Cultivated lettuce, Lactuca sativa, is closely related to several species of wild lettuce including Lactuca serriola (prickly lettuce), and Lactuca virosa (bitter lettuce). All of these species produce a milky latex in specialized cells called lactifers. The latex contains several bitter compounds called sesquiterpene lactones, including lactucin and lactucopicrin. Interestingly, the same compounds are responsible for the bitterness in chicory and radicchio (Cichorium intybus), also both closely related to lettuce. In these crops, a slight bitterness is expected.
During domestication and breeding, humans selected lettuces for reduced latex production and for less bitterness. In general, consumers prefer mild and sweet lettuces. Most growers have had the experience where lettuce has become bitter; often unpredictably, which may result in a bad customer experience.
So, what makes lettuce bitter? It turns out that the answer is not simple; several factors may be involved in bitterness.
Type and variety. Several researchers have shown that some types and some varieties of lettuce are more prone to becoming bitter than others. In general, crisphead (or iceberg) and romaine varieties are less likely to become bitter than butterhead or red and green leaf lettuces. Within the types, varieties differ in their tendency to become bitter. For example, in a 2002 study, Amy Simonne and her colleagues in Florida found that the varieties ‘Nancy’, ‘Big Curly’ and ‘Slobolt’ had much higher bitterness ratings than several other varieties (HortTechnology 12(4):721-726).
Bolting. Once a lettuce plant bolts, or transitions from vegetative to reproductive growth and starts to flower, it becomes more bitter. Therefore, the conditions that trigger bolting are likely to indirectly increase bitterness. In 2009, Korean researchers showed that the concentrations of bitter compounds were much higher in leaves on flower stalks than in leaves at the base of the plant. These changes in bitterness may happen before bolting is easily visible, especially for leaf lettuces. It can also happen when the plants are still very small. For example, when lettuces are planted very close together for salad mix production and are exposed to stresses, they can bolt when they are still very tiny.
Environment. Bolting is triggered by increased temperatures in the air as well as in the root zone. It can also be promoted by other factors, for example, exposure of the mother plant to cool temperatures during seed development can make the plants that grow from those seeds more susceptible to bolting (Scientia Horticulturae 38(304):223-229).
So, practically, what can be done to reduce this problem?
Stagger plantings. This falls under the ‘don’t put all your eggs in one basket’ rule. Once a lettuce plant bolts OR becomes bitter, it’s not reversible; e.g. it won’t get sweet again. Because environment plays such a big role in lettuce development and a hot spell can trigger bolting very quickly, the best insurance is to do a lot of sequence plantings, perhaps as often as one week apart. That way, even if one, or even two, plantings bolt or become bitter prematurely, you have another planting on the way.
Keep lettuce cool. In our warm summers, this can be very tricky. Many growers use white-on-black plastic mulch for summer lettuce production, to try to keep temperatures down. Research has shown that the difference between the white-on-black vs. black plastic or bare ground is slight, just a few degrees, but this can be enough to have a physiological effect on the plants and slow down plant development and the process of bolting. For those growing in protected environments like tunnels, maintaining very good ventilation is important, and it may be worth using shade cloth to reduce temperatures.
Choose varieties that are resistant to bolting and becoming bitter. This can be tricky. For example, in the Simonne study, “Slobolt” was one of the quickest to bolt. Unfortunately, I am not aware of comprehensive lists of the varieties least likely to become bitter. However, seed suppliers often indicate which varieties have some tolerance to heat and/or bolting. Also, if you have a planting that gets over-mature, you can take advantage of this and taste the different varieties – and then make note of which performed better or worse than others.
Harvest promptly. Even if they have not yet visibly bolted, over mature plants are likely to be bitter. It may be worth sacrificing an older planting and moving on to harvest a younger planting if there is any question.
Of course, there is one final strategy, if all else fails – use copious amounts of salad dressing, which is a good mask for bitterness!