Lettuce- Iceberg

How to Grow Lettuce from Seed

Family: Asteraceae
Binomial name: Lactuca sativa
Life Cycle: Annual

Lettuce varieties are classified by the way the leaves grow and are harvested. In this guide, lettuce plants are classified as either loose leaf or head-forming.  

Loose leaf: The leaves of this type of lettuce do not form heads. Plants can be grown as cut-and-come-again leafy greens, with leaves harvested as needed. This type of lettuce includes loose leaf, oak leaf and stem lettuces.

Head-forming: The leaves of this type of lettuce form round or elongated heads with tightly packed leaves. The entire head is harvested. This type of lettuce includes cos/romaine, butterhead and crisphead lettuces.

When to Sow Lettuce Seeds

Lettuce 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 lettuce in your climate.

Preparation

Lettuce plants are best grown in full sun or part shade. Choose a location that will receive at least 3 hours of full sun each day.

Lettuce 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.

Lettuce plants can be grown in containers. If possible choose a variety that’s recommended for container growing. Use a good quality potting mix and make sure your container is large enough for mature plants; a minimum of 10 litres litres is recommended for lettuce. During the growing season, keep in mind that container grown plants may need additional fertiliser to encourage healthy growth.

How to Sow Lettuce Seeds

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

Lettuce 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 3mm deep and 20cm apart, with rows 50cm apart.
2. Keep soil moist but never wet or dry.
3. Seeds should germinate in around 7-14 days at a soil temperature of 8-23°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 3mm deep.
3. Keep soil moist but never wet or dry.
4. Seeds should germinate in around 7-14 days at a soil temperature of 8-23°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 20cm apart, with rows 50cm apart.

Loose leaf only - Tip: Lettuce 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.

How to Grow Lettuce

Lettuce plants need regular 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.

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 20cm apart.

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 high-nitrogen fertiliser or one formulated for leafy greens or herbs 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 during the growing season.

How to Harvest Lettuce

Lettuce should be ready to harvest in approximately 50-85 days.

Loose varieties only - Leaves are ready to harvest when they are large enough to eat, and can be harvested as needed. Harvest leaves by pinching off the outer leaves, leaving some on the plant for future growth.

Head-forming varieties only - Leaves are ready to harvest when a head has formed and the leaves are tightly packed. Harvest whole heads by cutting them at ground level.

Eat lettuce leaves as soon as possible after harvesting. Store leaves short term in a perforated plastic bag in the fridge.

Common Problems when Growing Lettuce

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

• 

  Slugs and snails are molluscs that feed on tender leaves and shoots, mostly at night, leaving slimy trails behind them. Control them by removing their hiding places, keeping free range poultry, collecting them by torchlight or by placing traps. Read more about slugs and snails here.
• 

  Damping off is caused by a fungal growth that transfers from the soil to seeds or tender seedlings. Seeds may appear not to germinate, or young plants start to rot when they emerge from the soil and become soft and mushy at the base before dying. Use new potting mix if raising seedlings, do not water foliage and avoid waterlogged soil. Read more about damping off here.
• 

  Whitefly is a sap-sucking insect related to aphids. They are often found in large numbers on the underside of leaves and will swarm in clouds when disturbed. Plants may have yellowing leaves or may wilt, and growth will be slowed. Whitefly can be removed with a garden hose or sprayed with soap spray. Badly affected plants should be destroyed. Attracting beneficial insects that will prey on whitefly can be beneficial. Read more about managing whitefly 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.
• 

  Heads not forming is usually caused by warm weather or by growing plants too slowly, without adequate water or nutrients. Check to make sure you are growing a head-forming type of lettuce. In warm climates, grow lettuce in autumn and spring to avoid extreme temperatures. Water regularly and ensure adequate compost or fertiliser is applied.
• 

  Bitter taste can be caused by plants growing too slowly, suffering a setback in growth or being harvested too late. Enrich soil with aged manure before planting, ensure plants are watered deeply and regularly, and harvest when they are young and tender.