Companion planting involves placing different plant species near each other to maximize garden health and yield. The goal is to create a balanced ecosystem where plants provide mutual benefits, such as nutrient sharing or pest control. A common question is whether the cool-season vegetable pea (Pisum sativum) and the pungent bulb onion (Allium cepa) can be planted together effectively. These two crops have different growth habits and nutrient needs, making their proximity a subject of discussion among gardeners.
The Compatibility Question
Peas and onions are generally compatible and can be planted in the same garden bed, but success relies heavily on management. This combination is not a strict “do not plant” pairing, and real-world experience often shows they can coexist without issue, provided basic gardening principles are followed.
The primary risk of planting them too close is physical competition, as climbing peas can shade low-growing onions, hindering bulb formation. Their distinct nutritional requirements also complicate fertilization if they are intermingled. Onions require high nitrogen for leaf growth, while peas, as legumes, prefer lower nitrogen levels in the soil.
Understanding the Relationship
The relationship between peas and onions is a nuanced exchange of chemical and biological factors that can either benefit or hinder growth depending on proximity. Peas, as members of the legume family, engage in a symbiotic process known as nitrogen fixation. This involves Rhizobium bacteria living in nodules on the pea roots, which capture atmospheric nitrogen gas and convert it into plant-usable compounds like ammonia and nitrates. This fixed nitrogen is primarily released into the soil after the pea plant dies and its roots decompose, benefiting subsequent crops.
Onions, however, introduce a counteracting chemical factor. As alliums grow, they produce volatile sulfur compounds, such as thiosulfinates and sulphides, which are responsible for their strong scent. These compounds act as a natural insect repellent by disrupting the olfactory receptors of many pests. This strong odor is particularly effective at deterring aphids and other small sap-sucking insects that commonly attack pea plants.
The specific concern is that the allicin released by the onion’s roots may inhibit or harm the delicate Rhizobium bacteria responsible for nitrogen fixation in the pea roots. This potential allelopathic effect means the peas may not receive the necessary nitrogen compounds for their own growth, resulting in reduced vigor. Therefore, the pairing involves a trade-off between the onion’s pest protection and its potential chemical interference with the pea’s root biology.
Practical Planting Strategies
Successful planting of peas and onions together requires careful attention to timing and spacing. Peas are a cold-tolerant crop that should be planted in early spring as soon as the soil is workable. Onion sets or transplants can be planted simultaneously or shortly after the peas, as they also prefer cool weather for establishment. Both plants require well-drained soil to prevent root issues, and a neutral pH is generally suitable for both.
Spacing is the most important logistical factor to mitigate the potential for mutual inhibition. To allow for adequate airflow and sun exposure, rows of peas and onions should be separated by at least 12 to 18 inches. This distance helps reduce the physical shading of the low-growing onions by the climbing pea vines, which need a trellis or support system. The separation also helps ensure that the concentration of the onion’s root exudates does not significantly impact the pea’s nitrogen-fixing nodules.
Spring onions can be thinned out early in the season, which provides a harvest and increases the spacing for the remaining bulbs to grow larger. Peas are typically harvested over several weeks once the pods are full and tender. Once the peas are finished producing, cutting the vines at the soil level and leaving the roots to decompose will release the fixed nitrogen into the soil, creating a nutrient-rich environment for the following season’s crops.