When to Harvest Leeks:
Harvest leeks when they are big enough to use.
Some varieties mature at ½ to ¾ inches in diameter; others can grow to 1 inch (2.5 cm) or more in diameter.
• A leek is ready for harvest when its white stem or shaft is 3 inches (7 cm) long or greater. Harvest leeks before they start to widen too much at the base; don’t allow leeks to bulb.
• The top growth of a leek—called the flag—should be dark blue green at harvest. Unlike onions and shallots, leek tops do not die back as the plant matures.
• Most leeks mature 100 to 120 days after sowing seed, but a few varieties mature in as few as 60 days.
• Leeks can be harvested from late summer to early spring depending upon the variety and climate. In milder winter climates, leeks can be overwintered in the garden for spring harvest.
• Some varieties such as ‘Winter Giant’ and ‘Tadorna Blue’ are bred for overwintering. Temperatures as low as 20°F (-6°C) will not harm these varieties. If you leave leeks in the ground for winter harvest, hill up the soil around the plants and cover them with a heavy layer of mulch until you need them.
• Non-hardy leeks ready in 60 to 90 days will be milder flavored than long-growing, hardy varieties. Harvest non-hardy varieties in the summer and fall before the first frost.
How to Harvest Leeks:
Digging leeks is easier than pulling. Leeks have a large root system. Use a hand fork or garden fork to loosen the soil before lifting leeks. Where the soil is loose, you can harvest leeks by hand by gently twisting and pulling them from the soil.
How to Store Leeks:
Give harvested leeks a shake and brush off as much soil as possible then rinse the plant thoroughly.
Leeks are most flavorful used fresh.
Store leeks wrapped in a damp paper towel in a perforated plastic bag in the refrigerator for 7 to 10 days. Smaller leeks store better than larger leeks. Trim the roots and wash the leaves and stem before refrigerating.
Leeks store best at 32°F (0°C) and 95 to 100 percent relative humidity. Yellowing and decay develop rapidly at warmer storage temperatures. High relative humidity is essential to prevent wilting.
Trim the leaves and slice leeks in half lengthwise immediately before use and rinse out any remaining soil that is often caught between the tight leaves.
More tips: How to Grow Leeks.