Companion planting involves strategically placing different crops near one another to encourage beneficial relationships. However, this strategy also includes recognizing negative interactions, known as incompatibility or negative allelopathy. For a crop like garlic, which is in the ground for many months, identifying its poor neighbors is especially important. The goal is to avoid planting species that will either stunt the garlic’s development or suffer harm from the garlic’s presence.
Plants That Hinder Garlic Development
Garlic requires consistent access to nutrients and full sunlight to form large, healthy bulbs. Certain neighboring plants can severely inhibit this bulbing process. Large, leafy members of the Brassica family, such as mature cabbage, broccoli, and cauliflower, are heavy feeders that rapidly deplete soil nitrogen and other micronutrients. When positioned too close, they monopolize the available food, leaving the slower-growing garlic with insufficient resources to bulk up its underground bulb.
Competition also arises from tall, dense vegetation that casts shade over the garlic leaves. Garlic needs a minimum of six to eight hours of direct sunlight daily for its foliage to photosynthesize effectively. Plants like fennel or mature corn can grow tall and broad, intercepting the necessary light and resulting in smaller, underdeveloped garlic cloves. Perennials like asparagus also pose a risk; their thick, fern-like growth can shade the garlic, and their vigorous root systems compete intensely for soil moisture and nutrients.
Certain aromatic herbs, including sage and parsley, are poor companions because they compete directly with garlic for root-zone nutrients. Both are relatively heavy feeders, and their proximity can draw away phosphorus and potassium that garlic needs during its bulbing phase.
Furthermore, planting other members of the Allium family, such as onions, leeks, or shallots, creates a localized concentration of host plants for shared pests like onion maggots and onion flies. This grouping increases the risk of a widespread pest infestation that can damage the entire allium crop.
Crops That Suffer When Planted Near Garlic
Garlic’s presence can be detrimental to a different set of plants, often due to the strong biochemicals it releases into the soil. The most widely recognized crops that suffer near garlic are legumes, which include beans and peas. These plants rely on a symbiotic relationship with specific soil bacteria to “fix” atmospheric nitrogen into a usable form within root nodules. Garlic’s natural sulfur compounds, such as allicin, can interfere with these nitrogen-fixing bacteria, effectively suppressing the legume’s ability to enrich the soil. This interference leads to stunted growth, reduced vigor, and a poor yield of pods and beans.
Delicate herbs and certain root vegetables may also be negatively affected by the strong allelochemicals exuded by garlic’s roots, exhibiting poor development when grown in close proximity.
Even established perennial crops can be negatively impacted. Grapevines, for instance, are susceptible to nutrient competition from garlic, as both plants thrive in similar soil conditions and require the same types of resources. This competition can result in stunted growth for both the garlic and the developing grapevines.
Why Certain Plants Do Not Mix
One primary mechanism for negative interactions is allelopathy, where a plant releases biochemicals into the environment that inhibit the growth of nearby species. Garlic’s strong sulfur-based compounds, which are responsible for its flavor and pest-repellent qualities, act as allelochemicals. These compounds directly disrupt the physiological processes of sensitive plants, most notably interfering with the root nodulation necessary for nitrogen fixation in legumes.
Another significant issue is nutrient competition, which occurs when two or more plants demand a high quantity of the same limiting resource from the soil. Heavy feeders, such as large brassicas and herbs like sage, require substantial amounts of nitrogen and other macronutrients to support their fast growth. When planted near garlic, the more aggressive competitor will absorb the majority of the resources, resulting in a smaller harvest for both species.
Finally, conflicts in physical structure and growth habit play a role. Tall plants like corn or the mature ferns of asparagus create significant shading, preventing the sun-loving garlic from receiving the energy needed to form a large bulb. Furthermore, aggressive or dense root systems can lead to root structure conflict, physically impeding the expansion of the garlic bulb and root mass.