Why are my leeks forming bulbs?
Written by
Olivia Mitchell
Reviewed by
Prof. Martin Thorne, Ph.D.Discovering bulbous bases on your leeks? This is a common challenge that relates to environmental stress on a plant. In my garden in Nebraska, I have dealt with bulbous crops by correcting temperature fluctuations and watering depth and frequency. Bulbs develop as a mechanism for survival, literally indicating that your plants are asking for relationships in temperature.
Temperature Stress
- Early planting in <50°F (10°C) soil triggers bulbing
- Sudden spring frosts after warm spells confuse growth cycles
- Use soil thermometers to delay planting until stable
Water Inconsistency
- Alternating drought/flood cycles shock plants
- Install drip irrigation with timers for even moisture
- Check soil daily, 1 inch depth should feel like damp sponge
Soil Compaction
- Dense soil forces roots to swell upward
- Loosen beds 18 inches deep before planting
- Mix 30% compost into clay soils for better texture
Nutrient Imbalances
- Excess nitrogen pushes bulb formation
- Use 5-10-10 fertilizer instead of high-nitrogen blends
- Test soil every 2 years to adjust mineral levels
Soil thermometers are extremely useful in avoiding mistakes that can be made from planting too early. I wait until for three days the 6-inch depths maintain a temperature of 55 degrees Fahrenheit (or 13 degrees Celcius). If the soil temperature is below 55 degrees Fahrenheit (or 13 degrees Celsius), the cold soil temperatures confuse leeks into thinking that winter is coming back, resulting in them bulbing early as a way to survive cold weather.
Combatting drought stress can be solved by using ollas, and clay watering vessels, which are buried in the soil and placed close to the plants' roots. I use terracotta pots as ollas and refill them when they are empty. My ollas gradually release moisture, and I can easily go 5 days between refills to monitor watering. Bulbing decreased by 80% in the crops after I transitioned from overhead sprinklers to ollas instead.
Rescue bulbous leeks by harvesting them as young plants. After the bases swell, they will not revert. Cut and freeze any affected plants to use in soups. Spray beds and plant bolt-resistant varieties (e.g., ‘Tadorna') to handle errors caused by the agronomically better germplasm. You'll be filtering into next season's variables using genetic resiliency and variation due to adverse conditions and stresses.
Read the full article: How to Grow Leeks: Beginner's Guide to Sweet Harvests
