Leeks (Allium ampeloprasum) with a long white stalk have always been considered highly desirable by gardeners. To obtain such whiteness, the tradition has always been to either plant leeks in a trench that is then gradually filled in with soil (“trenching”) or to plant them at a normal planting depth, then to push up an increasingly high mound of soil in order to cover the stalks (“hilling”). In both cases, the stalk is deprived of sunlight and turns pale (blanches), giving the desired whiteness. Most leeks sold in markets and grocery stores are still blanched.
However, covering the stalk with soil is also what allows soil to work its way in among the leaves and down into the stalk. And finding grains of sand in their leek that has given many people a disdain of this otherwise wonderful vegetable.
But why not consider the alternative: growing your leeks without either trenching or hilling? After all, a white stalk is only aesthetic: blanching leeks is not at all necessary for their health or survival. Although gourmets may claim blanching improves their taste, that affirmation has been consistently refuted: in blindfolded taste tests, even the gourmets themselves were unable to distinguish between blanched and unblanched leeks! Also, when your guests bite into a leek that wasn’t blanched, they won’t risk breaking a tooth on a stone!
A unblanched leek still has a white base, but the upper part of the stalk will be pale green rather than white. Is that really the end of the world? I therefore suggest just letting leeks grow naturally: it’s certainly the most laidback method of growing them.
However, if you just can’t imagine a leek without a long white stalk, there are two other methods of reaching your goal: using self-blanching varieties and mulching.
Self-blanching leeks are just what the name suggests they are: they produce white stalks (or at least, nearly white stalks) even when exposed to the sun’s rays. So, no trenching or hilling is required. They’re better for your health than blanched leeks and far easier to grow. One cultivar you could try is ‘Takrima’.
Blanching Using Mulch
The other alternative is to blanch leeks, but with mulch rather than soil: straw, chopped leaves or whatever other organic mulch is available.
Simply transplant so your leeks into the garden as with any other vegetable, covering only the base of the stalk with soil (therefore, without trenching). Then cover their stalk with 4 inches (10 cm) of mulch. As the season progresses and the stalk lengthens, continue to add mulch, up to a depth of about 10 inches (25 cm). The mulch will prevent light from reaching the stalk, thus blanching it, giving you the whiteness your heart desires!
Mulching also cuts down on weeds and keeps the soil cooler and moister, much to the pleasure of leeks, which do best in cool, moist conditions.
The two other advantages of mulching leeks are that mulching requires far less effort than trenching or hilling with soil … and that mulch contains no soil particles that can work their way into your leek stalks. Therefore, your leeks will be snap to clean and there will be no sand particles to upset your guests when you serve them your homegrown leeks!
The only thing left to do is to learn how to protect leeks from their terrible enemy, the leek moth, now common in many areas. For that, I refer you to Leek Moth: Coming Soon to an Onion Near You.