How Big Do Leeks Get?


If you are planning on growing leeks in your garden this year, you might be wondering how big they will get.  That way, you can plan the number of plants and the amount of space you will need for your crop of leeks.

So, how big do leeks get?  The stems of leeks are 6 to 12 inches (15 to 30 centimeters) long and 1 to 2 inches (2.5 to 5 centimeters) in diameter.  The leaves of leaks can grow 12 to 24 inches (30 to 61 centimeters) tall with a spread of up to 12 inches (30 centimeters). Overwintering leeks are generally larger than summer leeks, and have a stronger flavor.

Of course, the quality of your leeks (if you get any at all!) depends on the care that you give your plants.  Let’s take a closer look at leeks, including size, growing conditions, and time to maturity.

How Big Do Leeks Get?

Leek stems (the part grown underground that is normally eaten) grow 6 to 12 inches (15 to 30 centimeters) long, with a diameter of 1 to 2 inches (2.5 to 5 centimeters) at harvest.

leek
Leek stems can grow 6 to 12 inches (15 to 30 centimeters) long. Although related to onions, leeks are much longer and thinner.

The leaves of a leek plant grow up from the stem and can reach a height of 12 to 24 inches (30 to 61 centimeters) above the soil, with a spread of up to 12 inches (30 centimeters).

Leeks are a member of leek family (allium), but they are generally smaller than leeks.  Leeks are often classified into summer leeks and overwintering leeks.

Summer leeks are smaller, and overwintering leeks have a stronger flavor.  They can be harvested early and used similarly to shallots.

How Long Does It Take Leeks To Grow?

Leeks can take 130 to 160 days to go from planting a seed to harvesting a mature leek stem.

Leeks usually take 14 to 21 days to germinate, assuming ideal conditions (consistently moist soil and the right temperature – more on this later).

Leeks are often sown directly into the ground.  If you start them indoors, they should be started 8 weeks (56 days) before transplanting them outdoors.

How Do You Know When To Harvest Leeks?

You should harvest a leek when the stem (at the base of the leaves) reaches 1 to 2 inches (2.5 to 5 centimeters) in diameter.  This will be 130 to 160 days after direct planting, or 75 to 100 days after transplanting outdoors.

Leeks should be harvested when they are 1 to 2 inches (2.5 to 5 centimeters) in diameter.

To harvest leeks, cut the roots below the stalks (leaves).  Then, twist the stalks back and forth until they are loose and pull them up out of the ground.  Cut off remaining roots and all but 2 inches of the leaves.

For more information, check out this article on leeks from Burpee.

Why Are My Leeks So Small?

One reason that your leeks are growing thin is due to overcrowding.  If you plant them too close together and fail to thin young plants, they will compete for water and nutrients in the soil.

leeks
If you plant your leeks too close together, overcrowding may result in smaller stems.

Leeks may also grow small due to water stress (dry soil due to drought) or lack of nutrients (especially nitrogen).

Assuming that the plant spacing, watering, and nutrition are all in order, there is one way to get larger leek stems.  Plant the leeks in a trench 8 to 12 inches (18 to 30 inches) deep and cover the stem with soil as the leek grows.

What Do Leeks Look Like?

The University of Minnesota describes leeks as overgrown green leeks, with a long white shaft (shaped like a cylinder).

The leaves of leek plants are thick, flat, and blue-green in color.

Are Leeks Hard To Grow?

Leeks like full sun, so be sure to plant them in an area where they get 8 or more hours of sunlight per day.  Avoid planting leeks in a place where they will be completely shaded by a tree or tall neighboring plants (such as tomatoes).

Leeks grow best in well-drained soil with a pH of 6.0 to 7.0 (slightly acidic).  However, there are many other factors that affect leek growth, including temperature, watering, fertilizing, and spacing.  Let’s start with temperature.

Temperature for Leeks

The minimum temperature for leek seed germination is 32 degrees Fahrenheit (0 degrees Celsius).  If the soil is any colder than this, you will see low germination rates – that is, if you can get any seeds at all to germinate!

This is nature’s way of protecting leek seeds from sprouting at a time when they will be unable to survive.  This is why it is suggested that you start leek seeds indoors to avoid cold soil temperatures in early spring.

The maximum temperature for leek seed germination is 95 degrees Fahrenheit (35 degrees Celsius).  If the soil is any warmer than this, germination rates will decrease.

Combined with high humidity, high temperatures can encourage the growth of mold, which is another threat to your plants.  So, don’t wait too long to plant your leek seeds and transplant your established plants outside!

Leek seeds are often sown directly into the soil outdoors, 2 to 4 weeks before the last frost.  You can find frost dates for your area on the Old Farmer’s Almanac website.

However, this may not be an option in a climate with a short growing season.  In that case, you should start your leek seeds indoors 8 weeks before transplanting them into the garden.

Leeks should be transplanted when they are 6 to 8 inches (15 to 20 centimeters) tall.  For more information, check out this article on leeks from the Old Farmer’s Almanac.

The ideal (optimal) temperature for leek seed germination is between 65 degrees Fahrenheit (18.3 degrees Celsius) and 85 degrees Fahrenheit (29.4 degrees Celsius).

Keep in mind that these temperatures refer to soil temperature, not air temperature.  If you want to find out the soil temperature, use a probe-type thermometer to check.

If the thermometer reads a temperature that is too low, then you have some options.  One option is to wait until the sun warms up the soil.

To speed up this process, clear away any debris, such as leaves or grass clippings, from the soil surface.  Also make sure to choose a location for planting that gets plenty of sun, so that it can warm up the soil faster.

If you are worried about a short growing season, you can also use a cloche (a plastic or glass cover) to trap some heat and warm up the air and soil near your leek seeds.

A cloche can be made from a plastic water bottle to retain warmth and humidity in the soil for seeds or seedlings as they grow.

For more information, check out the table below, and check out this article from the University of California on ideal seed germination temperatures.

Seed
Germination
Temperature
Temperature
(degrees
Fahrenheit)
Temperature
(degrees
Celsius)
Minimum 32 0
Ideal 65 to 85 18.3 to 29.4
Maximum 95 35

Leeks mature best at temperatures below 75 degrees Fahrenheit (23 degrees Celsius).  For more information, check out this article on leeks from Clemson University.

Watering for Leeks

Leeks need regular watering to maintain moist soil.  You may need to water more often if your soil is sandy (which means that it will drain quickly).

garden hose
You may need to water frequently to keep the soil moist for optimal leek growth.

Putting mulch on top of your soil will help to retain moisture, especially during periods of hot, dry weather.  If you find that you have a problem with dry soil, check out my article on how to treat dry soil.

On the other hand, over watering your leek plants (or any plants for that matter) can lead to root rot and eventual death.  The best way to decide when to water is to feel the soil with your fingers.

If the soil feels dry 2 or 3 inches below the surface, then go ahead and water.  For more information, check out my article on over watering your plants.

Try to water early in the morning, rather than at night, to allow water to soak into the soil before evaporating.

For more information, check out this article on growing leeks from the University of Minnesota Extension.

Fertilizing for Leeks

Adding compost to your soil before planting leeks is a good way to improve drainage for clay soil, improve water retention for sandy soil, and add nutrients to your garden.

compost bin
Compost adds nutrients and organic material to your soil, and can be made from recycled kitchen scraps and yard waste.

For more information, check out my article on making compost.

According to Utah State University, leeks require plenty of nitrogen, so consider using a fertilizer such as 16-16-8 at a rate of 1 to 2 pounds per 100 square feet.

You can also side dress with nitrogen-rich fertilizer, but be careful not to burn your plants, especially if using manure in your garden.

For more information, check out my article on over-fertilizing your plants.

Spacing for Leeks

Leeks should be planted 0.5 inches (1.25 centimeters) deep if direct seeding.  Leeks should later be thinned to leave 4 to 6 inches between plants.

Leave 24 inches between rows, to allow space for watering, fertilizing, weeding, and harvesting.  For more information, check out this article on leeks from the University of Maryland Extension.

Conclusion

By now, you have a much better idea of how big leeks get.  You also know a bit more about the care that is necessary to ensure a healthy crop of leeks in this year’s garden.

I hope you found this article helpful – if so, please share it with someone else who can use the information.  If you have any questions or advice about leeks, please leave a comment below.

jonathon.david.madore

Hi, I'm Jon. Let's solve your gardening problems, spend more time growing, and get the best harvest every year!

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