pairings help plants by managing pests, improving soil health, or optimizing space utilization. Onions are a valuable component in this system, often acting as a natural deterrent due to their pungent, sulfurous compounds. Understanding which plants make good neighbors and which do not is key to leveraging this technique for a more productive garden.
Plants That Thrive Next to Onions
Onions, as members of the Allium family, release strong aromatic compounds that repel several common garden pests, making them excellent protective companions. Carrots are a prime example of a mutually beneficial pairing, as the onion’s scent repels the carrot rust fly, a pest that targets carrot roots. Conversely, the carrot’s deep taproot helps loosen the soil, creating a better environment for the onion’s shallow bulb development without competing for nutrients.
Many members of the Brassica family, including cabbage, broccoli, and kale, benefit from nearby onions. Onions deter pests like the cabbage worm, cabbage looper, and flea beetles, which commonly damage the leaves of these crops. Planting onions in alternating rows with these leafy vegetables utilizes the strong odor to mask the scent of the target crops, reducing pest damage.
Leafy greens such as lettuce and spinach are compatible with onions because they occupy different vertical and subterranean spaces. Lettuce has a shallow root system, so it does not compete with the onion bulb for resources as it grows downward. Onions help deter aphids and other insects that frequently attack the tender foliage of these greens, allowing for a better harvest.
Tomatoes and peppers, which belong to the nightshade family, are protected by onions from general insect pests like aphids and certain fungal diseases. Planting onions near tomatoes may enhance the tomato plant’s resistance to diseases like Fusarium wilt due to increased anti-fungal enzyme activity in the soil. The herb chamomile is another beneficial neighbor, known for having anti-fungal properties that can reduce disease pressure on the onion itself, which is helpful in humid growing conditions.
Plants to Strictly Avoid
While onions are generally good garden neighbors, certain plants should be kept far away to prevent stunting, disease, or nutrient imbalance. Legumes like beans and peas are incompatible neighbors, primarily due to their nitrogen-fixing capabilities. These plants host bacteria in their root nodules that convert atmospheric nitrogen into a usable form in the soil.
Onions release substances that can inhibit the growth of these nitrogen-fixing bacteria, which stunts the growth of the legumes. Planting onions next to beans or peas can result in reduced yields for both crops. The high levels of nitrogen released by the legumes can also encourage excessive leafy growth in onions at the expense of bulb formation.
Asparagus should be planted separately from onions because both are heavy feeders, meaning they compete intensely for the same soil nutrients like nitrogen and potassium. This competition can result in stunted growth for both the perennial asparagus spears and the annual onion bulbs. Additionally, onions can introduce or share the Fusarium fungus, which causes crown and root rot, a serious issue for asparagus.
Certain herbs, most notably sage, can actively inhibit onion growth when planted in close proximity, a phenomenon attributed to allelopathy, where one plant releases chemicals that suppress another. Planting onions near other members of the Allium family, such as garlic, leeks, or chives, should be done with caution. Keeping them too close concentrates the risk of shared pests like the onion maggot or diseases that can easily jump between closely related species.
Practical Tips for Intercropping Onions
Successful intercropping with onions requires attention to the physical needs of the plants. A primary consideration is root depth; onions are shallow-rooted, so they pair well with deep-rooted companions like carrots or parsnips. This stratification allows both plants to access water and nutrients at different soil levels, maximizing the use of the garden space and avoiding underground competition.
Proper spacing is necessary to ensure the onion bulbs have enough room to expand laterally in the soil. Overcrowding can lead to smaller, underdeveloped bulbs. Companion plants should be placed with a buffer space of at least one to two inches from the developing onion. Fast-maturing crops, such as radishes, can be planted between onions, as they are harvested before the onion requires the full space for bulbing.
Onions are heavy feeders that require consistent moisture and a balanced slow-release fertilizer with equal parts nitrogen, phosphorus, and potassium (NPK) for optimal growth. Companion plants should have similar water and nutrient needs to simplify care, such as beets or celery, which thrive under the same conditions. Ensuring adequate phosphorus and calcium is important for strong root and bulb development.