A lettuce plant has a short stem initially (a rosette growth habit), but when it blooms, the stem lengthens and branches, and it produces many flower heads that look like those of dandelions, but smaller. This is called bolting. When grown to eat, lettuce is harvested before it bolts.
Commonly recognized types of lettuce include:
- Iceberg lettuces form tight, dense heads that resemble cabbage. They are generally the mildest of the lettuces, valued more for their crunchy texture than for flavor. Varieties of iceberg lettuce are the most familiar lettuces in the USA.
- Crisphead lettuces form moderately dense heads with a crunchy texture; this type is intermediate between iceberg and looseleaf types.
- Romaine, also called cos is a head-forming type with elongated leaves.
- Butterhead, also called Boston or bibb forms loose heads; it has a buttery texture.
- Batavia is similar to butterhead
There are hundreds of varieties of lettuce within these categories.
Some lettuces (especially iceberg) have been specifically bred to remove the bitterness from their leaves. These lettuces are have a high water content with very little nutrient value. The more bitter lettuces and the ones with pigmented leaves contain antioxidants.