Why lettuce gets bitter
In many parts of the gardening world, it is easy to grow all the components of a delicious salad all at once. In the Mojave, with its infertile salty soil, minimal humidity and wide range of temperatures, there are challenges. Nevertheless, when one succeeds in overcoming them, the produce is wonderful — as delicious as that grown anywhere.
Leafy greens come under the heading of “cool season” plants. They thrive when temperatures are no higher than about 75°F or lower than 40°F; in other words, late fall to early spring. As long as they receive protection from the most severe winter conditions, they can survive with little difficulty.
Last Halloween, I planted my cool season garden of delicious leafy annuals: five varieties of lettuce, three of spinach, and two each of chard, beets and kale. This provided such a bounty that I was able to harvest a fresh salad virtually every day starting around the end of December. It is a lovely thing to see a variety of leaf shapes and colors in the salad bowl. Lettuce leaves may be round, or oak-leaved, green or purple. Sadly, nothing lasts forever, and this period of lush harvests continued only until the end of March. The spinach, lettuce and chard have now started to bloom, which means my salad days have passed.
Once they have reached the end of their growing life, or when temperatures move outside of their comfort zone, many types of lettuce have the tendency to bolt — produce a flower stalk. At the same time, they may develop eye-crossing bitterness due to its production of certain chemicals, more commonly in Bibb and leaf varieties. The plants contain some of these bad tasting compounds at all times, but once they start to bolt, the production increases. This does not happen with most other cool season leafy vegetables, but one can usually expect it with lettuce. Some varieties will also change shape, from a spherical one to something more conical. Once the shape change is noticeable, the leaves have become inedible.
It costs the plant valuable resources to produce these compounds. In small amounts, they are a way for the plant to protect itself against pests. Students will occasionally ask me why a plant would spend so many of its resources to create large amounts of these terrible tasting chemicals. Here is a case of the plant protecting itself from herbivores, including us.
When annuals reach the end of their growing life, they must enter reproductive mode. One definition of annuals is that they are monocarpic, flowering and producing seeds once before dying. Therefore, they flower and ultimately develop seeds to create a new generation of the species. If an herbivore were to eat the plant’s leaves, flowers and seeds, then the species could not survive.
Turning leaves bitter is a clever means of deterring anyone from eating them. Visibly changing the shape gives a warning to potential pests to keep away. This way, seeds can form to create a new generation of delicious lettuce next cool season.
Email or call Angela O’Callaghan, Social Horticulture Specialist for Cooperative Extension, at 702-257-5581.