Garlic and onions can be planted together in the garden. These popular bulb vegetables belong to the same botanical family, Alliaceae, which also includes chives, leeks, and shallots. Because they are closely related, garlic (Allium sativum) and onions (Allium cepa) share nearly identical environmental preferences. This compatibility makes interplanting them a feasible and efficient strategy, provided growers account for the specific challenges of grouping members of the same plant family.
Shared Cultivation Requirements
Garlic and onions thrive under the same general growing conditions, simplifying the preparation of a single planting area. Both require a location that receives full sun, needing a minimum of six to eight hours of direct sunlight daily for optimal bulb development. This high light requirement fuels the photosynthesis necessary to store energy in the underground bulb structure.
The soil structure is the most important shared requirement for both crops. They perform best in a well-drained, loose, loamy soil texture that prevents water from pooling around the developing bulbs. A soil pH between 6.0 and 7.5 is ideal, and amending the bed with organic matter, such as aged compost, improves drainage and fertility. This environment is necessary because both bulbs are highly susceptible to rot in saturated conditions.
Their watering needs align, requiring consistent moisture without becoming waterlogged, especially during bulb enlargement. Both possess relatively shallow root systems, allowing them to coexist in the top layer of the soil without intense competition for nutrients at greater depths. This shared preference for surface-level nutrients means the soil must be regularly replenished with fertility.
Managing Shared Pests and Diseases
Grouping members of the Allium family in one location increases the risk of concentrating pests and pathogens. Because garlic and onions are genetically similar, they are susceptible to the same soil-borne diseases and insect pests. This shared vulnerability means that an outbreak in one plant can rapidly spread to the entire bed, potentially wiping out both crops simultaneously.
A primary concern for Allium growers is the fungal pathogen Sclerotium cepivorum, which causes white rot. This devastating soil-borne disease produces tiny, black, resting structures called sclerotia that can remain viable in the soil for decades. Another common threat is Fusarium basal rot, a soil pathogen that attacks the roots and basal plate of the bulb, often resulting in yellowing leaves and plant death.
Several insect pests also target both crops, including the onion thrips (Thrips tabaci) and the onion maggot (Delia antiqua). Onion thrips are minute insects that suck sap from the leaves, causing silvery streaks and stunted growth that reduces bulb size. The onion maggot is the larva of a fly that tunnels into the bulbs and stems, causing wilting and decay.
Preventative measures are crucial when interplanting, focusing on cultural practices to minimize disease spread. Ensuring good air circulation by maintaining proper spacing helps the foliage dry quickly, reducing the incidence of foliar fungal diseases like downy mildew and rust. Immediate removal and destruction of any diseased plant, a process known as roguing, is necessary to prevent the pathogen from spreading.
Spacing, Competition, and Crop Rotation
Successful interplanting depends on providing adequate space to manage competition for resources and mitigate disease risk. While their root systems are shallow, planting garlic and onions too closely together will cause them to compete intensely for surface nutrients, particularly nitrogen, and water. This competition results in stunted growth and the formation of smaller, less desirable bulbs for both crops.
Recommended spacing for garlic cloves is typically four to six inches apart, while onion sets or transplants need three to six inches between individual plants for proper bulb expansion. When planting them together, a minimum of four inches between any two Allium plants is necessary to reduce root overlap and ensure sufficient airflow. Creating alternating rows of each crop, rather than dense blocks, helps manage the area more effectively.
After harvest, adherence to a long-term crop rotation plan is the most effective strategy for maintaining soil health in an Allium-dedicated bed. Growers should avoid planting any Allium family member in the same spot for a minimum of three to four years. This extended break starves pathogens and prevents the buildup of pests like the stem and bulb nematode. Following Allium crops with a non-related plant family, such as legumes or cucurbits, helps break the pest and disease cycle and allows the soil to recover its balance.