How to grow leeks and best varieties

Aside from sowing and planting them out, leeks need very little attention, so they’re an easy veg to grow, plus they provide something to eat from the garden at times when there’s not much else to harvest.

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Best leek varieties

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When to sow

Leeks grow slowly and take up space in the ground almost all year, so make sure you have room. Around 20 mature leeks should fit in a square metre. Sow in April, either in module trays, putting several seeds in each module, or in a seed tray. Use a Best Buy compost for sowing seeds. If you sow in a seed tray, prick out the seedlings into individual pots when they're large enough to handle, using a Best Buy compost for raising young plants. Leeks sown in module trays and grown in clumps won't have long white roots, but all of the leek can be used for eating.

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Caring for your plants

Planting

If you sowed in modules, plant the whole clump with a trowel or dibber. If you sowed in a seed tray and pricked out individual plants, use a dibber to make a hole around 15cm deep. Drop in the plant, making sure the top is sticking out, and then fill the hole with water.

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Allow around 30cm between rows to make weeding easier, and space plants at 15cm intervals for full-sized leeks. If you’re short of space, plant closer together and harvest alternate plants as baby leeks, leaving the rest to grow on to maturity.

Protecting from pests

Leeks need little attention during the growing season. Keep the plot weed-free and well-watered in dry spells. Cover with insect-proof netting to avoid allium leaf miner and leek moth.

How and when to harvest

Harvest in: October to February

Leeks can be harvested from autumn until late winter. For individually-planted leeks, ease a fork into the soil next to the leek and gently prise from the ground. For leeks planted in clumps, use a trowel to cut through the clump and remove individual leeks as you need them. 

Common growing problems

Leek moth

The young caterpillars of leek moth burrow between the surfaces of the leaf, like a leaf miner, creating brownish-white patches. As they get bigger, they bore down into the shaft of the leek, or the bulb of the onion, causing more substantial damage and allowing in rotting fungi and bacteria. At this stage, the vegetables tend to develop yellow leaves and go slimy. 

The caterpillars are yellow-green with legs at the front and fleshy pro-legs at the back, they have a dark head and dark raised spots with hairs. They grow from 1mm, when newly hatched, to 10mm. As you might expect, leek moth is best known as a pest of leeks, but it can also attack chives, garlic, onions and shallots. There are two generations of caterpillars a year in the UK, in spring and late summer. The first attack is often mild, especially after a cold winter. The second one is the most damaging, and hot summers tend to favour the pest and make the problem worse.

Remove any caterpillars you find and cut back badly damaged foliage. Leeks can often recover and still produce a reasonable crop; onions are less likely to. Undamaged parts of the leeks can still be used. To avoid this problem, cover your leeks with fine insect-proof mesh when you plant them.

Allium leaf miner

Allium leaf miners tunnel through the leaves and stems in spring and autumn and you may find their brown, barrel-shaped pupae if you peel back the leaves in autumn and winter. To avoid it, cover your leeks with fine insect-roof mesh when you plant them.

The first sign of attack is often rotting towards the base of the leaves caused by fungi or bacteria invading the wounds caused by the miners. If visible, the mines themselves are narrow and linear. With onions, the leaves may become deformed, curled or wavy. If left unchecked, secondary rotting can soon destroy the whole plant.

Leaf mining takes place mostly from April to May and from October to November. However, the damage is most obvious later, once rotting has set in, so is usually seen in overwintering leeks between December and February, and in onions in June.

Once rotting sets in, the plants won’t recover. Undamaged parts can still be eaten, but affected onions can't be stored. If they're too badly damaged to be used, affected plants are best consigned to your local council's composting scheme, which will destroy the pest, or buried deeply so the adults can't emerge from the pupae.

Leek rust

Leek rust is a fungal disease that causes rusty-orange blotches on the leaves. It’s prevalent in warm, moist weather. Severe infections can make the leeks inedible, but this is rare and slight infections will affect only the outer leaves, which can be peeled off, and will usually disappear when the temperature drops in winter. There aren’t any chemical controls, so your best defence is to dispose of any affected leaves, ensure your plants aren’t crowded to improve airflow and grow your leeks (or garlic) in a different spot each year.

Bolting

This is when leeks start to produce flower spikes before the leeks are ready to harvest. It’s usually caused by the weather, with a late, cold spring often to blame. Very hot conditions can also cause bolting. Improving the soil so that it doesn’t dry out may help. Lift any affected plants and use what you can straight away.

White rot

White rot is caused by a fungus that destroys the roots, killing the plant. It then produces resting bodies that can last for many years in the soil.

Onions, shallots, garlic and leeks all suffer from this disease. Spring onions and overwintered onions seem particularly susceptible. 

Look out for yellow and stunted plants in May and June among your overwintered onions, and later in the other onion-family crops. When you pull up sickly plants, check for a white/grey mould around the base. In advanced cases, you will see black, pinhead-sized, regular spheres. These are the sclerotia, or resting bodies, that will carry the disease from one crop to the next.

When you see white rot, remove the plants and a few inches of surrounding soil. Burn or bin them. This will make sure that you don't add more sclerotia to the soil.

Avoid transplants grown in soil-based compost. Gather and use an attacked crop as soon as you can, as damaged bulbs will be unusable and won't store well. There are no chemicals available to gardeners to control this disease, and there are no resistant varieties.

How we test leeks

We selected 22 widely available varieties, including previous Best Buys and some that hold the RHS Award of Garden Merit. In early April, we sowed each variety in strip modules in a Best Buy compost for sowing seeds in the greenhouse at our trial grounds in Capel Manor, north London. Once the seedlings were large enough to handle, we potted them into long pots to grow on and harden off in a sheltered spot outside. At the end of May, we planted each variety in a single 2m row, allowing 15cm between plants and 30cm between rows. The whole trial was covered with a tunnel of Enviromesh netting to protect the crop from allium leaf miner and leek moth. We harvested half the leeks in November, leaving the remainder for a second harvest in February the following year. All the leeks were weighed and assessed for quality. During the trial we kept the plot free of weeds by hand weeding, and ensured that the crop was watered when necessary.