Blackcurrant, red fruit and citrus, with a wine-like acidity and a long finish. That's the Kenyan cup SL34 is known for, and the variety has been part of it since Scott Agricultural Laboratories selected it in the late 1930s.
Its history is a little less straightforward than the name suggests. The original SL34 tree was labelled “French Mission”, which led generations of coffee writers to classify it as Bourbon. More recent genetic testing says otherwise, though the labs don't agree on what it is instead. We know where the tree was found and what its label said. Exactly how those two pieces of history fit together is still unclear.
The Scott Agricultural Laboratories selections
During the 1930s, researchers at the Scott Agricultural Laboratories in Kenya began selecting individual coffee trees that showed promising characteristics for production and cup quality.
Selections made between 1935 and 1939 were given the prefix SL, followed by a number. Most disappeared from common use, but a handful became particularly important. SL28 and SL34 remain the two best-known examples.
SL34 was selected from a single tree at Loresho Estate in Kabete, near Nairobi. The tree was labelled “French Mission”, which would later become the source of much of the confusion surrounding its ancestry.
The French Mission connection
French Mission coffee has a well-documented history in East Africa. Spiritan missionaries established a mission at Bura in the Taita Hills in 1893, where Bourbon coffee brought from Réunion was planted. Seedlings were later moved to the Saint Austin mission near Nairobi in 1899, and seed from these plants was distributed to settlers.
Because that route from Réunion was documented, French Mission coffee became closely associated with Bourbon. For decades, SL34 was therefore assumed to be Bourbon as well.
Then genetic testing complicated the story.
World Coffee Research now classifies SL34 as Typica related, rather than Bourbon related. The original Loresho tree was labelled French Mission, but its genetic profile doesn't match the expectation that it should simply be a Bourbon descendant.
There are several possible explanations. The Loresho tree may have been incorrectly labelled. Typica material could have entered the French Mission population somewhere along the way. There may have been more than one source of coffee plants described as French Mission.
There's a wrinkle. Christophe Montagnon's 2022 study, which grouped coffees using a different set of genetic markers and reference samples, places SL34 in its “Ethiopian Legacy” group, alongside K7 and the little-known SL9. The two systems group coffee differently, and nobody has yet published work reconciling them for SL34.
We don't currently have enough evidence to say which explanation is correct. Either way, the old “SL34 is Bourbon” description shouldn't be treated as settled fact.
The SL34 family tree
The SL34 family tree
SL34
Selected from a tree labelled "French Mission" at Loresho Estate in Kabete, SL34 is one of Kenya's two most widely grown traditional varieties. It was long assumed to be Bourbon. World Coffee Research now classes it as Typica related, while RD2 Vision places it in an Ethiopian Legacy group. Known for winey acidity and dark fruit, SL34 remains central to Kenya's specialty coffee despite its susceptibility to disease and medium yields.
What does SL34 look like?
SL34 is a tall coffee plant with dark bronze tips on its young leaves. It has long lateral branches, large beans and a growth habit suited to the high-altitude areas where it is most widely grown.
World Coffee Research describes SL34 as adapted to high-altitude areas with good rainfall. It recommends 1,000 to 2,000 plants per hectare, which is low by Central American standards.
The cherries ripen to red, and the beans are large compared with standard Arabica varieties.
Growing SL34
SL34 is not an especially forgiving variety to grow. It has medium yield potential, is highly susceptible to coffee berry disease, and is also vulnerable to coffee leaf rust and nematodes. That makes it more demanding than modern disease-resistant Kenyan varieties such as Ruiru 11 and Batian.
World Coffee Research rates its quality potential at high altitude as exceptional, its top rating. Near the equator it recommends planting above 1,200 metres, weighing cup quality against the variety's disease susceptibility.
That's the trade-off. In Kenya's highlands, the combination of altitude, climate and established production knowledge can make SL34 worthwhile. Elsewhere, the balance may look very different.
What does SL34 taste like?
SL34 is best known for its acidity and complexity.
In good Kenyan coffees, the variety can produce a juicy, full-bodied cup with pronounced fruit character. Blackcurrant, red fruit and citrus are common descriptions, alongside a wine-like acidity and a lingering finish.
Not every SL34 tastes identical, though. Altitude, soil, rainfall, crop conditions and especially processing all contribute to the final cup.
Washed Kenyan coffee is particularly associated with the bright, structured profile that has made SL34 famous.
SL34 alongside SL28
SL34 is often mentioned alongside SL28, and for good reason. Both were selected by Scott Agricultural Laboratories during the same period and both became important parts of Kenya's specialty coffee industry.
They aren't genetically the same, though.
World Coffee Research classifies SL28 as Bourbon related and SL34 as Typica related. Montagnon's study draws different lines, putting SL28 with Typica and Bourbon and SL34 in Ethiopian Legacy, but both systems agree the two took different genetic paths. SL28 was selected in 1935 from a population called Tanganyika Drought Resistant; SL34 came from a single tree at Loresho.
Tasting the two side by side from the same producer can be a useful way to see how much variety can contribute to the final cup. The coffees share a growing region and processing tradition, but they don't necessarily taste the same.
SL34 today
SL34 remains widely grown in Kenya, usually alongside SL28, and is also found in other East African coffee-producing countries.
Cup quality is why it has lasted. Growers keep planting it despite the disease pressure and the moderate yields, because where the environment suits it and the coffee is well looked after, it produces the cups that made Kenyan coffee's reputation.
Quick varietal facts
Varietal: SL34
Species: Coffea arabica
Type: Single-tree selection
Genetic group: Typica related according to World Coffee Research; Ethiopian Legacy according to RD2 Vision
Origin: Kenya
Selection: Loresho Estate, Kabete, during the 1935 to 1939 Scott Agricultural Laboratories selection programme
Breeder/selector: Scott Agricultural Laboratories
Growth habit: Tall with long lateral branches
Leaf tip colour: Dark bronze
Cherry colour: Red
Bean size: Large
Yield potential: Medium
Planting density: 1,000 to 2,000 plants per hectare
Quality potential at high altitude: Exceptional
Disease resistance: Highly susceptible to coffee berry disease; also susceptible to coffee leaf rust and nematodes
High-altitude adaptation: Strong, particularly in areas with good rainfall
Typical cup profile: Bright, wine-like acidity with fruit character, full body and a long finish; commonly associated with blackcurrant, citrus and red fruit
Further reading
World Coffee Research: SL34 variety profile
The Loresho selection, the French Mission label, the Typica-related classification and the agronomic ratings we've used here.
World Coffee Research: SL28 variety profile
The other famous Scott Laboratories selection, from Tanganyika Drought Resistant trees, for a side-by-side comparison.
Montagnon et al.: Deciphering Early Movements and Domestication of Coffea arabica
The 2022 study that places SL34 in the “Ethiopian Legacy” group with K7 and SL9. Free to read.
Christopher Feran: The Lost Origins of “Inca Gesha”
Written about SL9, but full of detail on the Scott Laboratories selections, including where SL34 and SL28 came from.
Interested in SL34's family history? Read our guides to SL28, SL9, Typica and Bourbon.