PlantLife Online

Answer

There are a few reasons home grown lettuce may turn bitter, the most common is that the plants have started to set seed due to the warm summer heat. Because lettuce is a cool season vegetable warm temperatures will speed the lettuce into maturity causing seeds to set and sugars to be redirected to the growth of sprouts.

Lettuce may also turn bitter if the plants are not getting enough water, lettuce plants have big broad leaves that often require more water than other garden vegetables. When the plants reach wilting point or start to get brown tips it is often a sign the crop may taste bitter.

Lack of nutrients may also cause the lettuce to taste bitter. Lettuce is a fast growing plant and requires a steady supply of nutrients, I like to use an organic blend that can be applied every couple of weeks and wont build up in your garden's soil.