When you see leeks forming bulbs at the base, stress caused the problem in your garden. Wild swings in temperature or gaps in your watering trigger this response in the plants. The leek thinks hard times are coming and starts to store energy in a swollen base instead of building a long stem.
I saw this happen in my first garden during a brutal heat wave one July. The leeks I missed watering for three days all grew swollen bases that looked like onions. Plants in the same row that got steady water stayed uniform in shape. That taught me how fast stress can change the way leeks grow.
The next year I set up drip lines and a timer to water my beds on schedule. Not a single plant bulbed that whole season in my garden. The simple fix of steady water made all the gap in the world for this crop. I now tell every new gardener to focus on even moisture above all else when growing leeks.
True leeks should keep the same width from top to bottom of the white section when grown well. The leek bulbing problem shows up as a round swell near the soil line that looks like an onion. This is not normal growth for this crop. It means the plant switched from making stem to storing food for survival.
Texas A&M notes that unlike onions, leeks do not form bulbs when grown in good conditions. The thick, fleshy stalk stays about the same width at the base as it does higher up on the stem. When you see why leeks swell at base, you know something went wrong with the growing conditions in your garden.
Cold soil in spring can start the bulbing cycle before plants get going. Planting too early before the ground warms up stresses young plants beyond their limits. They respond by forming that onion-like base to store energy. Wait until soil temps reach 50°F (10°C) to move your transplants outside.
A leek not forming bulb is what you want to see in healthy plants in your garden. To get this result, keep your watering steady at 1 inch (2.5 centimeters) per week all season. Mulch around plants to hold soil moisture even in dry weather. Avoid planting in spots that get very hot then very cool during the day.
Bulbed leeks still taste fine even though they look odd in your harvest basket. Pick them right away before the texture turns tough and woody. Use the swollen base like you would use the white part of a normal leek in your cooking. The flavor stays mild and sweet as long as you cook it soon after picking from the garden.
Read the full article: Growing Leeks: Beginner-Friendly Guide to Sweet Harvests
