How to Grow Leek

Botanical Name: Allium ampeloprasum

Plant Type: Hardy biennial

The leek, Allium ampeloprasum, is a member of the plant family Amaryllidaceae, its closest relatives being onions, garlic, shallots, and chives. It is a biennial plant, running up to seed during late spring to early summer, and it is hardy and frost tolerant. The wild form is found throughout the Mediterranean and Middle East and may have been cultivated as early as the time of the first Egyptians. It was certainly used by the Romans and has an important place in the culture of the Welsh people. Through the process of selection, it has been developed into a multitude of varieties and several distinctive types of vegetable, including the familiar onion, shallot and potato onion. It is a close relative to the Welsh onion, A. fistulum, grown for its hollow stalks used in the kitchen as scallions.

The leek is an autumn to winter crop that is much hardier and more subtle in flavour than the onion. Some varieties produce a large bulb at the base of the shank which is an excellent substitute for onions in the kitchen. In fact, plants can be harvested well before maturity to utilise the bulbs and shanks in place of onion and shallots. It is a handsome plant in flower and deserves to be grown more widely in Australian gardens.

Varieties

Dozens of varieties of leek are available overseas, but the range is fairly limited in Australia. Important characteristics in selecting varieties include shank length, flavour and variations in the time to maturity. Late varieties in this case are generally preferable to the early ones in most climates, because while they take longer to mature, their ultimate size is usually larger than the early maturing types, so that their growth to a given size is usually faster.

Cultivation

Bed preparation, sowing and transplantation procedures for leeks are like those for onions. They prefer neutral soils rich in nutrients, and when these are lacking, produce plants of poor flavour. Seed requires temperatures in excess of 13°C for germination to occur. In temperate regions they can be established from early spring onwards, and in cold areas they are best established in early summer. Plants should be spaced out at intervals of 10 cm for smaller varieties, such as Blue Solaise, and up to about 25 cm for the larger types like Elephant. Closer spacing will produce smaller plants.

Sun: Full sun

Pollination: insect pollination, cross pollination

Soil: Leeks prefer rich, well-draining soil with a pH between 6.0 and 7.0. Soil can be enriched with mix in some manure/compost and organic pelletised fertiliser

Growing from Seed

Sow: can be sown direct or in punnets

Ease of Germination: medium

Germination Temperature: 13-24°C

Leek Germination Time: 7-10 days

Spacing: 10-20 cm

Harvest: 100-160 days

Yield: 6 shanks/m

Maintenance and Additional Care

To promote the development of longer shanks some gardeners blanch leeks in a manner similar to that described for celery, which involves growing the plants in a trench that is progressively filled with soil. This practice entails a lot of additional work, plus it tends to get a lot of annoying grit in between the leaves, making preparation in the kitchen more troublesome. An alternative is to pile up hay around the shanks, which also acts as a mulch. In our experience the better varieties produce ample shanks without blanching.

Growth can be rapid in our mild climate, in some cases reaching full-size 90 days after transplant, so that leeks established in spring may be ready to harvest in summer. Staggered plantings from spring through summer can produce a continuous crop for much of the year. When harvesting leeks exercise some care, for some varieties produce miniature plants around the base, which are known as pearls. Treat these babies with care and they can be transplanted to provide the gardener with a bonus crop.

Leeks are hardy and experience few diseases or pest problems. In particularly wet autumns they can experience the development of some slimy rots, which can affect the leaves. These are generally not serious and do not affect the shank.

These are heavy feeders, but they don’t need much more than compost-enriched soil. Work compost into the ground before planting. Some blood or bone meal will also encourage them to thrive. If you want to use fertilizer, choose a slow release balanced granular fertilizer. You can also use a liquid fish emulsion or just add more compost occasionally.

Companion Planting

Good companions: Carrot, Celery, Lovage, Marjoram, Parsnip, Strawberry, Onion and Onions.

Bad companions: beans, peas

Pests and Diseases

Leeks are mostly disease-free. The only diseases which typically impact them are those typically caused by humidity or moisture on the leaves. Thus, rust or powdery mildew can become a problem.

Growing in Containers

Leeks can be grown in pots or containers. Ensure there is the correct spacing, use a good quality potting mix and liquid feed regular with an organic fertilizer.

Harvesting

Can be left in the ground until required and harvested right through autumn, winter, and early spring.

Leek leaves and flowers are also edible. Removing a few outside leave when plants are young won’t affect the adult leek. These can be finely chopped and used in a range of different ways.

Seed Saving

Seed Longevity: 3 years

Isolation Distance: 400m

How to Save Seed: To save see you need to leave the plant in the ground until the second year when the flower head will grow. When most of the small flowers are open (so you can see the black seed), cut off the head and place it in a brown paper bag. Once completely dry the seed will drop and can be collected.

Leek seeds are more tightly encase in their seed pods than onion seeds, therefore immediate harvest of seed heads is not as critical.

Tips and Tricks for Success

• leeks need regular watering and an application of seaweed tonic every two to three weeks to promote quick growth and plump, tender stems.
• leeks have fragile shallow roots, so always weed around them by hand.
• mulching is essential in warm climates. Mulch the seedlings with a 1cm layer of chopped lucerne, hay or sugarcane. As they get stronger, increase the depth to 4cm. This also limits the amount of dirt that can find its way into their stems.
• to harvest, use a knife to cut stems off below ground level, or carefully dig them up with a long trowel.