Leeks are members of the alliums family.
Leeks grow best in a sunny position, in free-draining, fertile soil. There are essentially two things that leeks need to grown well: nitrogen and moist soil (sorry I know you hate those words, especially together!). Alliums are shallow rooted and cannot draw moisture deep from the ground. When leeks get ‘water-stressed’ they have a tendency to bolt. Therefore it is essential to keep them well watered, especially during dry spells.
Leeks occupy the ground for a long period of time, as much as 30 weeks.
The soil temperature needs to be at least 5 degrees centigrade for leek seeds to germinate. So, if growing from seed, it’s usually best to start them inside. Seeds can be sown from January through until April. Earlier sowings, however, increase the chances of plants bolting, so it’s probably best to sow seeds around March or April.
Fill pots or seed trays with multi-purpose compost and lightly pat down. Scatter seeds thinly across the surface. Cover with a few millimeters of compost. Water and keep moist! Cover with glass or place in a propagator and leave on a windowsill. Never allow the compost to dry out, check daily.
The seeds will take between 2 to 3 weeks to germinate. It will take a further 10 weeks to 12 weeks for the seedlings to grow strong enough to be planted out. They will grow to about 15cm to 20cm tall, pencil sized. Before planting out ensure the leek seedlings are hardened off for at least two weeks before transplanting.
Leek seedlings are usually planted outside anywhere from May through to mid July. Before planning out ensure the soil is well prepared and fertile.
To plant out, make some deep holes that are at least 6 to 8 inches deep. The hole should be deep enough to cover up the leek to its third leaf. The purpose of this is to maximize the amount of tender white stem the leek with produce. The holes should be 6 to 8 inches apart for decent sized leeks or 3 to 4 inches for smaller leeks. Rows should be 12 inches apart.
To promote root growth trim the leaves by a third and the tips of the root.
Place a leek into each hole ensuring the roots go to the bottom. Next water each hole; the soil should wash in and fill in the hole. Don’t be tempted to fill in the holes with soil by hand. Cover the crop with mulch to help the ground retain moisture.
As the leeks grow, draw up dry soil to cover the stem. This is known as blanching. This ensures the stems remain white. Try not to allow the soil to fall between the leaves.
Leeks are left in the ground for 30 weeks plus and they can be harvested over a long period of time. Some varieties are very hardy and can be over wintered. Harvesting can start as early as September with some varieties harvesting as late as March the following year.
Most plant growth takes place in the autumn. Ensure that plants are kept well watered, they require about an inch of water a week. Additionally, ensure the plants are given liquid feed weekly throughout the growing period. Apart from this there is very little maintenance required about from keeping the leek bed weed free.
Leek rust is a form of fungal disease. The fungus causes bright orange pustules on leaves. The infection can be worse on nitrogen rich soils with low potassium, so be careful with fertilizer applications. Most attacks occur from mid-summer until late autumn. If the pustules break open fungal spores can become airborne
Ensure the plants are spaced sufficiently to provide good airflow.
Avoid watering the outer leaves.
In the summer months, remove affected foliage when you see it. New foliage will be healthy.
Leek moth is a small brown moth, which can be a devastating pest, particularly along the eastern and southern coastal areas of England.
The young caterpillars tunnel into the leaves of the leek to feed leaving whitish brown patches. Older caterpillars tunnel down into the stem of the leek causing extensive damage, which may cause the leek to collapse or rot from within. After about a month, they crawl back up the leaves to pupate and spin white cocoons within the foliage.
There are two generations of caterpillar each year, the first around May to June, and the second around August to October. The second generation of pupae and moth hibernate in plant debris.
If you spot leaf damage, remove any caterpillars and cocoons that you see and destroy any badly infested plants. Remove all plant debris at harvest time and dig over the soil to disturb the hibernating moths and pupae. I
If leek moth is a known problem in the area, then cover the seedlings with horticultural fleece to prevent the adult moths from laying eggs on the leaves.