Growing Leeks From Seed & How To Plant Leek Seedlings

It’s hard to find a better crop to grow in the vegetable garden, leeks are ready for harvesting deep in the winter, the seeds are cheap, plants require almost no maintenance and suffer from essentially no diseases.

In this article I’ll teach you the secrets to growing leeks from seed and how to get a really long “show quality” white stem.

Best Leek Varieties To Try

There are quite a lot of different varieties to choose from if you want to be growing leeks from seed. 

Here are some varieties we would recommend you to try:

Musselburgh: This is a very popular leek variety, especially in parts where the winters are cold. This variety can be sown in spring and winter, so it is a great crop for the whole year. It has a sweet taste.

Blue Green Autumn Neptune: This leek variety is winter hardy and is very flavoursome. It produces long white stems.

Northern Lights F1: This leek is not only very tasty but will also look beautiful in the dull winter months. Its blue/greenish leaves will bring a smile to your face whenever you look out in the garden.

Now that we talked about varieties to try, let’s get to the nitty-gritty of growing leeks from seed.

Growing Leeks From Seed

leeks grown from seed

Leeks are best sown indoors in the middle of March or outdoors from the first week in April. Sprinkle approximately 15-20 seeds in a 15 cm (6in) pot of multipurpose compost and cover with a very fine layer of compost.

This is by far the best method as the compost really gives the seedlings a very good start.

Alternatively, the seeds can be sown into a seedbed for later transplanting or small seedlings can be purchased from most good garden centres.

Once sown, leeks can seem to take quite a while to germinate. At ideal temperatures, leek seeds will germinate in 10-14 days, but when in cooler conditions, this can be closer to three weeks.

If the seeds haven’t germinated after 3 weeks it really is time to consider resowing.

If the plants germinate too thickly, they should be thinned out early to avoid any root disturbance.

Initially, the leeks will look small and thin, like tiny onion seedlings. At this stage, the plants do often seem to slow down growing. A few weeks after gemination, using a good liquid feed will give a burst of green growth which will ensure the plants are really in the best condition before they are transplanted into the garden.

Once the seedlings have grown to be about the size of a pencil, they are ready to be transplanted.

Transplanting Leek Plants Outside

leek seedlings ready to be transplanted

Leeks are part of the onion family and, as such, will need good soil to grow well.

To ensure you can provide rich soil for your leeks, prepare the bed by mixing in a good amount of organic matter in autumn. 

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You can also plant green manure crops, that is then dug in before you plant out your leek seedlings. Read more about growing green manure crops in our article.

Free draining soil is essential as the winter months can cause rot problems. Adding a good layer of well-rotted manure to the final bed in autumn will give the very best results.

Leeks are really tough and shouldn’t need hardening off if planted straight outdoors unless extreme weather is expected.

The method for planting leeks is quite a unique one.

Plants should be spaced about 15cm (6in) apart in rows 30cm (12in) apart – planting further apart in very bad soils.

These spacings will give quite large leeks that are still ideal for the kitchen. However, if you are growing for the local show, double the planting distances.

To plant, you will be using quite a unique method.

Use the handle of a sweeping brush and make a mark 15cm (6in) up the handle. A brush handle is the perfect tool to use as a dibber for planting leeks. At each planting location, push the end of the handle into the soil until it reaches the 15cm (6in) depth mark, drop a seedling in the hole and water well.

Do not fill in the hole with soil, watering will fill the hole a little and over time the soil will fill the hole with loose soil. This hole and loose soil will allow the shank of the leek to grow over the coming months.

Some people recommend cutting the roots of the leeks before planting and trimming the tops, but research has shown this makes no positive difference to the growth.

Aftercare & Harvesting Leeks

leeks freshly lifted

Weeding the plants soon after planting will prevent any weeds having a chance of seriously competing against the leeks and save a lot of work later in the season.

Once established, the plants should make a canopy that will prevent most weed growth.

Blanching is the only extra step needed to get perfect leeks. You can successfully grow leeks without this, but to get really long stems you can blanch them.

There are two ways to do this:

  • As the leeks grow earth up the soil around the stem, this will block out most of the light and can be used to create a good stem.
  • In June or July, the plants should get up to 25cm (10in) tall and a 20cm (8in) length of cardboard tubes can be placed over the leek to block out sunlight. This is how professional show growers get incredibly long shanks.

Once this step is completed it’s time to just leave the plants to grow until they are ready for harvesting.

Although you will have to water them in prolonged dry weather. And you should keep their bed weed free.

Harvesting begins as soon as the leeks are big enough for cooking. In late summer and early autumn, small leeks can be picked and used in the kitchen.

Top Tip: To harvest leeks in heavy soils, use a fork to carefully lift them out of the soil.

In light soil, you should be able to just pull them out carefully.

The main crop should begin in October with harvesting continuing right the way through to early April.

Pick the leeks and use them within a week as they do not store for long periods once out of the ground. During winter, you can leave them in the ground until you need them.

And if you are not sure what to do with your leeks, I love to make buttered leeks. They go well as a side with a roast chicken. I have even done it for a Christmas dinner once. It was delicious!

Pests And Diseases Affecting Leeks

No guide about growing leeks from seed would be complete without discussing pests and diseases that could ruin your hard work.

Diseases

leave with rust disease

There are a few diseases that could infect your leek plants.

One of them is leek rust, which is a fungal disease which is most likely to strike between mid-summer and late autumn. 

It’s fairly easy to spot if your leeks are infected, because you will see bright orange pimples on the top and bottom of the leaves. These have given the disease its name.

They will burst open at some point and release the spores. As they are airborne, they can easily spread and infect other plants such as garlic or onions in your vegetable garden or allotment.

Leeks can tolerate mild infections, but if the attack is severe, it could lead to leaves turning brown, which will affect the health of the plant and impact on the yield.

Once a plant is infected, there is nothing to cure it. And when you have been growing leeks from seed, this can be devastating. So prevention is the best option. Here are some things you can do to minimise the risk of your leek plants getting infected:

  • Don’t feed them with liquid fertilisers. First of all, they won’t need it, as long as you have prepared the soil beforehand. But also, soils that are rich in nitrogen and low in potassium will increase the risk of infection.
  • Longer periods of wet weather will favour the disease, so make sure you give your leek plants enough space for good air circulation to allow the foliage to dry off quicker after rain.
  • After a plant has finished cropping, make sure you get rid of all the plant debris.
  • Make sure you practice crop rotation and include leeks, garlic and onions. Changing spots for three years.
  • If you have a plant that is badly affected, remove it immediately and destroy it, do not compost it.

Another disease that could hamper your efforts of growing leeks from seed successfully is onion white rot.

This is a soil-borne fungal disease, that will attack the roots of your crop of leeks. It affects leeks and onions, and is most prevalent between mid-summer and early autumn.

You will see the leaves wilting in dry weather. In wet weather, you won’t observe wilting, but because the disease destroys the roots, the plant won’t be anchored in the soil any more.

When you pull out an infected leek, you will see white mould at the bottom of the leek. If left in the soil, it will grow round black spores.

Once a plant is infected, there is nothing that can be done, you have to destroy it, do not compost it.

Because the disease is soil-borne, an infected bed will stay infected for a long time. So it’s important that you keep the disease from spreading to other parts of the garden.

If you have an infected bed, do the following:

  • Don’t grow leeks or onions in the bed for 15 years, as this is how long the fungus can stay in the soil
  • Clean tools and footwear after working on the infected bed to avoid transporting the disease to other beds.
  • If you have a small garden and fear that all the soil is contaminated, grow leeks and onions in containers. Again, make sure you always use fresh compost and clean tools.

Pests

moth on white background

One of the worst things when growing leeks from seed is having to watch them being eaten by pests.

One pest to look out for is the leek moth. It’s a fairly new pest, that was initially only found in the south and east of England near the coast. However, it has since spread further north.

It’s the caterpillar that does the damage, it borrows in the leaves to feed on them. It’s a white caterpillar with a brown head. You will see white/brown spots. A heavy infestation can cause the leaves to become yellow and start to rot.

Another problem is that an infested plant is also more susceptible to bacterial and fungal diseases, causing rot. And that’s the last thing you want when you have been growing leeks from seed.

Once you have noticed their presence, it will be too late to rescue the plant. You have to destroy the plant, do not compost.

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Like with most pests and diseases, prevention is the best way. 

The only way to keep the caterpillars away is by preventing the female leek moths from laying their eggs on your leek plants. Here are some tips to keep your leeks free of leek moth caterpillars:

  • Encourage the natural enemies of the pest, such as ground beetles and rove beetles. You can do this by having wood piles, compost heaps or leaf litter in the garden.
  • You can try companion planting. Carrots are said to be the perfect companion for leeks because they repel the leek moth. In turn, the leeks repel the carrot fly.
  • Or you can cover your leeks in insect-proof mesh, which will prevent the female moth from getting to your leeks.
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Another pest, that is often confused with the leek moth is the allium leaf miner. Again, it’s a fairly new pest, and it attacks leeks, onions, chives and garlic.

The actual pest is the larvae of the grey/brown fly, which are white headless maggots. They tunnel into the leaves, stems and bulbs. 

Like the leek moth caterpillar, the allium leaf miner larvae can also cause bacterial and fungal diseases.

The symptoms of an attack of this pest are:

  • lines of white spots on the leaves. These are caused by the female fly as she feeds on the sap of the plant after having laid the eggs.
  • holes from where the maggots have burrowed into the leaves.
  • rotting tissue caused by bacterial or fungal infections.

To prevent the female flies from laying their eggs on your leeks and ruining your efforts in growing leeks from seed, do the following:

  • practice crop rotation to prevent a build up of pests, as the pupae may overwinter in the soil. This is also recommended to prevent diseases.
  • Carrots as companion plants might also repel the allium leaf miner. So it is worth giving it a try.
  • Cover your leeks with insect-proof mesh, like the one recommended above.

Now that we have shown you the secrets of growing leeks from seed, you will be ready to enjoy homegrown leeks all winter. Happy growing!

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