Top Of The Crops - Leeks

Welcome to the top of the crops! Today you will learn how to grow leeks in a polytunnel.

Growing Leeks in a Polytunnel

Leeks are heathy vegetables that can be grown outside and tend to taste best once they have experienced a frost. But growing them in a polytunnel is also an option, and can help ensure you get a high quality harvest by providing a more controlled environment where inclement weather and pests are not as much of a problem. 

Key Information

Leeks, Allium ampeloprasum 'Leek group', is a common crop in the onion family, grown as a vegetable for its bundled leaf sheaths, sometimes wrongly referred to as stalks or stems.  The white bases, light green areas, and darker green leaf tops are all edible and have a range of culinary uses. 

When choosing which particular leeks to grow, you should go for leeks that are well suited to the growing conditions where you live.  

There are different leek varieties for early, mid and late season harvest and some varieties are better suited to certain conditions – such as heavy soils for example – than others. 

If you are looking for a perennial option then perennial leek varieties are available as an alternative to the annual crop. Giant leeks are also an interesting option.


The Preferred Conditions for Leeks

Leeks will do best - 

  • In full sun though they can be grown in partial shade.

  • In a fertile, moist yet free-draining soil or potting mix that is neutral or alkaline, with a pH above 6. 

They can be grown in the ground, in raised beds, or in containers where these conditions can be provided. 

What you will need to grow Leeks

To grow leeks, you will require:

  • Leek seeds or starter plants if you prefer. 

  • A suitable final growing location and perhaps a seed bed too. 

  • Facility to water your plants. 

  • Organic matter for mulch. 

  • Pots, or soil blocks and seed starting potting mix if sowing seeds indoors. 

  • A small spade/ trowel for planting.

  • A spade or garden fork for harvesting. 

How to Grow Leeks

Leeks are not particularly challenging to grow successfully, but you do need to get your timings right, select the right leeks and place them in the right place for them to thrive. 

Preparing the Ground

Remember that leeks will do best when they can grow in a soil or growing medium that is moist yet free-draining, with plenty of organic matter, and a neutral or alkaline pH. If your soil is poor, or acidic, you may need to amend the growing area before sowing or planting. 

Growing Leeks from Seed

Leek seeds should be sown some time between February and April indoors, or in March or April outside. Sowing seeds inside is a good idea if you wish for an earlier harvest, and want to get a jump start on the growing season. Sowing indoors can also be a good option if you are short on space in your spring garden. 

Sowing Indoors

Sow leek seeds from February onwards in a conservatory, greenhouse or on a warm windowsill somewhere that temperatures can be kept consistently above 7 degrees C.. 

Sow the leek seeds thinly, to a depth of around 1cm. Water them in and keep the seedlings where they receive enough light, and temperatures above 10 degrees C. 

If roots appear through the holes at the base of the containers, then move the young leek seedlings into slightly larger containers. 

Sowing Outdoors

If you prefer, you can also sow leek seeds outdoors. You can either sow them into their final growing positions, or into a temporary seed bed from which they can later be transplanted. 

The primes months for sowing leek seeds outdoors are March and April though you can also sow in May or June for later winter and spring harvests, or when growing a quick growing type for a harvest of baby leeks. 

Leeks are typically sown thinly, in drills around 1cm deep. If you are creating multiple rows then these should be around 20cm apart.

Transplanting Leeks

You can transplant young leeks that you started inside, or ones that you sowed into a temporary seed bed into their final growing positions after around 8 weeks, when they are around 20cm in height and there is no longer a risk of frost where you live. At this point, the young leeks should ideally be around the width of a pencil. 

Take each baby leek out of its pot or from the seed bed and trim the roots of each one to leave roots around 2.5cm long. 

Planting Out Leeks

Whether we are talking about planting out leeks you have grown yourself from seed into their final growing positions, or you choose to purchase young leek plants to plant out, the process is the same. 

In a suitable planting location, make a series of holes with a trowel handle, wooden stick or large dibber tool. The holes should be around 5cm in diameter and around 15cm deep. Planting leeks deeply will ensure a long white section and a shorter tougher green section. 

Make the holes around 10-20cm apart, depending on the specific leek variety that you have chosen to grow, and whether you want baby leeks or full-sized ones. 

Place the baby leeks in the holes, but do not fill the soil back in around them. Instead, fill the holes repeatedly with water and allow the soil to settle gently into place. 

Harvesting Leeks

Leeks will, depending on which variety you have chosen and when you choose to pull them, usually be ready to harvest between October and March. It is best to harvest leeks while they are still quite small as smaller leeks will tend to have better flavour. 

Do not try to pull leeks from the ground – use a fork to lift them as and when they are required. If the ground freezes solid where you live in the winter months, you can lift leeks and heel them into the soil in your polytunnel or in a warmer area next to a path or your home so they can be retrieved more easily as and when required. 

You can also lift all your leeks at once and chop and freeze them for longer storage, for use in a range of different recipes.

Preparation and Uses

From traditional soups and stews, to delicate haute-cuisine, leeks can be used in many different ways. They are often best sautéed with olive oil or butter which brings out their sweet and savoury flavour. 

Care Tips for Leeks

Leeks are not to challenging to care for as long as they are grown in a location where their basic needs are met. It will take a little more work if you are growing leeks in pots or other containers but even then, caring for leeks should not be too onerous. 


Water young leeks regularly and well until they become established in your garden. After this, you will only need to water during prolonged dry spells, unless of course you are growing under cover. Remember too that you will need to water a lot more frequently if you are growing leeks in pots. 

Try to keep the above ground portions of the plants dry when watering, watering the soil at the base of the plants where the water is needed, and not wetting the leaves. Wetting the leeks can make fungal problems more likely to occur. 


Mulching around leeks with organic matter such as homemade compost or well-rotted manure is very important in order to improve and protect the soil, conserve soil moisture, and suppress weed growth. 


Leeks will not thrive where the competition is too intense for water, nutrients, space and light. So make sure that you keep on top of weeding, and ensure good spacing between your plants. 

Protecting from Pests

Leeks may sometimes encounter problems with leek moths or onion flies or other pests. If these become a big issue then it is a good idea to cover your leek crop with fine mesh netting. 

Companion planting will also help to protect leeks from various pests. For example, intercropping leeks with carrots can help manage onion fly, while also protecting the carrots from carrot fly too. 

Varieties of Leeks

When choosing which leeks to grow, you should go for leeks that are well suited to the growing conditions where you live. Giant leeks are also an option. There are different leek varieties for early, mid and late season harvest.

Some highly regarded leek varieties to consider in the UK are:

  • ‘Blauwgroene Winter’ AKA 'Bandit'

  • ‘Cairngorm’ 

  • ‘Jolant’ 

  • ‘Krypton’ 

  • ‘Lancaster’ 

  • ‘Longbow’ 

  • ‘Mammoth Blanch’ 

  • ‘Mammoth Pot Leek’ 

  • ‘Oarsman’ 

  • ‘Pancho’ 

  • ‘Porbella’ 

  • ‘Porvite’ 

  • ‘Stamford’ 

  • ‘Toledo’ 

All of the above have an award of garden merit from the RHS. 

Common Problems for Leeks

Many of the problems encountered when growing leeks are due to improper placement or care. For example, fungal diseases such as leek rust and onion white rot are more likely to occur in damp or waterlogged conditions. 

Low temperatures in spring can cause bolting and flower stalks may form. Remove this while it is still in tight bud, and the leek will still form properly. The flower in bud can also be eaten. 

When growing leeks you also need to look out for insect pests as mentioned above, and may like to consider companion planting and netting where necessary. You should also look out for slugs and snails which may eat leeks while they are young. 

Top Tips for Growing Leeks in a Polytunnel

In a polytunnel garden, where space can often be at a premium, leeks can be grown along bed edges or in containers. These and other alliums can be useful for pest control and make good companion plants for a number of other crops. 

As an allium, leeks are a good companion plant for a range of other crops, including brassicas and carrots, whose pests will be confused by the scent of leeks, onions and other alliums. But avoid growing leeks and other alliums near legumes like peas or beans as they may stunt their growth.  

It is possible to grow leeks with other alliums for crop rotation purposes, but remember that leeks do share pests and diseases with other members of this family, so create diverse polycultures to reduce the chances of any serious issues spreading between them. 


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Blythman, J., Sykes, R., (2014) Why leeks are good for you. The Guardian. [online] Available at: 

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