How To Grow Leeks

How To Grow Leeks

Lovely long slender stems, this variety makes a lovely, tasty baby leek for harvesting from mid summer onward. The flavour is no less impressive when eaten full size in the autumn.
The leek is a great vegetable for cooler climates, it's easy to grow, useful and very versatile. The white elongated bulb at the base of the leaves, makes a very tasty, fresh vegetable, either on its own or in stews or casseroles. The green leafy tops of leek is excellent for flavouring soups and stews. They are also very nutritious and very rich in vitamin A. The leek is a member of the onion family, but is far easier to grow than the onion. The are able to grow in varied soil conditions, so long as it is not waterlogged.


Where to grow:
Although the leek vegetable is tolerant of a wide range of soil types, they grow best on a moist, light soil that has been heavily manured from a previous crop.  Freshly manured soil is not suitable, because leeks grown in very rich soil will be coarse and tough and with far too much leaf growth. If the soil is in need of organic matter, it is best to dig in well rotted garden compost.
Choosing the site for sowing leeks may be influenced by the fact that they are generally left in the ground to be dug as required during the winter months, and can remain in the ground for a year or more.  Don't grow leeks in the same place year after year as there will be an increased risk of pests and diseases.
In crop rotation, leeks follow lettuce, cabbage or peas, but it is not a good idea to plant them immediately after lifting early potatoes. This is because the soil will be too loose and disturbed and leeks do best on a firm soil.

When to grow:
Wait till early to mid-spring before sowing leek seed, depending on the weather.  They can either be sown in a seed bed for transplanting the following summer, or sown in their permanent positions.  If you sow in a seed bed you have the added bother of transplanting, but this must be balanced out by the fact that if they are sown in their permanent position, they will take up a lot of space for a long time before producing results.

Sowing Leeks

Sow the leek seed thinly (about 2.5cm / 1in apart), as germination is usually very good, in drills about 0.5 cm (1/4 in) deep and cover the seeds with fine sifted soil.  If the seeds are properly stored they will be viable for about four years, so you can keep seeds for future use.  After covering the seeds, firm the soil down and water if dry.  Drills should be about 15 cm (6 in) apart in the permanent bed.
Germination should take about 14-21 days and thinning should begin as soon as possible, when the pants are not more than thin green shoots, about six weeks from sowing. Thin the seedling to about 5cm (2in) the first time as some of the plants may die, and then thin again when everything seems to be going well, so that the plants are about 10 cm (4 in) apart.

Sowing Dates
There are 3 sowing dates for leeks which will give you fresh leeks from July right through till April.

  • Summer and Autumn varieties: (Hannibal), Sow in modular trays in a propagator mid February for planting out in mid April.
  • Autumn and Winter varieties: (Blue-green winter, Northern lights), Sow in modular trays in a propagator mid March for planting out in mid May.
  • Late Winter varieties: (Blue Solaise), Sow in modular trays in early May for planting out in early June.

Sowing in Modular trays.

Use a seed compost which has a finer texture and lower nutrients than your standard multipurpose compost. We use a seed module tray with each section being approx 2 inches deep. Here's what you do:

  • Fill the seed tray with compost and brush off any excess. When filling the tray rub the compost through your hands to break up any lumps. Give the tray a sharp bang on your table to settle.
  • With your fingers make small depressions in each cell about a fingernail or 1.5 cm deep.
  • Sow 2 seeds per module. We want 2 leek plants per cell so don't thin the second seedling out.
  • Cover the seeds with another layer of compost then scrape across the top of the tray with a stick to remove excess.
  • Gently water your seeds. A good tip is to use a plastic bottle with small holes punched in the cap. This is less likely to wash the seed around than the heavy spray from a watering can.
  • Place your trays in your greenhouse, polytunnel, cold frame or windowsill to germinate. They should be ready to plant out in about 8 weeks.

By mid summer when the plants are about as thick as a pencil and 20 cm (8 in) high, they will be ready for transplanting to their permanent position. If you are able to plant during showery weather the young plants will settle more quickly, otherwise water the bed the day before if the soil is dry.  To plant leeks in holes, use a thick dibber or trowel and make the holes 15 cm (6 in) deep and 15-23 cm (6-9 in) apart, depending on what size of leek you want.  Make sure the holes are vertical and move the dibber about from side to side so that they are slightly larger at the top.  The holes should be about 5 cm (2 in) in diameter. Cut back the roots until they are 2.5 cm (1 in) long and trim the tips of the leaves back slightly. Lower the young leeks gently into the holes and fill the holes with water.  The water will wash enough soil over the base of the plant to allow it to become established. As you hoe the ground from time to time the holes will gradually fill up with soil.

Another way of growing leeks is to plant them 25 cm (10 in) apart in a trench. This method is particularly good if you have deep, fertile soil.  The trench should be dug to a depth of about 30 cm (1 ft) and if there is going to be more than one trench they should be 75 cm (2 ft 6in) apart. If you try to dig the trenches too close together, the walls are likely to collapse.  Put in the bottom of the trenches about 7.5 cm (3 in) of well rotted garden compost and cover it with about 15 cm (6 in) of topsoil. Carefully plant the leeks so that they are absolutely upright in the bottom of the trenches, then water in as described above.

Crop care

Water the young plants well, especially in fry conditions, until they are well established. Soon after planting apply a liquid manure. Hoe between the rows regularly to keep down the weeds and also this will aerate the soil. Frequent hoeing will also create a dust which helps conserve moisture. Leaves which grow too long can be trimmed back slightly so that they do not rest on the ground. Cut the long dark leaves back by about 5 cm (2 in) in early summer and again in mid summer, and a third time if it is necessary, in early autumn.

Blanching Leeks
Blanching leeks increases the proportion of plant which is edible and improves the flavour which would otherwise be strong and harsh. Start blanching in mid August, this is a gradual process and should be done in several stages rather than all at once. As the plants increase in height you an earth them up which will blanch the stems. Scoop soil up around the plant which excludes the light from the bottom part of the stem and tuns them white. Don't earth them up so high or soil will become lodged in the leaves of the leek. You can earth them up twice during the growing season. There are several methods of blanching depending on which way the leeks are growing.
If you have your leeks growing in a trench, blanching consists of gradually filling in the trench with soil to the bottom of the lowest leaves each time until the plants have finished growing, which will probably be around mid to late autumn, this also depends on the weather.  This should give you at least 10-15 cm (4-6 ins) of blanched stems. The soil used for earthing-up must be dry and of a fine texture. If you use wet earth rot is liable to set in and if the soil is lumpy it will be difficult to handle and wont keep out the light properly.

If the leeks are grown on the flat surface, push the soil up around the plants increasing the soil depth by about 5 cm (2 in) each time. You can keep the stems free of soil by using collars, which are secured around the leeks up to the base of the leaves. Various materials can be used for the collar, lengths sawn from plastic piping, clay drain pipes or at virtually no cost, pieces of strong brown paper tied up with string or rubber bands. Whatever type of collar you decide on the minimum diameter should be 7.5 cm (3 in) and 30-37.5 (12-15 in) long. Attach the collars before carrying out the earthing-up process. As the plants grow, draw up more and more soil with a hoe fitting another collar above the first one.

Leeks are quite greedy plants and will benefit from a sprinkle of poultry manure spread widely around the roots. Our 'Seafeed seaweed-chicken manure pellets are an excellent source of nitrogen which will be perfect for leafy crops like kale. If you want to give the plant a quick boost a liquid seaweed feed is ideal.
Seamungus will release nutrients for up to 3 months so only needs to be applied once in the season.
Feeding will increase the thickness of the leek stem. You shouldn't feed overwintering leeks beyond August.

Pests and Diseases

Leeks affected by rust will have orange patches of spores on the leaf surface. The spores will slightly reduce the yield and affects the look of the plant but won't kill it. For over wintering varieties frost will kill rust so it won't be long term problem.
For Autumn Types you ca remove the affected leaves. Don't grow leeks on the same patch of ground the following year.

Onion Fly     Yellow, drooping leaves and tunnels in the plant tissue.
Leek Moth    White streaks on the leaves.
White Rot    Yellow leaves, white or grey fungus at the base of the plant.
White Tips    Leaf tips die back with white papery patches on the leaves.

White rot & Downy mildew
Leeks suffer from all the usual onion family diseases including white rot and downy mildew. They tend to be far less susceptible so you are not that likely to get them. A good crop rotation regime should protect you from most problems.


Leeks may be harvested from mid autumn through to the end of late spring, depending on the time of sowing and the variety. The hardier varieties are left in the ground until they are needed. Never pull the leeks out of the ground by force or they will more than likely break in two, leaving you with just a handful of leaves. Instead, lever them out with a spade or a fork. Dig up the largest ones first, if you leave the smallest ones in the ground until the spring, they will put on some more weight before they flower.

If the ground is likely to be frozen for a long period of time, it is a good idea to lift any leeks which are ready and store them in some sand in a cool place, where they will keep for about a month.  If by the end of the season you have a few leeks still left in the ground, but need to clear the plot, you can dig up the leeks and heel them in a shady place until they are needed. Lay them on their side in a shallow trench with the top part of the leaf stalk sticking out above the ground, covering the rest of the stalk with soil.  This also helps to stop them bolting.
You can if you leave the leeks in the ground nip out the flower stems and you will get a bonus crop, leek bulbs. These small white bulbs will form at the base of the plant, and if you harvest them in early summer you can use them as onions or shallots.

Harvesting Over Wintering Leeks
Leeks can take between 25 - 40 weeks to mature depending on the time of year you sow them. Overwintering leeks take longer to mature and should be ready about then. You can begin harvesting your leeks when still quite small to achieve a long cropping season. Very large leeks have an inferior flavour so don't let them get too big. Dig beneath the plants with a fork or trowel rather than pulling them from the ground. Leeks will happily remain in the ground over winter so only harvest them when you need them.


Leeks do not store as well as onions because they lack a tough outer skin, but can be kept in a plastic bag in the crisper of the fridge for up to a week. They are a good source of vitamins A and C, potassium and calcium.