Whether you’re interested in growing a mixed blend of baby lettuces, a few heads of romaine, or a tender bibb lettuce head, I find it’s helpful to know the different lettuce growing stages.
Understanding how lettuce goes from seed to a full lettuce head can help you determine whether or not your lettuce is on the right track. It also makes it a bit easier to look out for common problems and harvest at the proper time.
Over the years, I’ve grown thousands of lettuce plants from seed to maturity. Here’s what I learned about the various lettuce growth stages.
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Like with many vegetable plants, all types of lettuce start as a seed.
Lettuce seeds have a lanceolate shape and are about an eighth of an inch long. Depending on the variety, the seeds may be white, black, tan, brown, or gray.
Since these seeds are so small, it can be difficult to separate individual seeds to plants. After you plant thousands of seeds, the process becomes a bit easier.
However, no amount of experience can prevent the seeds from sticking together or increase their size.
If you want to make planting individual seeds a bit easier, you can look for pelleted lettuce seeds. These seeds are covered in a dissolvable coating that makes separating and handling them easier.
When you’re handling and storing lettuce seeds, it’s important to keep the environment in mind. Storing lettuce seeds in the wrong environment can render them unable to germinate.
To keep seeds viable, store them in a cool, dry place. This applies to both raw and pelleted lettuce seeds.
Germination: Emergence of Cotyledons
The next step in lettuce growth is germination. This is when the emerging seedling “wakes up” and begins to grow.
In my experience, getting lettuce seeds to germinate is one of the trickiest parts of growing lettuce. That’s because you must provide and maintain certain environmental conditions in order for germination to occur.
Lettuce seeds require moisture and proper temperature to germinate. They can germinate at temperatures as low as 40ºF, but their optimal germination temperature is 75ºF.
I can confirm that germination of seeds will become spotty when temperatures rise above 80ºF. If I’m planting lettuce seeds in the summer, I start the seeds indoors to provide an appropriately cool temperature.
Under the proper conditions, lettuce seeds will germinate in five to ten days, with lower temperatures leading to slower germination.
Now that I’ve covered the conditions that lettuce seeds need to germinate, let’s cover a bit more about what germination involves.
The first step of germination is the rapid uptake of water. This allows the outer coating of the seed to soften.
After the seed coat has softened, the seed begins to complete processes like respiration and metabolization. In essence, they prepare to grow.
At this point, cell division (aka growth) begins to occur! The first root (the radicle) emerges followed by the first above-ground tissue (the epicotyl). When this happens, you’ll see a little bit of red or green break through the soil surface.
As germination continues, two small leaves called cotyledons appear.
The appearance of cotyledons varies depending on the plant family. Therefore, if I forgot what type of seed I planted, I like to use cotyledons to help determine what type of plant is germinating.
Lettuce cotyledons are smooth and oval-shaped.
First True Leaves and Seedling Stage
As the lettuce seedling continues to grow, it will produce its first true leaves. While the cotyledons will stop growing and fall off as the lettuce matures, these true leaves will keep growing.
If you provide lettuce with the proper light, temperature, and moisture, the seedlings will continue to grow. The plant will produce more leaves and already present leaves will become larger.
During this stage, it’s essential that you maintain proper soil moisture. Letting the soil dry out for even a few hours can result in plant death, but keeping the soil wet can also lead to disease and death. Therefore, you should aim for slightly moist soil.
One thing to keep in mind about the seedling stage is that it’s the best time to thin lettuce seedlings. If you direct seeded lettuce outdoors and the plants are too close together, you can pull out seedlings by hand to ensure proper lettuce spacing.
The seedling stage is also typically when you harvest lettuce if you want a baby lettuce mix. During this point, the leaves are tender and also bite sized.
As lettuce plants continue to grow and produce new leaves, their leaves will develop a distinct circular shape. This is known as the rosette stage.
This stage will look different depending on the variety of lettuce you are growing. For example, romaine lettuce will have long, upright leaves while butter lettuce will have lower and more ruffled leaves.
Regardless of the type of lettuce, the rosette stage will last anywhere from 25 to 50 days.
During this time, it’s essential that you keep an eye out for pests as well as the development of disease. If you see pests like aphids, slugs, or armyworms, treat them ASAP to allow your lettuce to keep growing.
When lettuce leaves begin to curl inward, the plant is entering the cupping stage. This is a short period that occurs just before the lettuce plant begins to form a dense head.
This stage lasts about a week.
If you’re aiming for a nice dense head of romaine or a tender bibb head, the heading stage is the moment you’ve been waiting for! At this point, the outermost lettuce leaves begin to curl in and cover the plant’s small center leaves.
Depending on the variety and time of year, the heading stage can last anywhere from 20 to 45 days. During this time, it’s crucial that you check your plant regularly.
While harvesting your plant too early will only result in a smaller harvest, waiting too long can be detrimental. If plants overmature or begin to bolt, they can become tough and/or bitter.
One way I like to check to see if a lettuce plant is ready to harvest is by feel. As plants mature, the heads will become more dense — this is true for iceberg, bibb, buttercrunch, romaine, and more.
If you squeeze a lettuce plant and it easily collapses into your hand, it’s not yet formed a head. However, if the plant’s leaves hold up to a gentle squeeze, it’s ready to harvest.
If you’re growing lettuce to eat, you want to avoid the bolting stage. Once lettuce begins to bolt, the greens become bitter and/or tough.
With that said, this is a natural and important part of the lettuce plant’s life cycle.
So, what exactly is bolting? Bolting refers to the point when lettuce transitions from vegetative to reproductive growth.
You will know a plant is bolting when you see a rigid stalk emerge from the center of the plant. As the plant continues to bolt, it will transition from a short and squat plant into a towering form.
Eventually, flowers will form on the top of the plant’s stalk. If these flowers are properly pollinated, they will produce lettuce seeds within a few weeks.
If you want to save lettuce seeds to plant later, now is the time to do so! Simply wait until a dry period and then pick the seed heads off the plant.