Kohlrabi is a unique cool-season crop belonging to the cabbage family. Gardeners cultivate it for its edible, swollen stem, which is technically a corm, though often called a “bulb.” When a kohlrabi plant fails to produce this fleshy corm, resulting only in a large rosette of leaves, the cause is usually environmental stress or improper cultivation. This failure to swell is preventable by understanding the plant’s specific needs during its short growing period.
Temperature and Sunlight Requirements
Kohlrabi thrives in consistent, cool conditions, making it best suited for spring or fall planting. The ideal temperature range for the stem to swell properly is between 60 and 75°F. Fast, uninterrupted growth during this period is linked to producing a sweet, tender corm.
Prolonged heat stress, especially temperatures consistently above 80°F, can trigger “bolting.” Bolting causes the plant to prematurely shift energy from stem swelling to reproductive growth by forming a flower stalk. This diversion results in a small, woody, and bitter stem. Kohlrabi needs a minimum of six hours of full sunlight daily. In warmer climates, providing afternoon shade helps keep the plant cool and prevents bolting.
Improper Planting Depth and Density
Planting depth and spacing significantly influence corm development. The swollen stem must form just above the soil line, unlike root vegetables such as turnips or beets. If the seedling is planted too deep, the corm attempts to swell beneath the surface, often resulting in failure to develop or a misshapen, stunted stem.
When transplanting, ensure the base of the plant sits at or slightly above the soil surface. Overcrowding is another factor, as kohlrabi needs adequate space to swell freely. Plants should be thinned to a final spacing of 9 to 12 inches apart. Dense plantings create competition for resources, preventing the corm from expanding properly and resulting in smaller, tougher stems.
Nitrogen Overload and Soil Health
An overabundance of nitrogen is a frequent culprit behind excessive leafy growth and poor stem development. Nitrogen promotes vigorous vegetative growth, resulting in large leaves at the expense of the storage organ. While kohlrabi needs nitrogen early to establish its leaf canopy, too much nitrogen during the corm-swelling phase signals the plant to focus solely on foliage.
For proper stem development, kohlrabi requires sufficient phosphorus (P) and potassium (K). These elements are fundamental for storage organ growth. Avoid high-nitrogen fertilizers once the plant is established. Instead, opt for a balanced blend or one with a lower Nitrogen number relative to Phosphorus and Potassium. Consistent moisture is equally important; inconsistent watering stresses the plant, halting development and potentially leading to a tough, woody texture or causing the corm to split.
Actions to Encourage Bulb Development Now
If kohlrabi plants show excessive leafy growth but no corm, immediate actions can encourage development. First, if the stem base is buried, gently pull back surrounding soil or mulch to expose the swelling area. This releases physical restriction and signals the plant to begin corm expansion above ground.
To correct nutrient imbalance, apply a side-dressing of fertilizer low in nitrogen but rich in phosphorus and potassium, such as a 5-10-10 ratio or bone meal. This redirects energy toward stem growth. During intense afternoon heat, temporarily installing a shade cloth reduces heat stress, a common trigger for bolting. Finally, ensure consistent, deep watering to maintain even soil moisture, which is necessary for the corm to swell quickly and remain tender.