Normally we harvest our vegetables before the first fall frost, but the leek (Allium ampeloprasum porrum) is an exception to the rule. It doesn’t mind frost and in fact its flavor improves after a week or two of cold because frost causes the starches present in its stem and leaves to change into sugars. In fact, if you live in a climate with relatively mild winters (zone 7 or above), you can actually harvest “winter leeks” (extra hardy varieties) throughout the winter, as you need them, as if your vegetable garden was a large outdoor refrigerator.
In areas where the ground will be covered in snow for much of the winter, though, it’s not certain that you’ll be able to harvest leeks between December and March. After all, digging them out of the snow would be a hassle! But there’s still no hurry to rush out and harvest them as soon as the leaves change color. Keep on picking them until late fall, as you need them. Traditionally the final leek harvest used to be at the end of November, timed to put fresh leeks on the table at American Thanksgiving, but if the ground doesn’t freeze hard until later than that where you live, you can push the harvest date well into December.
Or you can wait until the following spring, when the snow melts. In many areas of Europe, leeks are considered the first vegetable of spring, earlier even than asparagus, and there is no reason you can’t grow them that way in your garden as well. Just mulch your leeks thoroughly this fall (use 8 to 12 inches/20-30 cm of mulch, such as shredded fall leaves or pine needles) if you don’t have reliable snow cover in your area. The purpose of the mulch is to prevent frost heave: winter leeks may be very hardy, but the repeated action of freezing and thawing can seriously damage them. Mulch will moderate the soil’s temperature and help them through the cold months.
However, when spring does come, harvest your spring leeks before temperatures warm up. That’s because leeks are biennials and will therefore soon start to bloom. When they do, they become tough and unpalatable.
How to Perennialize Leeks
And there is also yet one more possibility: you can also grow winter leeks as perennial vegetables, much like you would grow rhubarb. These perennialized leeks are called perpetual leeks, because you can harvest them at any season. Perpetual leeks have largely fallen out of favor over the last century, but the recent interest in permaculture has brought an increased interest in perennial vegetables. I suspect perpetual leeks will certainly be gaining renewed interest once people realize how easy they are to grow.
Any winter leek can be perennialized. Just leave it in the ground over the winter, but don’t harvest anything that first spring. Instead let the plant bloom. It will produce a tall flower stalk coiffed with a large ball of pretty pale pink or white flowers (leeks are grown as ornamentals in some countries). Before the blooms start to fade, offsets will begin appear at the base of the parent plant. Just let them grow. The result is that your single leek plant will form a clump, and a clump that grows larger over time. Think of how a daylily grows and you’ll get the picture.
Don’t harvest at all this year (the second year after you originally sowed or planted it). You’ll want the plant to multiply a bit first. The real harvest will begin in subsequent years, in year 3 and beyond, as the number of stems will increase massively. However, to keep your perennial leek going, make sure that when you do harvest, you always leave part of the clump in the ground each year to ensure the success of the next generation.
Once perpetual leeks start to produce, you can begin harvesting them at whatever season you choose, even in summer, but remember, they’ll be tastier when it’s frosty, therefore in fall, winter or spring.
Hardy Leeks for Cold Climates
There are many winter leek varieties and most are hardy enough to survive the winter in zone 4 under snow cover or a good mulch. All “winter leeks” can be allowed to perennialize. Here are a few you can try:
‘Alaska’, ‘American Flag’, ‘Autumn Giant’, ‘Bandit’, ‘Belge d’Hiver’, ‘Blaugrüner Winter’ (‘Bluegreen Winter’), ‘Bleu de Solaise’ (‘Blue Solase’), ‘Carentan’, ‘Elephant’, ‘Musselburgh’ (‘Giant Musselburgh’, ‘Large Musselburgh’), ‘German Winter’, ‘Lancelot’, ‘Lexton’, ‘Neptune’, ‘Northern Lights’, ‘Overwinter’, ‘Poireau de Liege’, ‘Siegfried’, ‘Winter Atlanta’, ‘Zermatt’.