Lettuce is botanically classified as Lactuca sativa, with Lactuca coming from the Latin word for milk or milkiness (lacteus). Lettuces can become bitter as the weather gets warmer and as the plants begin to bolt. You can reduce bitterness by cutting your lettuce in the field and then dunking it in a tub of cool water. That's what I do, right out in the field and I will let it sit in the water for a few minutes before taking it out and shaking the water off of it. Be sure to shade your tub of cool water. This reduces the field heat and keeps the lettuce from wilting. When I get the lettuce home, I wash it in cool water and then spin dry it (salad spinner) and put it in the fridge. If you will leave lettuce like this in the fridge for a couple of days, a sweeter taste will return.

There are types of lettuce that are better for summertime and warmer temperatures such as Jericho (an Israeli cultivar), Summertime and the Oakleaves (red oak leaf, green oakleaf). These lettuces are more tolerant of hot weather and are slow to bolt in the summer heat. You can also shade your lettuce or plant where the lettuce will get afternoon shade to reduce the risk of bolting and bitterness.