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How to Grow Leek from Seed

Family: Amaryllidaceae
Binomial name: Allium porrum
Life Cycle: biennial (usually grown as an annual)

When to Sow Leek Seeds

Leeks can be grown year-round in most climates. Avoid planting in extremely hot or cold weather which can affect germination and growth. Use the table below to identify the best time of year to sow leeks in your climate.

Preparation

Leek plants are best grown in full sun. Choose a location that will receive at least 6 hours of full sun each day.

Leek plants need a well drained soil enriched with plenty of organic matter. Prepare soil by weeding it thoroughly, digging it over to loosen it and adding aged animal manure or compost. Keep the area free of weeds until planting.

How to Sow Leek Seeds

Leek seeds do not require any treatment (eg soaking, stratification) before sowing.

Leek seeds can be sown directly into the garden OR seedlings can be raised in trays or other containers and transplanted to the garden once established.

Sow Direct

1. Sow seeds directly in the garden 5mm deep and 12cm apart, with rows 45cm apart.
2. Keep soil moist but never wet or dry.
3. Seeds should germinate in around 10-14 days at a soil temperature of 18-23°C.
4. Young seedlings will need protection from pests, pets and weather until they are established.

Raise Seedlings

1. Fill trays, punnets or jiffy pots with a good quality seed-raising mix, or use soil starter pellets.
2. Sow seeds 5mm deep.
3. Keep soil moist but never wet or dry.
4. Seeds should germinate in around 10-14 days at a soil temperature of 18-23°C.
5. Transplant seedlings to the garden once they have their first true leaves and are large enough to handle (usually 5-10cm tall).
6. Plant out, spacing plants 12cm apart, with rows 45cm apart.

Tip: Leek seeds can also be sown in the less formal ‘scatter seed’ method. Simply roughen the soil, scatter seeds evenly over the surface, then smooth the soil over lightly to cover the seeds.

Tip: Leek seeds are quite small. Handle them carefully to avoid them blowing away or being washed away. Mix seeds with sand or fine potting mix prior to sowing or use a seed dispenser, damp toothpick or tweezers to help space them evenly. Press lightly into the surface after sowing so that the seeds make good contact with the soil. Take extra care to make sure seeds and seedlings don’t dry out. Read more about sowing small seeds here.

How to Grow Leek

Leek plants may need watering during the growing season. Water when the soil is dry about 5cm below the surface (test this by scratching away a little soil with your finger). Water deeply in the early morning or late afternoon. Avoid watering the leaves of plants to avoid fungal diseases. Learn more about watering here.

If soil was well prepared no extra fertiliser should be necessary. In poor soil or to give your plants an extra boost, application of a balanced fertiliser or one formulated for fruit and vegetables can be beneficial:

• Apply slow release fertiliser at the recommended rate when transplanting or when seedlings are 5-10cm tall.
• Apply liquid fertiliser at the recommended rate and frequency while plants are fruiting or flowering.

Optional: To give plants room to grow, thin seedlings when they are large enough to handle. Pull out any weak or small seedlings so plants are spaced about 12cm apart.

Optional: Leek plants can be blanched to improve tenderness and flavour. Exclude light around plants by hilling soil up around their bases, mulching thickly with straw or placing opaque containers around the plant. Learn more about blanching here.

How to Harvest Leeks

Leeks should be ready to harvest in approximately 120-150 days.

Plants are ready to harvest when they are large enough to eat. Gently pull the whole plant from the ground and shake off any excess soil. Leeks are best left in the ground until needed. For short term storage, harvested leeks can be stored wrapped in a damp paper towel in a perforated plastic bag in the fridge. 

Common Problems when Growing Leeks

Like all plants, leek is susceptible to some pests, diseases and other problems. Below is a list of the most common problems gardeners encounter when growing leek plants:

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  Aphids are small (2-4mm long) sap-sucking insects that congregate on the new shoots or the undersides of leaves. They can cause leaves to wilt or become discoloured, and also excrete honeydew which can attract ants and other insect pests. To manage aphids, remove them by spraying with a garden hose, apply a soap or alcohol spray, or encourage predatory insects to your garden. Read more about aphids here.
• 

  Bolting is when a plant prematurely flowers and goes to seed. Bolting can be caused by a period of extreme weather. Avoid sowing seed until after the danger of frosts has passed or in very hot weather. Water plants regularly and deeply in hot weather to prevent them suffering heat stress.
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  Poor germination may be caused by sowing old seed. Seed for this species does not store well long-term, so use fresh seed each year. Make a note of the sowing date and the expected germination time. Seeds may take longer than the expected time, especially if conditions such as soil temperature are not ideal.
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  Rust (Puccinia sp.) is a fungal disease that causes brown to orange raised spots or patches to appear on foliage. Fungal spores are spread by wind or water to neighbouring plants, especially in temperatures of 10-20C and when humidity is high. To manage rust, space plants to avoid overcrowding, grow them in the recommended amount of light (eg full sun), do not over fertilise crops, remove dead plants and practice crop rotation. Read more about rust fungus here.
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  Onion thrips (Thrips tabaci) are flying insects less than 1.5mm long with slender pale yellow to light brown bodies. They suck sap from plants, leaving silver patches on the leaves and reducing the productivity and yield of plants. Thrips can live in the inner leaves and leaf folds of alliums, and may also infest bulbs. Wash thrips from affected plants using a garden hose, spread an organic mulch around plants, and encourage predatory mites and lacewings with companion planting.