Growing long white leeks is probably the most rewarding prize for any passionate gardener, but there are so many tricks to get to grips with that knowing how to grow big leeks can feel more than a little overwhelming.
The first time I grew leeks I was at university, with my first allotment. After seven months of careful tending, I had a single row of disappointingly tiny but very flavoursome leeks. Now, ten years later, we’ve played with our sowing schedule, mulching methods, and crop care, and want to share as much of our own process as we can to help you on your way to growing bigger, better, leeks.
The natural growing habits of leeks
Leeks are alliums, but unlike onions, garlic, or ornamental alliums they don’t form duplicating bulbs. Instead, they will naturally bolt in hot dry temperatures and attempt to set seed.
This seed falls to the ground when the leek has finished flowering, and the flower head has dried, similarly to walking onions. The seeds begin to germinate as soon as 14 days after hitting the soil, and the young plants will happily over-winter before being triggered into new and vigorous growth in spring.
In this article, we’ll explore how to harness the natural habits of leeks and the best way to duplicate nature in your own garden to grow the biggest leeks.
How & when to start leeks for bigger crops
As I explained above, leeks are happy to over-winter like all alliums and will begin growing and rooting into their permanent position in late fall. Because they are one of the most frost tolerant plants in the garden, and least likely crops to damp off as young plants, there are no hard and fast rules for when to do this.
In this section, we’ll explore how and when to sow leeks, and when to plant them in the ground:
When to sow leeks for bigger crops
Leeks should be sown from seed in early fall. They will usually germinate reliably within around 14 days, but it’s best to sow two seeds to a plug, or thinly in seed trays (they’re easy to prick out as they don’t grow particularly large roots at this stage).
By sowing early, in September or October, you’re giving your leeks a chance to develop a good root system before the growing season starts, and taking advantage of longer days.
If you try to germinate leeks in December, January or February, they will germinate very sparsely, so the September/October window is ideal to get a head start on the usual sowing time of March/April.
When to plant leeks out
If you’re asking yourself, ‘How big should leeks be to plant out?’ The better question is really ‘When are leeks big enough to handle?’. Leeks can be planted out as soon as they germinate, but they would likely get damaged by rough fingers, trodden on by garden wildlife, or eaten by slugs.
Leeks are ready to plant out when they are roughly the width of a pencil. At this stage, their flavour is developed enough to put off slugs, and their stems are strong enough to resist an early frost.
This year we actually planted our leeks on the allotment in a holding bed as an experiment, and they seem to have really strong roots in mid-winter, ready to be moved to a better position in spring. They were sown indoors in late September and planted into a basic unfertilized bed in early December (most years this would be November, but we had a cold spell in November which slowed their growth down).
Tricks for growing bigger leeks
Leeks come in two halves: whites and greens. All are parts of the leaves, but we generally refer to the tasty white part as the ‘stem’. Leek leaves are edible but tougher and take longer to cook, so when you plant them out, there are a few basic tips to increase the amount of white stem on your leeks, and in turn, grow bigger leeks.
How to plant leeks for bigger stems
Leeks are incredibly easy to plant. As I mentioned above, the tastiest part of a leek is its white stem before it spreads out into leaves at the top. To achieve more white, you need to blanch your leeks which we’ll cover in full later, but the first and most basic step is to drop them into holes in the soil so half the leek is buried and half is above ground.
Once your leek seedlings are roughly the same width as a pencil, they should be about 5” tall. At this point, dib a hole with your finger into rich but loose soil, drop the leek in and water the hole and don’t press on the soil around it.
The water fills the holes back in with soil, without compacting the leek or its roots. This means that whatever happens the base of your leek will be white, and the roots will be well anchored for the coming winter months.
The best fertilizer for leeks
Leeks are not fruiting or flowering crops. While they will naturally set flower if they go through a period of hot drought, it’s best that gardeners try to avoid this as the scapes (the flowering stems) are watery, and while they can be eaten, have a much more garlicky flavour.
The best fertilizer for leeks is a heavy nitrogen feed. All leafy crops need added nitrogen to boost their overall growth, which is best provided with liquid seaweed.
In spring, when the plants start to grow more vigorously you can add a light sprinkle of chicken manure pellets to the soil too. Their slow-release fertilizer is brilliant for bigger leeks, but don’t use this over winter as it can be too potent and kill off young plants.
Should You Trim Leeks?
If your leeks are growing well already there is no need to trim them. Trimming leeks should only be done if your plants are too thin. Thin leeks taste fine but are more likely to snap in high winds, so if you’re worried about the rate of growth in your leeks, trimming your plants can be useful.
Thin leeks are also a less than perfect candidate for storage, as they tend to dry out, so even more reason to try to grow big leeks. Check this article on storing leeks in the root cellar long term if you’re interested in alternatives to freezing them.
There is a common myth that trimming young leeks makes them stronger in the long term, but the results, while leeks grow thicker, and with more white, mean that leeks are much more likely to bolt and form scapes in late spring.
How to grow leeks with long white stems
Every part of a leek is edible, but the white parts of the stems are stronger and packed full of flavour, so the more you do to encourage these white stems, the better (and bigger) leeks you will have.
When you plant leeks out in the first place, dib them in (as described earlier), and water them into their planting holes. As they grow there are a few options to blanch the stems throughout the season.
The most common way of blanching is the traditional method of hilling-up leeks, which is the process of planting leeks in a trench and pulling soil back in around them. Personally, I’m not a fan of hilling-up leeks, as their leaves wrap around as they grow, which traps soil and insects inside the stem.
Growing leeks in toilet paper
My personal preference for blanching leek stems while they grow is toilet roll, or drain pipe. Both work just as well, and toilet roll will break down and be compostable at the end of the season, while plastic pipe will be re-usable every year.
The principle is the same, whether you use toilet roll to blanch leeks or plastic pipe. The tubes block light from the stems, which tricks them into thinking they’re underground. Using pipe rather than soil to hill-up leeks also reduces trapped soil when you harvest your crop.
The 3 best giant leek varieties
Now, if you’re really serious about giant leeks, and you’re growing for competition (or just to show off to friends and family), you’ll need another card up your sleeve. Most leeks will grow big if given the right conditions, but growing leeks from specially bred seeds can be the difference between growing big or growing prize-winning leeks.
Last year we tried Giant Winter, a generalized late cropping variety that grows at least a foot taller than other leeks in the same conditions, and the only difference in care was that the regular leeks had a toilet roll to blanch them, while Giant Winter Leeks had a 6” drain pipe around them, growing to 3” thick across the white stem.
We did lose to three others on the allotment at our annual show, who had grown other giant varieties from specialist retailers. They grew Giant Musselburgh (a wide but short variety) and Giant Early Leeks (which were just over 1/2” thick, but almost 2ft tall).
If you want to grow these leeks, it’s best to plan them out a little later, and plant them deeper too (8” is about right). Prepare the soil with phosphorus-rich fertilizers before planting, and rough up the bottom of the trench before dropping leeks in. This gives their roots the chance to establish a stronger, wider, root plate.
- Buy Giant Musselburgh Leek seeds online at True Leaf Market
- Buy Early Giant Leek seeds online at Eden Brothers
Growing the biggest competition leeks relies on a combination of factors. Big leeks need well-drained soil, good balanced organic fertilizers, and some careful garden trickery to grow well. But once you familiarize yourself with the growing habits of leeks you’ll have a show-stopping crop every year.