Leek is a biennial allium grown as an annual vegetable for its bundle of tightly wrapped leaf sheaths, which deliver a milder, sweeter flavor than onion. The edible portion is technically a pseudostem formed by overlapping leaf bases, not a true stem. It is one of the most cold-tolerant vegetables in temperate gardens, capable of standing unharvested through hard frosts and resuming growth when temperatures rise, making it especially valuable for extending the harvest from late fall into winter.
Leeks require a long growing season — typically 100 to 120 days from transplant — so most gardeners start them indoors 10 to 12 weeks before last frost. Setting transplants into 6-inch-deep holes or a trench and gradually hilling soil around the growing shaft blanches the lower portion white and reduces bitterness. Preferred soil pH is 6.0 to 7.0. Plants left in the ground through winter benefit from a light mulch in zones below 5.
Leeks are central to French, Welsh, and Belgian cuisines, used in soups, gratins, and braises. They are high in vitamins K and A, folate, and the flavonoid kaempferol, which has been studied for cardiovascular protective effects. The sulfur compounds common to all alliums have demonstrated antimicrobial activity in laboratory studies, and traditional herbalism uses leeks to support digestion and as a mild diuretic.
White Rust
White, blister-like pustules on the undersides of leaves with yellow spots on the upper surface; severe infections distort leaves and stems.
Remove and destroy infected leaves, improve air circulation, avoid overhead watering, and rotate brassicas to a new bed each year.
Downy mildew
Yellowing on top of leaves with grey-purple fuzz underneath. Spreads rapidly in humid conditions.
Remove affected leaves. Improve air circulation. Avoid overhead watering. Copper spray as preventive.
Botrytis (grey mould)
Grey fuzzy mould on petals and stems, worst in cool wet conditions.
Remove affected parts immediately. Improve air circulation. Avoid overhead watering. Apply copper fungicide if severe.
Damping Off
Seedlings collapse at the soil line with thin, water-soaked stems; seeds may fail to emerge or rot before sprouting.
Sow in well-draining mix, avoid overwatering, and ensure good airflow. Remove affected seedlings and let the surface dry between waterings.
Rust
Orange to brown raised pustules on the undersides of leaves, with yellow spotting on the upper surface. Heavy infections cause leaves to yellow and drop.
Remove and destroy infected leaves. Avoid overhead watering and improve air circulation. Apply a sulfur or copper-based fungicide if it spreads. Clear plant debris in fall.
Onion Thrips
Thrips tabaci →Silvery streaks and stippling on leaves, distorted growth, and tiny pale insects hiding in leaf bases during hot, dry weather.
Spray plants with water to dislodge thrips, encourage predatory insects, and apply insecticidal soap if populations build. Keep plants watered to reduce stress.
Allium Leaf Miner
Phytomyza gymnostoma →Rows of small white feeding punctures run along leaf surfaces, followed by pale winding mines inside leaf tissue. Infested shafts may contain small white maggots and show rotting or distortion at the base; plants can be killed outright in heavy infestations.
Cover plants with fine-mesh insect netting before adult flies emerge in early spring and again in early fall. Remove and destroy all infested plant material. No chemical treatment is effective once larvae are inside the tissue. Rotate alliums to a new bed each year to disrupt the overwintering population.