Iceburg Lettuce Growth Stages

Did you know iceberg lettuce was first bred as a hardy option for cool-climate gardeners in the northern United States? Over 100 years after it first hit the market, the iceberg is still America’s most popular lettuce variety!

Growing lettuce is a great way to dip one’s toes into vegetable gardening. Most of us don’t eat enough leafy greens, and iceberg lettuce is a neutral, easy-to-grow option that can bulk up any healthy meal.

In this article, I want to walk you through the iceberg lettuce growth stages and offer tips for a perfect harvest.

Conditions for Growing Iceberg Lettuce

Regardless of the variety, it’s a hard and fast rule that lettuce likes cool temps. In most climates, this leafy green is grown in the early spring, but some gardeners may also plant lettuce in the fall or even through winter.

Since growing lettuce is often a race against summer heat, there’s no time to waste when it comes to planting. Seeds are best started when your soil is workable in the spring. If you’re shooting for a specific soil temperature, 40°F is the minimum germination requirement.

Iceberg lettuce grows best in full sun, but there are some exceptions. For example, providing some afternoon shade may extend the growing season by several weeks, especially in warmer climates. Too little sun, however, results in leggy plants.

Aim to keep the soil consistently moist throughout the lettuce’s growing period. Dry soil can impact flavor and promote premature flower production. 

Most gardeners know that nitrogen equals vigorous top growth. Iceberg lettuce grows best in soil rich in organic matter. Feed with a balanced liquid or granular fertilizer every two weeks after planting.

I can’t stress enough how important average daily temperatures are when growing iceberg lettuce. The most common issues associated with heat (temperatures above 75 to 80°F) are bitter leaves and bolting. There are some cultivars that are bred for increased heat tolerance, so keep this in mind.

Iceberg Lettuce Growth Rate

On average, iceberg lettuce can grow about 1 inch per week. But this rate can vary greatly based on the weather and other growing conditions. It’s also important to remember that your lettuce is not just growing taller — it’s also hard at work getting wider and thicker.

Support vigorous growth by keeping your lettuce plants well-watered throughout the growing season. Applying a rich liquid fertilizer throughout the season will also ensure a rapid growth rate.

Growth Stages of Iceberg Lettuce

Your iceberg lettuce will take about 70 to 80 days to mature during the peak growing season. The lettuce life cycle can take up to 130 days in cooler temperatures. 

There are several unique cultivars beneath the iceberg lettuce umbrella. Some popular examples include ‘Sun Devil’, ‘Ithaca’, and ‘Mission’. These varieties all have similar life cycles and cultural requirements, so there’s no need to differentiate between them when discussing iceberg lettuce growth stages.

1. Seed Germination

Lettuce plants grow from small seeds. The seeds are slender and pointed (I think they look a lot like turf grass seeds). 

Some distributors offer pelleted lettuce seeds that are easier to sort and handle. Pelleted seeds are biologically identical to ‘normal’ seeds but have been covered with a dissolvable, man-made coating.

Lettuce seeds require very specific conditions to germinate and sprout. The two most important factors to consider are moisture and soil temperature.

Consistent moisture is necessary for seed germination. Seeds absorb water from the environment to trigger cell division and sprouting.

Lettuce seeds prefer soil temperatures around 75°F. However, germination is possible if the soil temperature stays between 40 and 80°F.

Lettuce seeds germinate in about 5 to 10 days if these requirements are met. Germination starts as the seed imbibes (absorbs) water and essentially ‘wakes up’. The primary root (radicle) emerges from the seed coat within hours or days. At this stage of your lettuce plant’s growth stage the first shoot and leaves emerge from the seed and push toward the soil’s surface.

2. Seedling Emergence

Again, you may see young lettuce sprouts appear just 5 to 10 days after planting. These sprouts emerge with two small leaves called cotyledons.

lettuce Seedling Emergence

Cotyledons are unlike all other leaves that the lettuce plant will produce from this point on. Cotyledons develop inside the seed embryo. They have limited (or no) photosynthesis abilities and instead serve as a very basic energy storage system. 

The cotyledons are vital to early seedling development because the plant cannot yet make its own energy via photosynthesis.

In ideal growing conditions, a lettuce seedling will develop its first ‘true’ leaves in less than ten days. These leaves emerge from between the cotyledons and resemble typical lettuce foliage. Additional true leaves will grow until the lettuce plant is harvested or naturally dies off.

 Seedling Emergence lettuce

3. Vegetative Growth

Now that the seedling has established and has a few sets of true leaves, it can put all its energy into vegetative growth. This growth stage is typically divided into a few different phases based on the size and shape of the lettuce plant and its leaves:

Rosette Stage

Lettuce leaves grow in a rosette shape. This means that all of the leaves grow up from a central point (like the petals of a rose) on the stem that is typically very close to the ground. 

This phase can last 25 to 50 days, depending on the average daily temperature and other environmental factors.

Note that the leaves may appear loose and lanky during this growth period. This is fairly normal early on in the lettuce plant’s life cycle. It should look more like a typical lettuce head in the following stages.

Cupping Stage

The so-called cupping stage is a brief period where the lettuce leaves start to curl inward but have not yet formed a head. In most lettuce varieties, this stage lasts about seven days.

Heading Stage

This is when your iceberg lettuce plant will start to look like… well, lettuce! The leaves will curly tightly inwards, forming a densely packed head while continuing to grow bigger.

Head formation normally takes 20 to 45 days. 

There’s little harm in harvesting the lettuce early in this stage — the head will be smaller than its full potential, but the texture and flavor will still be great. Waiting too long, however, can result in a low-quality harvest or premature flowering.

4. Flowering

Lettuce flowering is commonly known as bolting. This is because flower development is rarely desirable in lettuce. Early flowering, which can essentially ruin a crop of unharvested lettuce, is a frequent problem in home gardens and commercial farms.

High daytime temperatures and lengthening daylight hours are usually responsible for lettuce flowering. Because of this, it’s hard to say exactly how many days after planting lettuce is likely to flower.

Bolting is an issue because it marks the point where lettuce quits, focusing on leaf growth and redirecting its energy into reproduction. The plant saps energy from the leaves, resulting in bitter and tough lettuce.

lettuce Flowering

Lettuce flowers form on tall, thick stalks that emerge above the foliage. The flowers themselves consist of clusters of small, bright yellow florets. Each cluster is called a capitulum.

5. Pollination

Lettuce plants are generally self-fertile. This means that a single flower can pollinate itself to create viable seeds. 

Pollen is normally spread (either within the same flower or between two different flowers) via flying insects or the wind. 

6. Seed Development

Each successfully pollinated floret is capable of producing one lettuce seed. The seeds mature an average of 12 to 21 days after flowering occurs.

According to Virginia Tech, fresh lettuce seeds are attached to a pappus structure. The pappus is similar to the tuft famously seen on dandelion seeds and helps with wind dispersal of the seeds. This structure is not part of the seed itself and is unnecessary for germination later.

When to Harvest Iceberg Lettuce

Pick your iceberg lettuce when the heads are large, crisp, and tightly packed. Watch for the outermost leaves to start fading to a pale green. This is a good sign that the rest of the head is ready to harvest. (Just pick the lettuce before those outer leaves turn brown!)

I highly recommend watching the weather forecast when deciding when to harvest your lettuce. If your lettuce is close to ready and there’s a heatwave in the forecast, it’s best to get outside and harvest early.

For more articles about plant life cycles, here is a link to Radish Growth Stages.

FAQ Iceberg Lettuce Plant Stages

What causes misshapen lettuce heads?

There are many potential causes of misshapen lettuce heads in the garden. Disease and pest damage are common culprits that can usually be diagnosed by carefully inspecting affected plants. Malnutrition or drought stress can also cause similar issues.

Citations

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Ben's horticultural interest grew when graduating from Hertfordshire University in 1997. Having contributed to numerous publications including Better Homes & Gardens, Garden Design Magazine, and The English Garden. He is also the author of Propagating Houseplants Made Easy.