Anytime lettuce growth is abnormally slowed, leaves can become tough and bitter. This can be caused by planting late, when hot weather slows growth and changes chemical content. The same change in quality will result if plants are crowded or if soil is low in nutrients, organic matter or both. Soil with low moisture content for even a short time can cause bitterness.
Mixing organic matter into soil and mulching will help remedy this. Lettuce not only tolerates cool temperatures but produces its most tender and flavorful leaves in early spring, as soon as hard-frost danger is over, or in fall before heavy frost.
A feeding of a balanced plant food as soon as leaves emerge helps maintain flavor and tenderness. Some varieties tolerate heat more than others, and most seed catalogues indicate which ones these are. Buttercrunch is a variety that has tremendous heat resistance. -