Kohlrabi is a unique cool-season crop, a member of the cabbage family known for its swollen, bulb-like stem that grows above ground. Like other brassicas, kohlrabi thrives in rich, well-draining soil and requires consistent moisture, making it a heavy feeder. Companion planting involves strategically placing different plant species near one another to provide mutual benefits, such as enhanced growth or natural pest management. This approach creates a more balanced garden ecosystem, reducing the need for chemical interventions.
Beneficial Planting Partners
Aromatic herbs serve as effective natural deterrents against common kohlrabi pests like cabbage worms and flea beetles. The strong scent of herbs such as dill, mint, and sage can confuse pests, masking the attractive odor of the kohlrabi foliage. Dill is particularly beneficial because its umbrella-shaped flowers attract predatory insects like hoverflies and parasitic wasps, which feed on aphids and young cabbage worms, offering biological pest control.
Certain plants offer physical benefits, primarily by improving soil conditions or providing shade. Bush beans, for example, are valuable companions because they fix atmospheric nitrogen into the soil through a symbiotic relationship with rhizobia bacteria, which becomes available to the kohlrabi. This gentle addition of nitrogen supports leafy growth without causing excessive nutrient imbalance. Shallow-rooted crops like lettuce, spinach, and beets are ideal neighbors because they utilize different soil depths than kohlrabi, minimizing competition for water and nutrients.
These low-growing companions also act as a living mulch, covering the soil to help retain moisture and keep the root zone cool, which prevents kohlrabi from bolting prematurely in warmer conditions. Nasturtiums function as trap crops, meaning they are more attractive to pests like aphids, drawing insects away from the main crop. Planting a border of nasturtiums can effectively sacrifice these flowers to protect your kohlrabi harvest, while their roots also help aerate the soil.
Plants to Avoid
Avoid planting kohlrabi near its close relatives in the Brassica family, such as broccoli, cauliflower, kale, and cabbage. These plants share genetic vulnerability, attracting the exact same pests, including cabbage loopers and diamondback moths, leading to concentrated infestations. Furthermore, they are all heavy feeders with similar nutrient requirements, resulting in intense competition for soil resources and water, often stunting the growth of all plants.
Plants with aggressive root systems or high nutrient demands also make poor neighbors for kohlrabi. Tomatoes are notoriously heavy feeders that rapidly deplete soil nutrients, leaving little for the kohlrabi to develop its bulb. Their extensive root systems and large canopy can also shade out the lower-growing kohlrabi, inhibiting development. Similarly, corn, with its deep and widespread roots, competes fiercely for water and nitrogen, which can reduce kohlrabi yields.
Fennel should be kept separate from kohlrabi due to its allelopathic properties. This plant releases chemical compounds into the soil that inhibit the growth of many nearby vegetables, including brassicas, leading to poor development and reduced harvests. Strawberries are also a bad pairing because they can harbor specific diseases and pests, such as Verticillium wilt and nematodes, which easily transfer to the neighboring kohlrabi.
Practical Placement and Timing
Effective companion planting relies on selecting the right partners and strategic placement, often utilizing intercropping techniques. Intercropping involves planting different crops simultaneously in the same space, such as block planting herbs throughout the kohlrabi patch instead of just along the edges. This dispersal ensures the pest-confusing scent is spread evenly across the planting area, maximizing its protective effect.
Spacing is important when intercropping to ensure kohlrabi’s growth is not constrained; maintain eight to twelve inches between kohlrabi plants and their vegetable neighbors. Fast-maturing companions like radishes or leaf lettuce can be planted between the slower-growing kohlrabi plants, utilizing succession planting. These quick-to-harvest crops are removed within a few weeks, freeing up space and nutrients before the kohlrabi bulbs begin to swell and require the full space.
Timing the planting of companions is crucial, especially for cool-season kohlrabi. For a spring crop, quick-growing companions should be planted simultaneously to ensure they are harvested before summer heat arrives and kohlrabi needs more room. Conversely, for a fall crop, companions like dill can be planted later in the season. They will bolt less readily in cooling temperatures, maintaining their pest-deterring properties throughout the kohlrabi’s development.