How to grow leeks

Leeks are part of the allium group of plants that includes onions, garlic, chives, shallots and many non-edible ornamental varieties too. They generally have a milder, sweeter flavour than onions.

Leeks are usually ready to harvest in late autumn and through the winter. Many varieties are very hardy so they can stand in the ground all winter for harvesting as required. When buying your seed check the packet for details of hardiness.

Unlike onions that are often grown from sets, leeks are grown from seed. They can be started under cover from late winter or sown direct in early spring.

Sow leek seeds under cover

For an early harvest leek seeds can be sown under cover from January to March. Using a forty cell seed tray I sow three or four seeds per cell at about 1cm deep. Water gently and place in a light place. Seeds will germinate in a week or two. A minimum temperature of 7°C is required, though slightly warmer temperatures will speed up germination. A heated propagator or heat mat will get things going but is not essential, a warm window sill will do, as will an unheated greenhouse as long as the temperatures aren’t too cold.

Once germinated keep the seedlings moist but do not over water.

I thin to one seedling per cell as the seedlings begin to grow. Alternatively, the leeks can be planted out as a bunch and harvested when they are small and very sweet.

If the seedlings outgrow the cells before the weather is suitable for planting out they can be potted on into slightly larger pots. This should be done if the roots begin to grow through the bottom of the modules.

Leeks should be ready to plant out about eight weeks after sowing. They should be about 6mm in diameter and 15 - 20cms tall.

Sow leek seeds outside

Sow leek seeds in the allotment or vegetable garden in March or April. Sow in a 1cm deep drill in a seed bed. Cover and water. Ensure the soil stays moist as the seedlings grow. As with plants grown under cover the leeks will be ready for transplanting into their final growing bed when they are about 6mm in diameter and 15 - 20cms tall.

Transplanting leeks

When your leeks are ready for transplanting lift them carefully from the soil with a hand fork or trowel. Take a few at a time and keep them in the shade or in water so they don’t dry out. Using a large dibber (a repurposed fork handle is ideal!) make a row of holes in the soil about 15cms deep, 5cms diameter and 20cms apart. Drop one plant in each hole. Planting deeply blanches the plant giving it a long length of sweet, white root. Fill each hole with water to settle in the roots but don’t backfill the hole with soil. This lets the root swell.

Keep the newly transplanted leeks well watered until they are established and continue to water in dry spells throughout the summer.

For plants started under cover, remove from their pots and tease the roots apart carefully if there is more than one per pot. Otherwise, the process is the same as above. Alternatively plant as small groups for harvesting as baby leeks.

Some gardeners recommend trimming the roots of the leeks a little when transplanting, this is said to stimulate growth. I’m not sure if this is true but in my opinion it does no harm and it certainly makes getting the roots into the holes easier.

Growing leeks

Keep your leek bed as free of weeds as you can to reduce competition for nutrients and allow good air flow. Water regularly until the plants are well established and continue to water if the summer is particularly dry. Mulching with homemade compost or well-rotted manure will help keep moisture in and reduce the need to water.

If any of your leeks produce a flower stalk (known as bolting) it should be removed as growth will slow down or stop if the plant is allowed to flower.

Harvest leeks

Leeks can be harvested whenever you think they are big enough to use. They should reach a useable baby leek size about eight weeks from planting out but can take three or four months to reach their full size. Many varieties are very hardy and will happily stand in the soil all winter for harvesting as needed. Check the details on your seed packet to find the hardiness of the variety you are using.

Leek problems

Leek rust is a fungal disease that causes the leaves to discolour and develop spots. It is prevalent in wet conditions so grow leeks in well drained soil if possible and avoid overwatering. Keep your leek bed weed free to allow good air circulation.

Onion white rot is a fungus that affects the root of leeks and other alliums. It resides in the soil for many years and is difficult to get rid of. Crop rotation can help.

Leek moth, onion fly and allium leaf miner can all adversely affect your crop. If they are known to be in your area an insect proofing net like enviromesh will help.