When to sow and plant out leeks

When you sow depends on when you want to eat. Summer leeks, harvested from late June to August, need the earliest start: sow them indoors in January or February. Autumn leeks are sown from mid-March to early April, and winter leeks in April, for picking from January into May.

In the UK, and anywhere you don't want to raise plants indoors, sow into a seedbed or pots outdoors in March and April; late winter varieties can go in until May.

Keep indoor-sown summer leeks at 61°F or above; spells around 41–46°F are the riskiest. Colder spells tell the young plants that winter has passed, and they may run to flower instead of thickening.

Sow thinly, about 0.4–0.6 in deep, in trays, modules or a seedbed. The seed takes two to three weeks to come up.

After eight to twelve weeks the seedlings are about 8 in tall and as thick as a pencil: that is the moment to plant them out. Summer leeks go out from April to mid-May, autumn leeks from late May to June, winter leeks in July and early August.

Choose a sunny spot and work in two buckets of compost per square metre, because leeks are hungry. Make holes about 6 in deep with a dibber, or draw a furrow the same depth. Many gardeners trim the roots to 0.8–1.2 in to make the plants easier to drop in.

Drop one plant into each hole and fill the hole with water instead of soil. The water settles enough earth around the roots, and the hole fills in by itself over the following weeks. The deeper the stem sits, the longer the white part.

Space the plants 6 in apart in rows 12 in apart. A 120 × 80 cm raised bed then holds three rows of eight, 24 leeks. Winter leeks grow bigger and like rows 16 in apart: two rows, 16 plants. For quick summer leeks you can plant closer, down to 4 in in the row.

Water young leeks until they are growing strongly, and after that in longer dry spells. Keep the bed weeded, carefully, so you don't damage the stems.

Once the holes have filled, draw soil up around the stems a few times over the summer. Earthing up keeps light off the lower stem, which gives you a longer, whiter and more tender shaft.

Summer leeks are ready from late June to August, autumn leeks from late August to December, and winter leeks from January to May. You can start picking while they are still slim, which stretches the season.

Loosen each leek with a fork before lifting it; pulling on the leaves tends to snap the stem.

Winter varieties are the point of growing leeks: they stand in the bed through frost, and you lift them whenever you need one. Frost may even sweeten the flavour. Autumn leeks are meant to be finished by the end of the year; any left in November can be lifted and heeled into a sheltered spot. Once picked, leeks keep in the fridge for a few days, or you can slice and freeze them.

Leek varieties are sold in three groups, named after when you harvest them. Summer leeks are the fastest and are eaten from late June to August; they need the earliest sowing and the most warmth while young.

Autumn leeks follow from late August to December. Winter leeks are sown last, grow slowly and are bred to stand in the bed through frost until spring. For one sowing that feeds you through the cold months, choose a winter variety; the packet or catalogue always says which group a variety belongs to.

Leek moth caterpillars leave white patches on the leaves and tunnel into the stems, and damaged leeks often rot. There are two generations: caterpillars feed in May to July and again from August to October, the second more damaging.

Allium leaf miner is now the more common problem in the UK. Its larvae tunnel down into the stems; the flies lay eggs in March–April and again from September to November, so winter leeks are exposed. Fine insect mesh over the bed during the flight periods keeps both pests out.

Leek rust shows as orange pustules on the leaves, worst in humid weather, in crowded beds and on plants given lots of nitrogen but little potassium. Space well, feed in balance and clear away old leaves.

Leeks belong to the onion family with onions, garlic, shallots and chives. Leave three to four years before any of them grows in the same bed again, and never plant alliums where white rot has appeared.

German mixed-planting lists pair leeks with cabbages, carrots, celery, lettuce, spinach, parsley and tomatoes. These pairings are tradition; we found no trials showing that any of them protects leeks or raises the yield. Leeks next to carrots is the classic, based on the same pest-confusion idea as onions and carrots, and it does no harm.

The rule with a real reason: don't plant leeks next to or straight after other onion-family crops, because they share leek moth, leaf miner, rust and white rot.