How To Grow Leeks From Seed To Harvest

Leeks, sometimes referred to as the poor man’s asparagus, are members of the onion family that, instead of forming a bulb, form a long shank. Not only are these hearty vegetables easy to grow – they also provide lots of vitamins and minerals: C, A, folate, and the phytochemical kaempferol, which is believed to help prevent cancer.

Like their allium cousins, leeks can be regrown from the root end, though they are most typically grown from seed; they are simple enough for even the beginning gardener to attempt. While they may not seem terribly versatile, leeks can be used in a variety of dishes: their sweet, delicate flavour makes a great addition to creamy soups, and they can be served steamed or braised, or even chilled in a salad.

Leek varieties

There are many different varieties of leeks, with some coming to harvest in as little as 50 days and others in as many as 140 days. These longer-to-mature varieties are excellent for winter gardens as they’re quite hardy, but their quickly-maturing brethren don’t tolerate the cold nearly as well.

From the summer months onward, leeks can be affected by rust, a fungal disease; however, many varieties are ‘rust resistant’ – and leaving space between your plants for air circulation will also help prevent rust.

A couple of varieties i have found grow well for me were called “Blue Solaise” and “American Flag” . There are many other varieties available, although these seem to be among the most common ones sown.

Where to plant leeks

As a rule, leeks prefer a sunny, sheltered site with well-drained soil. Though they can be planted in heavy soil, we recommend mixing in some horticultural sand for aeration and drainage as they typically thrive in a slightly moist, light soil.

For best results, remove all weeds from the planting beds and rake in some well-rotted manure in the fall; if the soil is too rich, your leeks may be coarse and tough with lots of leaf growth but a thin shaft. If the soil is in need of organic matter, feel free to dig in some compost; and about a week before sowing, spread a balanced fertilizer over the bed and rake it in.

It’s best to rotate your crops, with leeks following cabbage, lettuce, or peas; growing them in the same place year after year can increase the risk of pests and diseases. Planting leeks immediately after harvesting early potatoes is not ideal, as the soil will be too loose and disturbed for the leeks to thrive – they do best in a fairly firm soil.

When to plant leeks

Depending on the weather, sow your seeds outdoors in early to mid-spring. If you plan to begin the seeds indoors, there are 3 sowing dates that will provide you with fresh leeks from July through to April:

  • Summer and autumn varieties can be sown in mid-February to be planted out in mid-April
  • Autumn and winter varieties should be sown in mid-March to plant out in mid-May
  • Late winter leeks can be sown in early May for planting in early June

You can, of course, sow your leek seeds directly into the planting bed, but as they take several weeks to germinate and sprout sufficiently, the space may best be used for another plant in the meantime.

Growing leeks in pots

If sowing in trays indoors, choose a seed compost which has a finer texture and lower nutrients than standard multipurpose compost. Use a 2”-deep, sectioned tray; fill it with loose compost and brush off any excess. Allow the compost to settle – you can also give it a sharp bang on your planting table.

Use your finger to make a small depression about the depth of a fingernail in every section and plant 2-3 seeds in each. Cover the seeds with another layer of compost then gently water them – it’s best to use a light spray that will not wash the seed around the tray.

Leave the trays in a greenhouse or cold frame, or place them on the windowsill to germinate; it will take about 8 weeks until they are ready to transplant.

Thinning, trimming, and transplanting leeks

If after 3-4 weeks, two or more seedlings sprout with about ½” of each other, pluck at least one. It’s better to have one seedling with plenty of room to grow rather than 3 straggly seedlings competing with each other.

Using a sharp pair of scissors, trim off each plant at 4-5” in length; this will help encourage new leaf growth at the base of the leek and prevent your leeks from becoming leggy and tangled. Your seedlings may need a second trimming before transplanting. The leek trimmings can be used instead of chives many dishes like pasta, quiche, or soups.

When your plants are about 8” tall and the thickness of a pencil, they’re ready to transplant to their permanent position. Planting during showery weather will help your young plants to settle in more quickly; if the weather has been dry for some time, water the planting bed well the day before planting.

Prepare holes 6” deep and 2” in diameter that are slightly larger at the top; they should be between 6” and 9” apart depending on your chosen variety. Cut the leek roots back to about 1” long and trim the tips of the leaves slightly before gently lowering them into the holes. Fill the holes with water, which will wash soil over the base of the plant, allowing it to become established; hoeing the ground from time to time will allow the holes to gradually fill up with soil.

If you have a very deep, fertile planting bed, you could also transplant your leeks into a 1’-deep trench. The plants should be about 10” apart and each trench should be 2.5” feet away from the others; trying to dig the trenches too close together may cause the walls to collapse.

Put 3” of well-rotted garden compost in the bottom of the trench and cover it with about 6” of topsoil. Then carefully plant the leeks 10” apart so that they are upright in the bottom of the trench and water in as described above.

How to grow leeks

You’ll need to water the young leeks well until they’re established. It’s also important to hoe between your rows regularly to control the growth of weeds and keep the soil light and aerated for better drainage. Your new transplants will also benefit from a light application of liquid manure to help them get established and begin to flourish.

If the leaves grow too long, trim them back occasionally to prevent them from resting on the ground – they will typically need to be cut back about 2” approximately 4 weeks after transplanting and again a month later; longer-to-mature varieties may need a third trim before harvesting.

Blanching leeks

The edible white portion of the leek, with its delicate flavour, can easily turn green and become stronger and harsher in flavour as it is exposed to the sun. Therefore, we recommend blanching your plants.

There are a few different ways to achieve this and the most traditional gardeners prefer to hill up their plants: as your leeks increase in height, scoop fine, dry soil lightly around their base, which will block out the sunlight and help your plant remain white; but beware – hilling your plants too high will cause soil to lodge between the leaves.

If you’ve chosen to grow your leeks in a trench, simply gradually fill the trench with soil to the bottom of the lowest leaves periodically until your plants have finished growing. If you transplanted your leeks into holes, you will need to push the soil up around them approximately 2” at a time until the leeks are ready to harvest.

Collaring leeks

Many experts recommend collaring your leeks before hilling them up – applying anything from heavy paper to plastic piping or clay drainpipes around the leek to keep the soil from lodging in the growing plants.

For some, this has evolved into a new way to blanch their leeks; gardeners can use tubing to keep the sun off their growing plants: some use toilet paper or paper towel rolls, while others favour transplanting their leeks within reusable plastic tubes such as thin-walled PVC piping. Your tube should be approximately 3” in diameter so as not to restrict the leek’s growth.

As soon as your transplants are well-rooted, or your outdoor-sown plants begin to thicken and thrive, place the tubing around the base of the plant at soil level; the tube should be just long enough to reach the point where the leaves fork off from the base of the leek.

You can either do this by gathering the leaves and sliding the tube gently down around the plant, or by cutting the tube lengthwise before placing it around the stem and securing it with tape or elastic bands. Staking your tubing will make it stable and avoid damage to your plants.

Pests and diseases in leeks

 The most common problem for any allium is rust, a fungus that will cause orange patches to spring up along the leaf surface – though rust affects the look of the leek, and can reduce your yield, it should not kill your plants.

If you’ve chosen a varietal that overwinters, the frost will kill off the fungus which causes rust; otherwise, simply remove the affected leaves. As suggested earlier, rotate your leeks to a new location next growing season to prevent the fungus from affecting them – you should also avoid placing onions or garlic in the affected area.

You may find white tips, an issue where the leaf tips yellow, then become bleached and white before dying back and white papery patches form on the leaves. This infection is caused by contact with infested soil and severe infections can be catastrophic. White tips can be controlled through the use of pesticides which can help prevent crop losses.

White rot in leeks

Leeks can also suffer from white rot or downy mildew – also common diseases for the onion family – though these are less common problems and leeks are most resistant than their cousins, which means they are far less likely to succumb.

Plants affected by white rot will have yellowed leaves with white or grey fungus at the base of the plant. Downy mildew enters the plant through wounds and natural openings and first appears on older leaves, as white, yellow or brownish spots on the upper surfaces and downy grayish mold on the corresponding undersides.

These eventually release more spores, causing havoc in your planting bed.

Allowing for good air circulation and drainage in addition to a good crop rotation schedule should help to avoid these diseases, which can be treated with commercial or homemade fungicides.

Pests can also be a problem: onion fly can tunnel into the plant tissue, causing the leaves to droop and yellow and leek moths can cause white streaks on the leaves.

Harvesting leeks

Remember that what you see of the leek at harvest is merely the tip of the iceberg – the majority of the edible part of the plant is under the soil, and if you simply pull on the leaf end, your leek is likely to break in two.

It’s best to use a gardening fork or spade to gently lever out each leek. Begin with the largest and most robust stems as removing them will leave more space and nutrients for the smaller plants to mature.

When to harvest leeks

Depending on the varietal and the time you sowed your leek seeds, harvesting can begin in late summer, through autumn (and even winter in warmer climates) and into late spring. Hardier varieties can be left in the soil and pulled as needed. Don’t, however, allow them to grow too large, as very big leeks can have an inferior flavour.

In areas where the ground freezes for quite some time, you should lift any leeks which are ready and store them in some sand in a cool spot. If you still have leeks in the ground by mid-spring and need to prepare your planting beds for new sowings, you can dig up the leeks and place them on their sides in a shallow trench in a shady area.

Cover the stalk lightly with soil, which also helps prevents them from bolting. If you choose to leave your leeks in the soil long enough to flower, nipping the flower stems will generate a bonus crop: leek bulbs that you can use as onions or shallots!

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