Mustard greens (Brassica juncea) are a fast-growing, cool-weather leafy vegetable prized for their pungent flavor and quick harvest time. Gardeners often use companion planting to group beneficial plants together. However, some plants can actively hinder growth by competing for resources, sharing vulnerabilities, or releasing growth-stunting chemicals. Identifying these incompatible neighbors is a preventative step that can significantly improve the health and yield of your mustard greens.
Identifying Incompatible Plants
Successful gardening with mustard greens requires keeping them separated from other plants that fall into three main categories: heavy nutrient feeders, allelopathic species, and members of their own family. High-demand crops like tomatoes, corn, squash, and eggplant should be planted elsewhere because they rapidly deplete the soil of nutrients that mustard greens need for their leafy growth. This competition for food and water can quickly stunt the greens.
Certain plants release chemicals into the soil or air that actively suppress the growth of nearby species, a phenomenon known as allelopathy. Fennel is a powerful allelopathic plant that should be isolated from nearly all garden vegetables, including mustard greens. Sunflowers also exude compounds from their roots and decaying leaves that can inhibit the growth of surrounding crops. Furthermore, all members of the Brassica family, such as cabbage, broccoli, kale, and collard greens, are poor neighbors for mustard greens because of shared biological vulnerabilities.
Competition for Essential Resources
Mustard greens are light to moderate feeders, thriving on a steady supply of nitrogen, which encourages lush leaf production. Heavy feeders like tomatoes and corn require significantly more nitrogen and other macronutrients throughout their long growing season. When planted too closely, these high-demand crops effectively starve the mustard greens, resulting in stunted, pale, and less flavorful leaves.
Physical competition for light also creates issues, especially with tall, sprawling plants. Sunflowers and corn can cast a dense shadow over the low-growing mustard greens. Mustard greens require partial to full sun to grow vigorously, and excessive shading will slow their growth and lead to a less productive harvest. Plants with large, aggressive root systems, such as squash or potatoes, can also interfere with the shallow root structure of mustard greens, limiting their ability to absorb water and nutrients.
Shared Vulnerability to Pests and Disease
Planting mustard greens near other members of the Brassica family creates a concentrated breeding ground for shared pests and diseases. Pests like flea beetles, cabbage worms, and aphids readily move between a patch of mustard greens and nearby cabbage or kale. A shared planting area allows these insect populations to multiply quickly and sustain themselves across multiple crops.
Certain diseases also spread easily among related plants, making crop rotation and separation a necessary preventative measure. Soil-borne diseases such as clubroot, which causes swollen and distorted roots, can persist in the soil for years and infect all nearby brassicas. Black rot, a bacterial disease, and fungal issues like downy mildew are also easily transmitted when family members are grouped together. Avoiding the planting of other brassicas nearby creates a biological barrier that helps contain any outbreak.