SL* — currently believed to contain SL34 and SL9

The 'SL' prefix stands for Scott Agricultural Laboratories, a research station established in 1903 under British colonial rule in Kenya. In the 1930s, the Kenyan government commissioned the laboratory to survey and catalog coffee varieties that could offer superior drought resistance, high productivity, and exceptional cup quality. Researchers collected diverse genetic material from across the coffee-growing world, including French Mission Bourbon, Mokka, and Kents, and assigned numerical identifiers to the most promising single-tree selections identified during their trials.

SL28 and SL34 emerged from this rigorous selection process as the most successful outcomes, eventually defining the profile of Kenyan coffee for nearly a century. These varieties are celebrated for their vibrant, 'electric' acidity and complex flavor notes, often described as blackcurrant and tomato leaf. Because of their success, they became the standard for commercial production in Kenya and were eventually exported to other regions, including Hawaii and parts of Latin America, where they are prized for their ability to express unique terroir.

In contrast, other selections like SL9 represent the experimental nature of the Scott Labs project. Historical records, including a 1956 bulletin from the Coffee Board of Kenya, indicate that SL9 was a single-tree selection derived from a block of unknown origin. Unlike its more successful counterparts, SL9 was characterized as a 'shy bearer'—meaning it produced low yields—and was found to be highly susceptible to coffee berry disease. Consequently, the variety was largely dropped from formal cultivation and breeding programs, leaving it as a rare, historical footnote.

In the field, SL34 is recognized as a tall plant with dark bronze-tipped leaves and a semi-erect growth habit that tends to droop on older branches. It is specifically adapted to high-altitude environments with consistent rainfall. While it remains a favorite among roasters for its cup quality, its high susceptibility to diseases like coffee berry disease remains a significant challenge for producers, requiring careful management and specific environmental conditions to thrive.

Today, the term 'SL' is sometimes used colloquially or in marketing to describe rare or heirloom-like selections, such as the 'Gesha Inca' (SL9) found in Peru. However, it is important to distinguish between the widely established, commercially viable varieties like SL28 and SL34 and the obscure, often poorly documented selections that were discarded during the original 1930s trials. Genetic verification for these rarer 'SL' tags is often difficult, as many do not exist in modern, standardized genetic databases.

For the producer, the choice to plant SL varieties is a balance between the potential for high-quality, premium-priced coffee and the reality of agricultural risk. Because these varieties are often susceptible to common coffee diseases, they require intensive care compared to modern, disease-resistant hybrids. Nevertheless, the enduring reputation of the 'SL' name ensures that they remain highly sought after by specialty roasters and consumers who value the distinct, bright, and complex profiles that these historic Kenyan selections consistently deliver.