SL28 started 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. That tree went on to become one of the most important varieties in Kenyan coffee, known for its pronounced acidity, fruit character and exceptional quality at high altitude.
Ninety years later, SL28 is still widely grown in Kenya and has also travelled well beyond East Africa. It produces relatively little coffee and is susceptible to the major diseases of coffee, but its quality potential 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 Moduli 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, where the drought tolerance was confirmed.
The population became known as Tanganyika Drought Resistant and was distributed to farmers.
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 describes it as the standout selection from this period of work. The original Tanganyika Drought Resistant material was eventually superseded by its progeny, including SL28.
The Scott Agricultural Laboratories
The place where SL28 was selected has its own history.
The buildings at Kabete were constructed in 1913 as a sanatorium and were later used as a military hospital during the First World War. They were named after Dr Henry Scott, a Church of Scotland missionary.
The colonial government established the Scott Agricultural Laboratories in 1922, and the Coffee Section moved there in 1934. Twenty-four acres were allocated to coffee research, giving the researchers space to grow and compare the large number of selections arriving from different parts of the coffee-growing world.
The Coffee Section later moved to the Coffee Research Station at Ruiru in 1949. The former Scott Agricultural Laboratories site is now part of Kenya's National Agricultural Research Laboratories.
What the genetics say
SL28 is classified by World Coffee Research 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.
That is worth stating clearly because SL28's history is sometimes mixed up with the breeding work that came later.
Rume Sudan, for example, became an important source of disease-resistance genetics in later Kenyan breeding programmes. It appears in the ancestry of varieties including Ruiru 11 and Batian. It isn't part of SL28's own lineage.
SL28 therefore isn't the product of a carefully designed modern hybridisation programme. It was a selection from existing material, identified because that particular tree had characteristics worth keeping.
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.
The branches tend to become more horizontal or drooping after several cropping seasons, giving mature SL28 trees their characteristic spreading shape.
The cherries ripen to deep red and produce relatively large, elongated beans. WCR uses SL28 as its reference for large bean size in Arabica, while also rating its cherry-to-green-bean outturn as high.
Growing SL28
SL28 is particularly well suited to medium and high elevations. Within five degrees of the equator, World Coffee Research recommends growing it above 1,200 metres, with lower minimum elevations at higher latitudes.
The variety is also notably drought tolerant. That characteristic was one of the reasons its original Tanganyika Drought Resistant population attracted attention in the first place.
Its nutritional requirements are relatively low, and the trees can be remarkably persistent. WCR reports SL28 trees in Kenya that are between 60 and 80 years old and still producing coffee. It describes this as rusticity: the ability to survive long periods of neglect and return to production when conditions improve.
Yield is a different matter. SL28 has low yield potential compared with varieties such as Caturra, Bourbon and K7, which WCR uses as its medium-yield references.
The disease problem
SL28's biggest weakness is its susceptibility to disease.
It is susceptible to coffee leaf rust, Coffee Berry Disease and nematodes. That makes it considerably more demanding than newer Kenyan varieties such as Ruiru 11 and Batian, which were developed partly to address those problems.
The breeding work that followed the severe Coffee Berry Disease outbreaks of the twentieth century produced varieties with much stronger disease resistance. Ruiru 11 was released in 1985, while Batian followed in 2010.
Ruiru 11 is an F1 hybrid whose male parent includes K7, SL28 and Rume Sudan among other varieties. Batian has an even broader composite background, including SL28, SL34, Rume Sudan, N39, K7, SL4 and Timor Hybrid.
Neither variety has made SL28 irrelevant. The cup quality of the older selection remains one of the reasons farmers continue to grow it.
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 sitting underneath. The body can be full and juicy, and the finish can be remarkably persistent.
Those are useful tendencies rather than a fixed recipe. The final 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 specialty coffee is fully washed, and this style of processing tends to emphasise clarity and acidity. When an SL28 lot is picked carefully and processed well, its fruit character can be exceptionally vivid.
The variety also responds to its environment. Coffees from different Kenyan counties can taste quite different even when the same variety and broad processing method are involved. There are general regional patterns, but they aren't rules.
Why Kenyan coffee often isn't a single variety
There is an important practical detail behind the way Kenyan coffee is normally sold.
Smallholder farmers typically deliver cherry to a cooperative washing station. Those 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 botanical variety.
That means 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 the washing-station level.
Single-variety Kenyan lots do exist, but they require enough separation at farm and factory level to make it practical.
SL28 beyond Kenya
SL28 has travelled surprisingly well.
World Coffee Research lists it as common in Kenya, Malawi, Uganda and Zimbabwe, and it has also been planted in several Latin American countries including Colombia, Guatemala, El Salvador, Costa Rica, Peru and Panama.
One of the best-known examples outside Africa came in 2017, when British barista Dale Harris won the World Barista Championship using a fully washed SL28 from Ernesto Menéndez's Finca Las Brumas in El Salvador.
The coffee was grown at around 1,800 metres in the Santa Ana region. Harris used it as his signature espresso, describing pomegranate, Ethiopian honey and caramelised orange alongside a silky texture and a Pinot Noir-like mouthfeel.
The coffee had originally been prepared for the Cup of Excellence, but the lot was instead exported to the UK and ended up in Harris's competition routine. Menéndez's SL28 later became part of the wider story of Finca Las Brumas, which returned to Cup of Excellence competition and won again in 2024.
It's a useful demonstration of what happens when a Kenyan variety is grown somewhere else. The variety's identity travels, but the environment changes the result.
Why SL28 is still being planted
On paper, SL28 has plenty working against it.
It is low yielding and susceptible to the major diseases of coffee. Modern varieties can produce more coffee, mature earlier and offer better resistance.
SL28 has something those varieties are still trying to reproduce: exceptional quality potential at high altitude. WCR gives it the highest quality classification in its catalogue and describes it as a variety with very good cup quality potential.
For producers who have the right environment and a buyer willing to pay for quality, that can be enough to justify keeping it in the ground.
And after nine decades, SL28 remains one of the clearest examples of how much a single selected coffee tree can influence an entire coffee-growing region.
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 Moduli, 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: Exceptional 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.