Why Have My Leek Plants Stopped Growing?

Leek plants that were growing steadily and then abruptly stalled — remaining the same size week after week, not producing new leaves, not developing shank diameter — have had something interrupt their growth. Leeks are slow-growing by nature, but even slow growth should be measurable week on week during the main growing season (June through September). A plant that makes no visible progress for three to four weeks in summer has a problem worth investigating.

Root damage from pests

Onion fly larvae feeding on the root plate and shank base cause sudden stunting followed by wilting and plant death. If stunted plants also have yellowing, drooping leaves and pull out of the ground with soft, rotted bases, onion fly maggots are likely present. Check the base of a stunted plant — maggots (5–8 mm, white) are usually visible. Remove affected plants, cover the remainder with fine mesh netting immediately, and disturb the surrounding soil to expose any pupae.

Waterlogged or compacted soil

Leeks in waterlogged ground stop growing because their roots cannot take up water or nutrients efficiently without adequate oxygen. Soil compaction has the same effect — roots cannot penetrate compacted layers and the effective rooting zone becomes limited. Mid-season stunting that worsens after wet weather and improves slightly in dry periods points to waterlogging. Check drainage and improve it for future seasons with organic matter and grit incorporation.

White rot developing at the base

White rot (Sclerotinia cepivorum) causes gradual stunting as the basal plate and roots are progressively destroyed. Plants look like they are simply not growing. Exposing the base of a stunted plant — carefully clearing a small amount of soil — reveals white fluffy mould with black sclerotia if white rot is present. Remove all affected plants and treat the area as white rot-contaminated for future allium rotations.

Nutrient depletion in dry conditions

In a dry summer, nutrients dissolved in soil water become unavailable to roots even when the soil is not bone dry. Leeks in free-draining soil without supplementary watering or feeding can plateau in growth during extended dry spells. Watering and a liquid feed combined should restart growth within a week if dry nutrient lockout is the cause.