Bitterness

This is a characteristic sign of imminent bolting, but it may also be caused by water stress or unusually warm weather. Once it happens there isn't much you can do about it.

Vernalization

Lettuce will bolt if the plants are vernalized while small. This happens if a plant with a stem diameter greater than ¼˝ is exposed to temperatures below 50˚ F for two weeks. When the weather warms up it senses that winter has passed and so goes into flowering mode.

Snails and Slugs

These molluscs love the tender young leaves and are the commonest problem you will face when growing Lettuce.

Mammals

Deer, Rabbits and Groundhogs can quickly devastate even a mature Lettuce patch. A fence may be necessary if you have these problems.

Cutworms

These can be a real problem for young seedlings in spring. Some gardeners use individual Cutworm collars of cardboard. If you find plants laying on the ground, dig in the soil around them and you can usually locate the culprit. If you find it you can prevent it doing further damage.

Bolting

Lettuce flowers when the day length gets up to 14 or 16 hours (the exact day length depends upon the variety), even if the weather is cool. Warm weather (above 75 to 80˚ F) frequently accompanies the long days of midsummer and may hasten bolting, but it isn’t the primary cause. Bolting will also occur when a plant reaches full size and has all the resources it needs to flower. When the plant has enough large leaves they signal the plant that it is ready to flower, so it may be possible to slow it down by the frequent picking of single leaves (this won’t stop it though).

When a plant starts to bolt, it turns bitter, the head elongates and the new leaves begin to take on an elongated shape. Then the flower stalk appears. Lettuce plants are quite beautiful at this stage and if left alone will soon produce an abundance of seed. If you allow them to produce seed, you'll have plenty available for growing cut and come again salad or Microgreens.

Tip Burn

Burnt looking leaf tips may indicate a shortage of calcium, or night temperatures over 65˚ F.