SL varieties

The SL varieties—most notably SL-28 and SL-34—originated from the Scott Agricultural Laboratories, a research facility established by the British colonial administration in Kenya in 1903. During the 1930s, the laboratory was tasked with identifying and breeding coffee plants that could thrive in the Kenyan highlands while offering superior commercial viability. The 'SL' designation simply refers to the institution where these selections were cataloged and developed between 1935 and 1939.

SL-28 is widely considered a Bourbon-type cultivar. While its exact parentage remains a subject of debate, it is frequently cited as an Ethiopian selection that was brought to Kenya via Tanzania due to its notable drought tolerance. In the field, it is a tall plant that was specifically selected for its ability to withstand dry conditions and maintain high productivity, making it a foundational variety for the Kenyan coffee sector.

SL-34, the other prominent member of the series, is also a tall cultivar selected from Bourbon stock. Its genetic origins are similarly ambiguous; some researchers suggest it is a simple Ethiopian selection, while others believe it to be a mutation of Bourbon. Compared to SL-28, SL-34 is often described as being heartier and possessing greater resilience against certain pests and diseases, though it remains demanding in terms of cultivation requirements.

For the specialty coffee drinker, these varieties are prized for their distinct sensory profiles. SL-28 is particularly famous for its intense, bright acidity—often described as citrusy or reminiscent of black currants—paired with a silky mouthfeel and complex fruit notes ranging from mango to red berries. This clarity and expressive character have made SL-28 a benchmark for high-quality Kenyan coffee, frequently sought after by roasters and consumers alike.

Producers choose to plant SL varieties primarily because they balance the difficult trade-off between agricultural performance and cup quality. While modern breeding programs have introduced more disease-resistant hybrids, the SL varieties remain the standard for quality in Kenya. Their ability to produce a high-scoring, complex cup in the specific climate of the Kenyan highlands ensures their continued dominance in the region's coffee production.

Despite their success, these varieties are not without challenges. They are generally susceptible to common coffee diseases, such as coffee berry disease and leaf rust, which has led to the development of newer, more resistant varieties like Ruiru 11 and Batian. However, the legacy of the SL series remains secure, as they continue to define the 'Kenyan profile' that is celebrated globally in specialty coffee competitions and high-end retail markets.

Today, the SL varieties are documented in the World Coffee Research (WCR) Coffee Varieties Catalog, which serves as a practical guide for producers worldwide. This inclusion highlights their status as essential genetic resources. As the industry continues to evolve, the story of the SL varieties stands as a testament to the impact of early 20th-century agricultural research on the modern specialty coffee landscape.