Leeks (Allium ampeloprasum var. porrum) are a member of the onion family, cultivated as a cool-season annual vegetable. Unlike onions, leeks do not form a distinct bulb but instead produce a thick, edible cylinder of tightly wrapped leaf sheaths, known as the shaft or stem. Their cold tolerance means that once leeks have reached maturity, they can remain in the ground for a significant period, using the soil as a natural, long-term storage facility.
Identifying Leek Maturity
Recognizing when a leek is fully mature is key to determining how long it can stay in the ground. Leeks are a long-season crop, typically requiring between 70 and 150 days to reach a usable size, depending on the specific cultivar planted. Early-season varieties, like ‘King Richard,’ mature more quickly than late-season or overwintering types such as ‘Bandit’ or ‘Tadorna’.
A leek is considered ready when the stem diameter measures approximately one to two inches. The desired edible portion, which is the blanched white or pale green shaft, should also be firm and substantial. Leaves should appear dark green and upright; yellowing or wilting foliage may indicate the plant is past its peak or experiencing stress. Once these physical signs are present, the leek has completed its primary growth phase and is ready to be utilized or left for storage.
Maximizing In-Ground Storage
Leeks possess exceptional cold hardiness, allowing them to be left in the garden throughout the winter and harvested on demand. This practice, known as overwintering, means leeks can remain in the ground for several months past their initial maturity date. Overwintering varieties are bred for this purpose, often featuring a bluish foliage and a greater ability to withstand severe cold.
In many temperate regions, leeks can survive temperatures as low as 10 degrees Fahrenheit without additional protection. Certain hardy varieties, like ‘Autumn Giant’ or ‘Tadorna,’ have been known to survive temperatures dropping to negative 10 degrees Fahrenheit. The key strategy for maximizing this in-ground storage window is the application of deep mulch over the planting area before the harshest cold sets in.
A heavy layer of straw, shredded leaves, or compost, often six inches or more, serves two important functions. First, it insulates the root zone of the leek, protecting the plant from deep-soil freezes that could compromise its integrity. Second, the mulch prevents the ground surface from freezing solid, allowing for easy harvest. In mild climates, where the ground rarely freezes deeply, leeks can successfully remain viable for consumption well into the spring months.
Why Leeks Must Be Harvested
While leeks are highly flexible, there is a biological deadline that eventually forces the harvest: the onset of bolting. Bolting is the process where the plant sends up a thick, rigid flower stalk, signaling its switch from vegetative growth to reproductive growth. For leeks, this is typically triggered by vernalization—a period of cold temperatures followed by the return of warmer spring weather, which mimics the natural biennial life cycle of the plant.
Once the flower stalk, or scape, begins to form, the leek draws energy and moisture away from the desirable white shaft to support the rapidly growing seed head. This internal stalk causes the edible stem to become tough, fibrous, and notably woody down the center, rendering the core unpalatable. Though the plant remains technically edible, the quality degrades significantly, often becoming bitter as well.
If left unharvested into the warmer spring and early summer, leeks also become increasingly susceptible to pests and diseases that thrive in warmer conditions. The plant may begin to propagate itself vegetatively, causing the base to split and form small bulblets, which compromises the integrity of the single, large shaft that is the primary goal of the harvest. Therefore, leeks should be harvested in early spring, just as the weather begins to warm consistently, to avoid the irreversible damage caused by bolting.