Kale (Brassica oleracea) is a leafy green vegetable that thrives in cool weather and produces a steady harvest. Like all members of the cabbage family, kale requires rich soil and consistent moisture to flourish. Companion planting involves strategically placing different species near each other for mutual benefit. Successful gardening requires understanding the negative relationships between plants, as well as the beneficial pairings. Identifying neighbors that compete with or harm kale prevents stunted growth, nutrient deficiencies, or pest infestations.
Nutrient and Water Competitors
Kale is known as a heavy feeder, demanding significant amounts of nitrogen and other macronutrients from the soil to support its large, leafy growth. Planting kale next to other vegetables with similarly high demands can result in a direct competition for these finite resources. When two heavy feeders are placed in close proximity, the result is often a diminished yield for both plants as their root systems vie for the limited supply of nutrients.
Plants with extensive root structures, such as tomatoes, are especially problematic near kale. Tomatoes require large volumes of water and a constant uptake of nutrients like calcium and phosphorus. This relentless resource extraction can effectively starve a nearby kale plant, which needs substantial nitrogen to produce its characteristic foliage.
Large vining plants like squash, pumpkins, and cucumbers consume copious amounts of water and soil nutrients. These plants compete aggressively beneath the soil, and their dense foliage can spread out and shade the kale. The combination of root competition and light deprivation severely limits the kale’s ability to photosynthesize and absorb resources for healthy leaf development.
Plants That Attract Shared Pests
A significant danger to kale comes from planting it near species that act as reservoirs for its most common pests and diseases. Kale belongs to the Brassicaceae family, and its close relatives share a genetic vulnerability to the same destructive insects. Placing these plants together creates a concentrated target area, allowing pests to move easily from one host to another.
Other members of the Brassica family, including cabbage, broccoli, cauliflower, kohlrabi, and Brussels sprouts, should be kept separate from kale. These plants are primary hosts for the cabbage white butterfly (cabbage worms), cabbage loopers, and flea beetles. These pests chew characteristic small holes in the leaves of all Brassica crops.
When these relatives are planted together, a pest infestation is more likely to reach epidemic levels because the entire planting area provides a continuous, abundant food source. For instance, the fungal disease clubroot, which affects the roots of Brassica plants, can easily spread through shared soil. Separating these susceptible plants reduces the chance of a garden-wide crop failure from shared biological threats.
Growth-Inhibiting Neighbors
Some plants negatively affect kale by actively suppressing its growth via chemical means or physical dominance. This chemical suppression is known as allelopathy, where one plant releases biochemical compounds that inhibit the growth of another.
Fennel is an allelopathic plant that should never be grown near kale or almost any other vegetable. It releases compounds through its roots and decaying foliage that are toxic to many nearby species. The volatile oils and chemicals released by fennel effectively stunt the growth of garden plants.
Aggressive herbs like mint can physically choke out kale by forming dense, shallow root mats. Mint’s vigorous, spreading root system rapidly colonizes the top layer of the soil. This dense root network inhibits the kale’s ability to establish its own roots and access surface moisture, leading to a suppressed, unhealthy plant.