Why Is My Leek Flowering and Going to Seed?

A leek that sends up a tall, round flower stalk — topped with a globe-shaped head of purple-white flowers — is bolting. Like all alliums, leeks are biennial plants: in their natural lifecycle they grow vegetatively in the first year, overwinter, and then flower and set seed in their second year. The bolting problem in the garden arises when overwintered leeks are triggered to flower prematurely in spring or when conditions cause even young plants to behave as if they are in their second year. A bolted leek develops a hollow woody flower stalk running through the centre of the shank, making it inedible.

Why leeks bolt

Leek bolting is triggered by vernalisation followed by warming — the same mechanism as in onions. When a leek plant of sufficient size experiences a prolonged cold period (typically several weeks below 10°C, especially below 5°C), it registers this as having completed a winter cycle. When temperatures then rise in spring, it attempts to flower and set seed. This is entirely natural for an overwintered leek and is why many leeks that have been in the ground since autumn will bolt in the following spring if not harvested before the warming begins. The critical factors are the size of the plant when cold arrives, the duration and depth of the cold, and how quickly temperatures rise afterward.

Variety matters enormously

Leek varieties are bred and selected for specific harvest windows: early season (August–November), mid-season (November–January), and late season (January–April). A variety selected for early harvest, if left in the ground through winter and into spring, will almost certainly bolt. A late season variety is bred to resist bolting through winter and remain harvestable into spring. Always check the harvest window when buying seed and choose a variety whose intended window matches when you want to harvest. Growing an early variety and trying to keep it through winter is the most common cause of unexpected bolting.

What to do with a bolted leek

Once a flower stalk has emerged, the leek cannot be saved as a culinary vegetable in the normal sense. The central stalk will be hollow and woody. However, if you catch it early — as soon as the stalk is visible but before it has elongated significantly — you can harvest the leek immediately. Trim away the woody central stalk and use the surrounding tissue in cooked dishes (soups, stews) where any slight bitterness or textural difference from the stalk is not noticeable. Do not leave bolted leeks in the ground — they attract pests and diseases.