SL34, SL28

The SL28 and SL34 varieties were born from a deliberate, state-sponsored effort in the 1930s by the Scott Agricultural Laboratories (SL) in Kenya. Tasked with improving the economic viability of the region's coffee sector, researchers sought to identify and propagate plants that could thrive in the unique conditions of the Kenyan highlands while resisting the frequent droughts that plagued local crops. The resulting selections, released in the 1930s, were so successful that they effectively established the modern identity of Kenyan coffee, moving the industry away from reliance on chance and toward scientific selection.

SL28 was selected in 1935 from a drought-tolerant population originally sourced from Tanganyika. While its exact genetic lineage has been the subject of historical debate, it is widely recognized for its Bourbon-like characteristics and deep root system, which allows it to withstand periods of water scarcity. SL34, meanwhile, was selected for its robustness and productivity. While early literature often grouped both as Bourbon-related, recent genetic analysis has indicated that SL34, along with other SL varieties, may share a closer lineage with Typica-related strains.

In the field, both varieties are tall, vigorous plants that thrive at high altitudes, typically between 1,400 and 2,100 meters. They are visually distinct, often displaying broad leaves with characteristic bronze tips. While they were bred for environmental resilience, they are not immune to modern agricultural threats. Both varieties are notably susceptible to major coffee diseases, including Coffee Berry Disease (CBD) and coffee leaf rust. This vulnerability has led many contemporary farmers to interplant them with more resistant, modern hybrids like Ruiru 11 or Batian to ensure long-term crop security.

For the roaster and the consumer, these varieties are prized for their extraordinary cup quality. SL28 is perhaps the most famous, celebrated for its intense, bright acidity and complex flavor profile, which frequently includes notes of blackcurrant, grapefruit, and a distinct tomato-vine quality. SL34 is often noted for providing similar high-quality acidity but with added body and depth. Together, they produce the wine-like, juicy, and highly expressive cup that has become the hallmark of the Kenyan auction system.

Producers continue to cultivate these varieties despite their disease susceptibility because the market demand for their specific flavor profile remains unmatched. The economic incentive is clear: high-quality lots of SL28 and SL34 consistently command premium prices at auction. Because the trees are long-lived and can remain productive for decades, many smallholder farmers continue to maintain original plantings, viewing them as a vital, albeit high-maintenance, asset to their farm's portfolio.

Today, the legacy of Scott Labs is firmly cemented in the global specialty coffee lexicon. While the industry has moved toward breeding programs that prioritize disease resistance, the SL varieties remain the benchmark against which all other Kenyan coffees are measured. They represent a pivotal moment in coffee history where scientific research successfully aligned with the terroir of a specific region to create a product of enduring, world-class quality.