Blackcurrant and dark berry, citrus and red fruit, a wine-like acidity and a long, juicy finish. That's SL28 at its best, and it began with a single coffee tree.
In 1935, researchers at the Scott Agricultural Laboratories in Kenya selected one tree from a population known as Tanganyika Drought Resistant. It went on to become one of the most important varieties in Kenyan coffee, known for pronounced acidity, fruit character and high-altitude quality.
More than ninety years later, SL28 is still widely grown in Kenya and has travelled well beyond East Africa. It produces relatively little coffee and is susceptible to the major diseases of coffee, but its cup quality has kept producers and buyers interested for generations.
Where did SL28 come from?
The story begins with a collecting trip. In 1931, A.D. Trench, the senior coffee officer at Scott Agricultural Laboratories, travelled through Tanganyika, now Tanzania. In the Monduli district he came across coffee that appeared to tolerate drought, disease and pests better than other plants he had seen. Seed was collected and taken back to the laboratory in Kenya.
The population became known as Tanganyika Drought Resistant. A few years later, Scott Laboratories began selecting individual trees from different coffee populations. Between 1935 and 1939, 42 trees were selected and evaluated for characteristics including yield, cup quality, drought tolerance and disease resistance.
Each selection was given an “SL” number. SL28 was selected in 1935 from a single tree within the Tanganyika Drought Resistant population. World Coffee Research says it was considered the prize selection of this period of intensive breeding.
The Scott Agricultural Laboratories
The place where SL28 was selected has its own history. The buildings at Kabete opened in 1913 as a sanatorium, named after Dr Henry Scott of the Church of Scotland Mission in Kikuyu, and were used as a military hospital during the First World War.
In 1922, after the closure of the Kabete Experimental Farm, the property was set aside for the Agricultural Department and the buildings were converted into laboratories. After Kenya's independence, the laboratories' functions were absorbed into the Coffee Research Institute and the National Agricultural Research Laboratories.
What the genetics say
World Coffee Research classifies SL28 as Bourbon related, within the Bourbon-Typica genetic group. Its documented lineage is a selection of Tanganyika Drought Resistant, with a Bourbon-like genetic background. SL28's history is sometimes mixed up with the breeding work that came later, so it's worth being clear about where each variety fits.
Rume Sudan, for example, became an important source of disease-resistance genetics in later Kenyan breeding programmes and appears in the ancestry of Ruiru 11 and Batian. It's not part of SL28's documented lineage. SL28 is a selection from existing material, picked because that tree had characteristics worth keeping, not the product of a modern hybridisation programme.
What does SL28 look like?
SL28 is a tall coffee plant with large leaves and long primary branches. Young leaf tips are mainly green, although bronze types can also occur. After several cropping seasons the branches tend to become more horizontal or drooping, which gives mature trees their spreading shape.
The cherries ripen to deep red and produce large, elongated beans. World Coffee Research rates the bean size as large and the cherry-to-green-bean outturn as high.
Growing SL28
SL28 is well suited to medium and high elevations. Within five degrees of the equator, World Coffee Research gives an optimal altitude of above 1,200 metres, with lower minimum elevations at higher latitudes.
The variety is also drought tolerant, which is why its original Tanganyika Drought Resistant population attracted attention in the first place. Its nutritional requirements are low, and the trees are persistent. WCR reports SL28 trees in many parts of Kenya that are 60 to 80 years old and still productive, and describes this as rusticity: the ability to survive long periods of neglect and return to production when conditions improve.
Yield is the trade-off. SL28 has low yield potential compared with Caturra, Bourbon and K7, which WCR uses as its medium-yield references.
The disease problem
SL28's biggest weakness is disease. It's susceptible to coffee leaf rust, Coffee Berry Disease and nematodes, which makes it considerably more demanding than newer Kenyan varieties such as Ruiru 11 and Batian.
Those varieties came out of the breeding work that followed the severe Coffee Berry Disease outbreaks of the twentieth century. Ruiru 11 was released in 1985 and Batian in 2010. Ruiru 11 is an F1 hybrid whose complex male parent includes K7, SL28 and Rume Sudan among other varieties. Batian is a composite with an even broader background, including SL28, SL34, Rume Sudan, N39, K7, SL4 and Timor Hybrid.
Farmers still grow SL28 alongside them, and the cup quality of the older selection is one of the main reasons.
What does SL28 taste like?
SL28 is famous for acidity. The best examples can have intense blackcurrant and dark-berry character, with citrus, red fruit and a wine-like quality underneath. The body can be full and juicy, and the finish long.
Those are tendencies rather than a fixed recipe. The cup depends on where the coffee is grown, how ripe the cherries were when picked, how they were processed and how the coffee was roasted. Most Kenyan coffee is fully washed, which tends to emphasise clarity and acidity, so a carefully picked and well-processed SL28 lot can have very vivid fruit.
The variety also responds to its environment. Coffees from different Kenyan counties can taste quite different even when the variety and broad processing method are the same. There are general regional patterns, but they aren't rules.
Why Kenyan coffee often isn't a single variety
Smallholder farmers in Kenya typically deliver cherry to a cooperative washing station. The cherries are processed together, and the farmers supplying the station may be growing SL28, SL34, Ruiru 11 and Batian on the same plots.
After processing, coffee is graded using categories such as AA, AB and PB. These grades refer primarily to bean size and shape, not variety. A Kenyan AA coffee isn't necessarily an SL28 coffee, and a lot labelled with several varieties isn't automatically lower quality. It reflects how coffee is produced and processed at washing-station level. Single-variety Kenyan lots do exist, but they need enough separation at farm and factory level to make them practical.
SL28 beyond Kenya
World Coffee Research describes SL28 as important in Uganda, and it has spread to Malawi, Zimbabwe and Latin America.
One of the best-known examples outside Africa came in 2017, when British barista Dale Harris won the World Barista Championship in Seoul using a washed SL28 from Ernesto Menéndez's Finca Las Brumas in El Salvador. The farm sits between three volcanoes, and its Cup of Excellence lots have come from around 1,750 to 1,850 metres. Finca Las Brumas returned to the Cup of Excellence in 2024 and took first place in the Experimental Coffee category.
Why SL28 is still being planted
On paper, SL28 has plenty working against it. It's low yielding and susceptible to the major diseases of coffee, and some modern varieties can produce more coffee with much stronger disease resistance.
What it has in return is cup quality. World Coffee Research rates its quality potential at high altitude very highly. For producers with the right environment and a buyer willing to pay for quality, that can be enough to keep it in the ground.
Quick varietal facts
Varietal: SL28
Species: Coffea arabica
Type: Individual tree selection
Genetic group: Bourbon-Typica group, Bourbon related
Lineage: Selection of Tanganyika Drought Resistant; Bourbon-like genetic background
Origin: Kenya
Selection: 1935, Scott Agricultural Laboratories, Kabete
Source population: Tanganyika Drought Resistant, collected in Monduli, Tanganyika, in 1931
Growth habit: Tall with large leaves and branches that become drooping after successive cropping seasons
Leaf tip colour: Mainly green, occasionally bronze
Cherry colour: Red when ripe
Bean size: Large
Cherry-to-green-bean outturn: High
Yield: Low
Nutrition requirement: Low
Drought tolerance: Strong
Disease resistance: Susceptible to coffee leaf rust, Coffee Berry Disease and nematodes
Quality potential: Very high at high altitude
Typical altitude: Above 1,200m within five degrees of the equator
Notable characteristic: Rustic; mature trees can remain productive for decades
Key growing regions: Kenya, Uganda, Malawi and Zimbabwe, with plantings also reported across Latin America
Typical cup profile: Bright acidity, blackcurrant and dark berry, citrus, red fruit, juicy body and a long finish
Further reading
- World Coffee Research: SL28 Variety Profile
- World Coffee Research: Ruiru 11
- World Coffee Research: Batian
- Alliance for Coffee Excellence: Finca Las Brumas
Want to explore the varieties around SL28? Read our guides to SL34, Bourbon and Sudan Rume.
