A quick guide to leeks
- Most leeks require a long growing season of about 120 to 150 days.
- Start seeds indoors and transplant in early spring.
- Hill the plants to produce a longer white shaft, or plant in a furrow and fill it in.
- Leeks have shallow root systems and need plentiful watering.
- Leeks are tolerant of cold, so you can delay harvest until after the first frosts.
Easy-to-grow gourmet vegetable
Leeks (Allium ampeloprasum var. porrum) are a gourmet vegetable that you can grow easily in Minnesota. They have a mild onion flavor. You may eat them in soups, as well as raw, braised or in casseroles or quiche. You can eat both the green leaves and the white shaft.
Leeks look like overgrown green onions, with a long, cylindrical white shaft. The leaves are thick, flat and folded. Plants grow two to three feet tall, and can have a width of two inches.
Soil pH and fertility
- Have your soil tested.
- Leeks grow best in well-drained soil with pH between 6.0 and 7.0, and high organic matter.
- Apply phosphorus (P) and potassium (K) according to soil test recommendations.
- Many Minnesota soils have enough phosphorus. Unless your soil test report specifically recommends additional phosphorus, use a low- or no-phosphorus fertilizer.
- You can increase your soil’s organic matter content by adding well-rotted manure or compost in spring or fall. Do not use fresh manure as it may contain harmful bacteria and may increase weed problems.
- In mid-summer, side-dress with a balanced fertilizer such as 10-10- 10 at a rate of one cup per 10 feet of row. Spread the fertilizer alongside the row, about six inches away from the plants, and scratch it into the soil.
- Do not use any fertilizer containing a weed killer ("Weed and Feed"), as it may kill your vegetable plants.
Most leeks require a long growing season of about 120 to 150 days, and a minimum of eight hours of bright sunlight daily. Some newer cultivars require as few as 90 days to maturity, and these may be most suitable for Minnesota conditions.
In northern climates, start seeds indoors in late February or March.
Transplants should be no more than 10 to 15 weeks old when set out in the garden.
- Harden off the plants for five to seven days before transplanting by putting them in a cold frame. If you do not have a cold frame, set them outside for longer periods each day while returning them to the shelter of your home or garage at night.
- Transplant leeks as soon as early spring weather has calmed down and daytime temperatures are at least 45°F.
- Trim the roots of the transplants to one inch to help you transplant, if necessary.
- Plant two to six inches apart, with 12 to 36 inches between rows.
- A transplant solution of half-strength 20-20-20 or 10-10-10 fertilizer will get the plants off to a good start.
- Close spacing of two to four inches between plants works well if the plants are set out so the leaves will grow into the between-row space, rather than towards the plants on either side. This spacing will make the best use of space, light and air circulation.
To produce long white shafts, some gardeners plant leeks in furrows. Set transplants at the bottom of a six-inch deep furrow. As the plants develop, raise the soil level along the stems up to the leaves, gradually filling the furrow.
Another method is to hill the plants by planting them at normal soil level, then mounding compost or soil around the plants several times during the growing season.
How to keep your leek plants healthy and productive
- Leeks have shallow root systems. They will produce the best yields and quality if supplied with enough water, whether by rainfall or irrigation.
- Plan to irrigate during dry spells.
- Too much water can lead to fungal diseases.
- Water the soil when you irrigate, rather than the leaves of the plant.
- One inch of rainfall per week is enough.
- An inch of water will wet a sandy soil to a depth of ten inches, a heavy clay soil to six inches. If your soil is sandy, it is better to water more often than once a week.
- Use a trowel to see how far down the soil is wet. If it is only an inch or two, keep the water running.
- Frequent, shallow cultivation will kill weeds before they become a problem.
- Leek roots are fibrous and shallow. Take care not to damage them by hoeing close to the plant.
- Cultivate just deeply enough to cut the weeds off below the surface of the soil.
- Mulching with herbicide-free grass clippings, weed-free straw or other organic material to a depth of three to four inches can help prevent weed growth, decreasing the need for frequent cultivation.
- Leeks have many of the same pest problems as onions.
- For assistance in diagnosing unknown problems, visit the University of Minnesota Extension diagnostic site “What’s wrong with my plant?”
- Use good cultural control practices to reduce disease problems to a good level and allow for a successful harvest.
Most leek varieties fully develop when the stem width is bigger than one inch. Some smaller varieties mature at one-half to three-fourths of an inch diameter. A quality leek should have a firm, white shaft more than three inches long. Swelling at the base, called "bulbing," is undesirable.
Unlike their onion and shallot cousins, leek tops do not die back as the crop matures. The top growth, called the flag, should be dark blue-green.
Harvest leeks by either gently twisting and pulling them from the earth, or digging and lifting them. Trim the leaves to a more manageable length at harvest, if desired.
Thoroughly clean leeks before cooking. There is often a small amount of soil held tightly between the leaves, so slice the whole leek lengthwise, separate the layers and rinse thoroughly to remove any soil.
Leeks are fairly frost tolerant, so you can delay harvest until after the first few frosts. Temperatures as low as 20°F may not harm some varieties. Mound mulch around your leeks to protect them, and you can enjoy digging fresh vegetables out of your garden into late fall.
Reviewed in 2018