Leek

Start leeks indoors, 4 to 10 weeks before your last frost

Sow seed a quarter inch deep in a moistened soilless mix indoors, counting back 4 to 10 weeks from your average last spring frost (in northern climates that lands in late February or March). Keep the mix near 70F until germination, then move the flat under lights or into a very bright window. When the grass-like seedlings reach 6 to 7 inches, cut them back to 1 1/2 to 2 inches, and use the trimmings like chives. A leek needs a long season to size up, so the head start is what decides whether you get a fat shaft or a pencil. Trimming the tops keeps the seedlings from flopping and pushes growth into the roots. Counting back from YOUR last frost rather than a fixed date is what makes this portable across zones 2 to 11.

Drop transplants into a deep hole, and do not fill it in

Once daytime temperatures hold at 45F or above (Minnesota Extension's threshold), harden the seedlings off for five to seven days, then drop each one into a hole about 6 inches deep, up to the first leaf notch. Space the holes 6 to 9 inches apart in a home garden; 2 to 6 inches works if you want more, thinner shafts. Do not backfill the hole - just water them in and let the soil collapse in on its own over the following weeks. The white edible shaft is only the part of the stem that grew in darkness, so the depth of that hole is literally the length of your harvest. Backfilling packs soil into the leaf notch and grits the layers you will later eat.

Hill soil up the shaft two or three times

As the plants grow, bank 2 to 3 inches of soil or compost against the stems, two or three times over the season, stopping at the point where the leaves fan out. Blanching is what makes the shaft tender and mild rather than tough and oniony, and it only happens where light is excluded. Hilling past the leaf fan buries the growing point and traps soil between the layers.

Sidedress nitrogen twice while the shaft is bulking

Sidedress a balanced fertilizer such as 10-10-10 at one cup per 10 feet of row, spread about six inches out from the plants and scratched in. Utah State splits this as a quarter pound of 21-0-0 per 100 square feet, applied twice about six weeks apart during the bulking stretch. Leeks are shallow-rooted and spend a long season laying down leaf after leaf, and every leaf is a future ring of shaft, so a nitrogen lull in midsummer shows up as a thin stem in fall.

Trap for leek moth, then count 7 to 10 days

Hang a pheromone trap and check it weekly. There are three flights a season, and eggs hatch roughly 7 to 10 days after you start catching adults - that lag, not the calendar, is when a treatment is worth making. Pull and destroy allium debris at season's end so the adults have nowhere to overwinter. Leek moth larvae mine inside the folded leaves and down into the shaft, so once you can see the windowpaning damage the caterpillar is already somewhere a spray cannot wet. The trap catch is the only warning you get while the eggs are still on the leaf surface.

Thrips and leek moth: spinosad, and aim into the axils

If thrips silvering or leek moth mining forces your hand, spinosad (Entrust) or spinetoram (Radiant) are the home-scale materials - add a spreader and aim down into the leaf axils, because both pests shelter where the leaves fold together. Bt kurstaki is the gentlest option for leek moth but often will not hold it alone, since the larvae mine inside the leaf where Bt cannot be eaten. Thrips rasp the leaf to silver under hot dry spells and leek moth larvae tunnel the shaft, and both hide in the tight leaf axils - a spray that only wets the outside of the flat leaf does almost nothing.

⚠ WAIT 1 DAY after spinosad or spinetoram before pulling leeks. Bt kurstaki is 0 days. Spinosad and spinetoram are both moderately toxic to bees - if you are letting any second-year leeks flower for pollinators, do not spray those plants at all, and spray the rest at dusk when bees are not flying.

Cover for allium leafminer before the flies find you - both flights

Put insect netting or floating row cover over the bed before the first adult flight, and keep it on until the flight ends roughly five to seven weeks later. Penn State's field cue is the bloom of daffodils, forsythia, or ornamental pear - once those open, the flies are due. Do it again in fall: Penn State reports a second 5-to-7-week flight beginning in late September, which is the one that hits an overwintering leek crop. As a matter of rotation, do not lay cover over ground that grew infested alliums last year, or you will trap the emerging pupae underneath it. Allium leafminer females stipple a neat row of round punctures down a leaf and lay eggs there, and the maggots then mine down into the shaft where no spray can reach them. Exclusion is the only control that works before the damage is already inside - and because leeks stand in the ground into winter, the late-September flight matters as much as the spring one.

Dig leeks as you need them, and let the frost come

Start digging once the shaft is over an inch across, or a half to three-quarters of an inch for baby types, looking for a firm white shaft at least three inches long. Leave the rest standing and lift them one at a time as you cook. Hill the soil up and pile on a heavy mulch before the ground freezes, and you can keep digging through winter. A leek does not bolt or split the way an onion does if you leave it, so the ground is the best storage you have - and frost genuinely sweetens the shaft. The mulch is not there to save the leek but to keep the soil soft enough to get a fork into.