Companion planting is the agricultural practice of growing different plants together for mutual benefit, enhancing garden productivity and health. The concept is based on the subtle biological interactions between species, often involving pairings where one plant provides a protective or nutritional advantage. Planting onions with cabbage is highly compatible. This combination is particularly beneficial for the cabbage, offering a natural form of defense against common insect pests. Careful planning regarding their respective needs is necessary for a successful harvest.
The Protective Relationship: Pest Deterrence
The primary benefit of planting onions near cabbage lies in the onion’s ability to deter specific insect pests that target the cabbage family. This protective effect is a result of the strong aroma and volatile sulfur compounds released by the onion plant, which belongs to the Allium genus. These compounds, such as thiosulfinates and diallyl sulfides, create a pervasive scent that confuses or repels insects, masking the attractive smell of the cabbage. This protection is effective against several common cabbage enemies, including the cabbage worm, cabbage looper, and the cabbage aphid. Studies have shown that intercropping cabbage with onions can significantly reduce the prevalence of these insect populations and the resulting leaf damage compared to growing cabbage alone.
Optimizing Growth: Practical Planting Considerations
Successfully growing onions with cabbage requires specific attention to physical layout and timing to maximize the protective effect without hindering growth. The most effective planting strategy is intercropping, where the smaller onion plants are positioned strategically between or around the larger cabbage heads. Planting onions around the perimeter of the cabbage bed or in alternating rows creates a scent barrier.
Spacing
Cabbage is a sprawling plant that requires significant room, typically needing 12 to 24 inches between plants depending on the variety. Onions have a shorter root depth and can be planted much closer together, sometimes as close as 3 to 4 inches apart for bulb varieties, or 1 inch for bunching onions. Placing two to three onion plants between two cabbage plants is a common and effective intercropping model.
Timing and Sunlight
Timing is also an important factor, as both are cool-season crops that enjoy similar temperature ranges. Onions can be started from sets or seeds early in the season, and the cabbage seedlings can be transplanted simultaneously or shortly after. Using bunching onions or scallions is often easier for intercropping than large bulb onions, as they take up less space and are harvested earlier. Adequate sunlight is necessary for both, so ensure the mature cabbage canopy does not completely shade the lower-growing onions as the season progresses.
Resource Management and Competition
While the pest-repelling benefits are clear, managing the shared resources of the soil is necessary because cabbage and onions have differing requirements. Cabbage is a heavy feeder, meaning it requires a high and consistent supply of nutrients, particularly nitrogen, to form its dense head. Onions also require fertile soil, but their nutrient needs are less demanding than the vigorous cabbage.
Water and Nutrients
A key management task is balancing the watering schedule. Cabbage requires consistent, abundant moisture to thrive, while onions prefer well-drained soil and can suffer from rot if kept too wet. Gardeners should aim for a moist, but not waterlogged, soil environment, often achieved through drip irrigation or careful monitoring. Competition for nitrogen can be mitigated by regularly amending the soil with organic matter or a balanced fertilizer, ensuring the heavy-feeding cabbage gets what it needs without starving the onions.
Yield Considerations
It is worth noting that some studies suggest the presence of onions can slightly decrease the final dry mass of cabbage, indicating a small degree of resource competition may exist. However, this potential reduction in vigor is typically outweighed by the increased protection from pest damage and the higher overall yield of undamaged produce. Careful spacing and nutrient applications are the best ways to ensure both crops have the resources they need to reach maturity.