When to Start Leeks From Seed to Get a Big Harvest?

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Knowing when to start leeks from seed to get a big harvest is one of the holy grails of vegetable gardening. 

In this article, I’m going to share some family secrets to the perfect leek, and some wisdom of my own from trial and error. Fundamentally though, this article aims to give you a definitive answer to when to start leeks from seed, depending on where you live.

When to sow Leeks for bigger crops

Leeks are easy to germinate and pretty easy to grow on, so it’s all about timing. Leek roots, like most alliums, grow outwards rather than downwards, especially at the seedling stage, so free draining soils with good moisture retention are best to provide them with the best start

When to sow leeks depending on your climate

Leeks are hardy down to -12°C/10°F, so can be grown outdoors through winter in zones 7b – 11b. Cooler regions can still grow leeks over winter but should do it under glass, or indoors, while warmer regions are unlikely to be able to grow leeks with any real flavour as they benefit immensely from a chill for full flavour in spring.

	When to sow leeks indoors (autumn sowing)	When to sow leeks indoors (spring sowing*)	When to plant leeks outdoors*
Zones 1-5	N/A	April – May	May – June
Zones 6-7	August – September	March – April	March – April
Zones 8-9	August – October	March	November, or March – April
Zones 10-11	September – November	February	November, or February – March
Zones 12-13	Any time (keep well-watered and harvest young)	Any time (keep well-watered and harvest young)	Any time (keep well-watered and harvest young)

*These guides are given for temperate northern hemisphere regions – for the southern hemisphere, replace September with March, and work forwards.

I would never advise direct sowing leeks, as you will always get stronger plants by sowing indoors and allowing them to strengthen up in seed trays or plugs. This levels the playing field, as all climates above USDA zones 1-5 can sow indoors right through fall and early winter, provided the day lengths are long enough to germinate seeds (they need 6 hours of sun per day).

When to sow leeks, zones 6 & 7:

The colder your area gets in winter, the later into spring you should plant them out. For example, zones 6-7 are reasonable temperatures but have colder winters than most are used to. This can rot leeks that have been left outdoors uncovered. Sowing leeks indoors, and growing them on until spring gives you a head start.

When to sow leeks, zones 8, 9, 10 & 11

In zone 8, 9, 10, and 11 you should plant leeks outdoors in October or November so they have a head start in spring – meaning you’ll be harvesting leeks all summer long. To achieve this you should be sowing your leeks in late summer or early autumn to achieve pencil-thick leeks in time for planting out.

In zones 8 & 9 it’s a good idea to cover your leeks over winter to protect them from very bad frosts, but in zones 10 & 11, it will only freeze lightly for a few days, and nowhere near enough to harm leeks.

When to sow leeks, zones 12 & 13.

Zones 12-13 are extreme weather conditions and are best described as tropical to sub-tropical conditions. Leeks do not do well in these conditions and will only grow to a short size before the high temperatures and sudden heavy rainfall causes them to wilt. When growing leeks in these extreme conditions, give them short seasons and harvest them as baby leeks after just 1-2 months of growth.

Is it best to sow leeks indoors, in a greenhouse, or direct?

How to sow leeks indoors

Sowing leeks indoors couldn’t be easier and is much more reliable than sowing leeks directly in the soil. Follow our step-by-step guide to sowing leeks for a bigger harvest:

1. Prepare seed trays or module trays half-filled with loose, free-draining seed compost.
2. Plant two seeds per module, or sow thinly in a tray.
3. Cover with sieved seed compost.
4. Water well, then leave covered in a sunny spot for 7-14 days to germinate.
5. After germination, water lightly whenever the soil is dry to touch.
6. When seedlings reach 1-2inches tall, thin them to 1 seed per module/inch.
7. Continue caring for your leeks indoors, or in the greenhouse until they are 5mm thick (the width of a pencil).

How to sow leeks in a greenhouse

In zones 6-10, you can sow leeks outdoors in a greenhouse to overwinter, just make sure to have your greenhouse heater on, or light a few candles on cold nights when frosts are predicted.

Besides that, the process of sowing leeks indoors vs. sowing leeks in a greenhouse is identical. Follow the steps above.

When to plant out leeks for the biggest harvest

While leeks are seen as frost-hardy plants, they should be planted out as young, pencil-width plants when you know the temperatures won’t drop below -6°C/21°F. Very cold winters or extended cold snaps will damage young leeks before they establish roots, so in zones 5-7 leeks should only ever be planted out in spring, having overwintered indoors.

Leeks are frost hardy right down to -12°C/10°F, so in milder climates (Zones 7-10) they can be left in the ground over winter rather than harvested all at once. A cold snap gives them a much more buttery flavour and sweetens their zingy onion tones to a flavour more like roast garlic.

Final thoughts on the best time to sow leeks

Once you’ve got your head around those conflicting dates, and worked out your hardiness zone, there’s nothing stopping you from growing prizewinning leeks. When you’re finished and ready to harvest follow our guide to storing leeks for winter to extend the enjoyment of this vegetable garden staple.

Knowing when to start leeks from seed depends on where you live, but as a rule of thumb, they should always be started when the days are getting shorter. Provided there are still six hours of light left in the day, they will germinate and continue growing until early winter.