Leeks

• Seeds germinate in seven to ten days. They are not rapid, often germinating at a varied rate.

Sowing time

Mid-spring is ideal. You can sow earlier, say at the same time as onions in late winter. However, that brings a risk of premature flowering in the summer.

• I have sown seeds in February and then lost some plants to bolting by July.
• It’s worth waiting until early April, in this climate at least. I sow a few varieties at the same time, for all seasons of harvest.

An issue with early sowing is that people who do it can make you feel that you missed the boat! This is terrible because, with leeks for example, you can sow as late as May, in the latter third of spring. You will have a lovely harvest – it will just be a little smaller, and later.

Sowing method

I used to sow seeds in rows in a garden bed, around the middle of April/mid-spring. Now I multisow in modules only – why the change? Partly it was from erratic germination, partly from the time and space needed; the old saying is ‘plant leeks when their stems are of pencil thickness’.

Leek transplants can actually be smaller, and, once you have the kit for propagation, it does not take long to fill a module tray and multisow leeks. Four seeds give an average of three plants per module, depending on the age and quality of seeds. In a 60 cell tray that results in 180 leeks, enough perhaps for a family in winter.

• Each leek at harvest, from a multisown clump, can weigh 100–200 g/4–7 oz, after trimming its outer green leaves. You can use the leaves as well if you wish – they are great in soups and stews.
• Results from propagation under cover are reliable and have less pest damage – see leek moth and leaf miner information below.

Push compost firmly into the trays and, before watering, use fingers to make small hollows or indents. Drop four to five seeds in each. Then level the compost, or drop a little new compost on top, and water thoroughly after sowing. Sowing depth is about 0.5 cm/0.2 in.

Four reasons to multisow

• Grow more plants in the same area of propagating space.
• Use less compost to propagate the same number of plants.
• Save time because you are planting two or more leeks for each motion when transplanting.
• Plants grow better when in clumps alongside their mates, from germination to harvest.

All in all, more from less!

Pot on?

You can transplant module-sown leeks in late spring or early summer, even when quite small. Or, if there are vegetables growing in the bed you want for eventual leek planting, simply keep the module-sown clumps growing – pop each one into a 7 cm/3 in pot.

Use the same multipurpose or potting compost at all stages.

Plants grow fast after potting on, and if after two weeks your ground is still not ready, pop out the contents of the 7 cm/3 in pots into a 10 cm/4 in pot with more compost. You will be repaid for the time and compost needed to do this, by a larger harvest in autumn and winter.

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Your time options for transplanting leeks are considerably more wide-ranging than almost any other vegetable. They are simple to keep growing by potting on. They are not damaged if you leave them in modules for two or three weeks too long – they would just stop growing for a while, and your leeks at harvest would be smaller, but still worthwhile.

Transplant size and time

• Transplant at 10–17cm/4–7 in high, from six to fourteen weeks after sowing.
• You can also transplant leeks in late summer to early autumn, but harvests will be noticeably less.

Transplant method

Planting depth is much debated.

Shallow planting

This results in a less white but still long shank, and has three advantages:

• It’s quicker and easier to set plants in the ground in shallow holes.
• Leeks are easier to harvest and with less soil disturbance, because the base of their stem is not deep in the ground.
• Plants grow better, for having their main roots immediately in contact with more soil microbes near the surface, compared to the amount 15–20 cm/6–8 in below surface level.

Deeper planting

When plants go into 15 cm/6 in deep holes, or even deeper trenches, they will have more white at the base of their shank when you harvest. If you like that, plant deep.

When you raise leek transplants in open ground, it’s feasible to set them deeper than plants raised in modules. You may need a trowel or spade to set them deep. Harvesting will also be a spade or fork job.

• The photo below shows 100% blanching achieved by a collar of cardboard, or a pipe around the shank. In contrast, the leeks in my crate had grown from shallow planted, multisown modules, with no blanching at any stage. Peeling off an outer sheath or two reveals paler shanks.

Spacing

Clumps of two to four leeks can space at 25–30 cm/10–12 in, according to how large you like them to grow.

Set individual leeks at 15 cm/6 in apart, or closer if your wish is for small leeks at harvest. From a close spacing, you can harvest every other leek, leaving the rest to grow more.

If leek moth is prevalent in your area, best cover as soon as planted with some mesh, wide enough to offer no entry point at the sides. It may or may not be on hoops.

Interplanting

The photo above shows an interplant that broke the rule of not having plants compete with each other. Both were needing to grow strongly at the same time. The cabbage won!

A better option would be to interplant leeks between, say, spring-sown carrots, coming to the last month of their harvest. Or transplant leeks between spring-sown lettuce, which gives the final harvest about a month later.

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Leeks grow largest in moist soil. They also tolerate dry conditions, when their leaves turn darker and growth slows.

Always give water to transplants. Often they follow vegetables such as potatoes, which have sucked soil quite dry, plus there is hot sun in summer. Watering in any dry periods until mid-autumn is highly worthwhile. Give a decent amount as well.

Extra mulch to retain moisture?

Leeks are little troubled by slugs, and grow well when soil is mulched with a thin layer of straw. Hay is possible too but may have weed seeds, and possibly some weedkiller.

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How to judge readiness

You can harvest small or large, at any time. If you like a decent-sized leek, allow at least three months of growth from the time you transplanted.

In early spring you need to finish harvesting before flower stems become tough. They shoot up in the middle of shanks from mid-spring, hidden inside at first, so don’t delay too long. Last harvests here are in early May, depending on variety.

How to pick

The deeper you transplanted your leeks, the deeper you need to go to dig them out.

With the planting depth I recommend for module-sown clumps, you may be able to pull out whole clumps without using any tool. Otherwise, twist and pull individual leeks or use a trowel for undercutting roots, as I show in the video.

• Before the main harvest period for leeks growing in clumps, best make a first mini-pick in early autumn of any tiny leeks, because they won’t grow large. They are the runts.
• This thinning out also reduces rust a little.
• From mid-autumn, usually, remove individual plants or clumps of leeks here and there, to thin the planting and enable more growth in remaining leeks.

When to pick and how often

Leeks store well. You could harvest, say, two clumps twice a week, for eating leeks every day through autumn. In the cool of winter, weekly harvests are good.

If your soil is about to freeze hard with cold weather arriving, harvest beforehand with some roots left on. Place leeks in a container with a little soil or compost at the bottom, somewhere with some light. They may or may not freeze and are available as needed.

Use these tables for an idea of how much harvest to expect, and how often.

No dig bed of trial – 1.5 m/5 ft wide

• Leeks Philomene, multisown on 5th April
• Transplanted on 23rd June after Casablanca potatoes
• Two rows across the bed, each with six clumps of three to four leeks
• 21 leeks per row; harvests lasted for two months

DateWeight (kg)Number of leeks3rd October0.90415th October1.32      620th October1.25       45th October1.79       926th November1.69       91st December1.9210TOTAL8.8742 leeks

Three Strip Trial, from Strip 3’s bed of 1.2 x 2 m/ 4 x 6.5 ft

• Leeks Philomene, multisown on 5th April
• Transplanted on 9th July after Charlotte potatoes
• Four rows along the bed, which is 1.2 m/4 ft wide
• Eight clumps per row, of three to four leeks in each
• Therefore 28 leeks per row and 112 leeks in a bed
• Later planting than the table above; harvests slightly later and smaller

DateWeight (kg)Number of clumps29th October2.15612th November2.14719th November2.3479th December2.09617th December2.776TOTAL11.4932 (112 leeks)

Weights are for trimmed leeks with most of the long leaf cut off. Each leek on the Two Bed Trial (first table) was on average twice the weight of those in the Three Strip Trial. The difference is from earlier transplanting and wider spacing.

The middle photo below shows a higher yield in the same Three Strip bed, three years earlier. I used the same spacing, but transplanted the leeks 18 days earlier.

Storing

Leeks keep for one to three months in winter, when harvested with roots on and stored in a container or bucket, out of the weather and cool, even freezing a little.

Or, if you trim the roots and keep leeks in, say, a cool shed below 10 °C/50 °F, they can be good for two to three weeks. Before cooking them, you need to peel a sheath or two from each leek, because they turn brown from the outside yet are still good in the middle.

Saving seed

Long ago, I did this from one plant, and growth from those seeds was weak, due to inbreeding from a lack of genes. Subsequently, I learnt about the need for cross-pollination, with six to ten leeks flowering together. Stems need support from early summer as they grow heavy, up to the harvest in late summer to early autumn. More details from Real Seeds.

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Which pests are likely, and when

Leek moth is common in warmer areas. In the UK, for example, it’s in southern and central England, but I have not heard of it affecting Scotland.

Allium leaf miner causes worse damage, twice a year. Plus it’s harder to prevent.

White rot is a big issue if present in your soil.

Rabbits eat the stems of young leeks, potentially causing much damage. Bird netting should keep them off – make it very secure at the sides if there are more than a couple of rabbits.

Slugs and snails sometimes live and/or hide in the crevice where leaf meets shank. I have not known them to be a problem.

Leek moth (Acrolepiopsis assectella)

The caterpillars are small and mostly stay well hidden inside the newest leaves, right in the middle of affected plants.

At first there is no damage visible, but in late summer you see leaves with vertical gashes, different from the punched holes made by slugs. Growth stops and some plants may become a rotten mass of brown mush.

• Prevention is by covering new plantings with mesh throughout summer.
• Besides mesh, a possible remedy is to spray a small jet of Bacillus thuringiensis into the centre of each leek plant, every three weeks from midsummer to early autumn.

Allium leaf miner (Phytomyza gymnostoma)

Allium leaf miners are horrible pests, and are now spreading around the south and east of the UK.

• A cover of mesh from February to early June can help, and again from September to November.

These timings are to protect allium plants from the two generations of egg-laying flies, which are present mainly in early spring and again, more importantly, during mid to late autumn. However, when pupae are already in the soil, I don’t know how to protect leeks from them, except to transplant later rather than sooner, say in July, and cover with mesh throughout – also in the case of leek moth!

The white maggots look similar to leek moth and bore into and eat foliage and shanks for two weeks or so, before turning into brown pupae. Pupation occurs within stems and bulbs, in both summer and winter after the respective hatchings.

From any leaves and plants, the 3 mm/0.1 in long pupae can end up in soil. They hatch as small flies in the spring.

• I would feel safe about putting all damaged leaves and plants in the middle of a compost heap with heat of, say, 50 °C/122 °F plus.
• Rotation is worthwhile, with a gap of approximately three years.

White rot (Stromatinia cepivora) – see photos in Lesson 21 on onions

This fungus is specific to allium plants. Stromatinia exists in soil for 5–15 years, as dormant sclerotia. They spring to life in temperatures of 15–20 °C/60s °F, and when they detect allium roots nearby.

Signs of damage are leeks falling over, sometimes with bright yellow leaves, and roots covered in white mould. Best not grow any alliums in that soil for perhaps three years or more.

However, like clubroot, white rot can be prevalent where soil has been poorly looked after. Check this observation from Jayne Arnold, market grower in central England at Oxtons Organics. She wrote in an email of 20th November 2020, after their third year of practising no dig:

• ‘For the first time in 30+ years we have not had a single leek with white rot. A disease we inherited and obliged us to practice eight-year rotations, to enable us to grow alliums’.

Other possible difficulties

Leek rust (Puccinia poori) is a fungal parasite (biotroph) on leaves, but does not kill them. Therefore it’s less serious than, say, late blight on potato leaves. It makes bright orange pustules, similar to or the same as garlic rust.

Common advice is to burn or dispose of rusted leaves. At Homeacres I always put them on the compost heap, and, after eight years of doing this, I see no more rust here than when I arrived. Leeks suffered rust here in 2013, growing in ‘virgin’ soil of 20+ years pasture and no vegetables.

Rust is sometimes claimed to be a soil problem but I am not convinced. Generally, it likes moist and warm conditions – not too hot. Plants in the leek bed below, now in its sixth consecutive year of growing leeks (middle photo), suffer no more rust than they did in year one.

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