SL34 is one of Kenya's best-known coffee varieties. It was selected by Scott Agricultural Laboratories in the 1930s and has become closely associated with the bright acidity and fruit character found in some of Kenya's finest coffees.
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 puts SL34 in the Typica-related group 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.
We don't currently have enough evidence to say which explanation is correct. The important point is that the old “SL34 is Bourbon” description should no longer be treated as settled fact.
The SL-34 Family Tree
The SL-34 Family Tree
SL-34
Selected from a tree labelled "French Mission" at Loresho Estate in Kabete, SL-34 is one of Kenya's most celebrated coffee varietals. For decades believed to be Bourbon, recent genetic testing reveals it actually belongs to the Typica group, creating an intriguing botanical mystery. Known for producing exceptional cup quality with bold, winey acidity and complex dark fruit notes, SL-34 remains a cornerstone of Kenya's speciality coffee identity despite being susceptible to disease and relatively low-yielding.
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. Its recommended planting density is relatively low by Central American standards, reflecting the larger trees and the multi-stem pruning systems commonly used in East Africa.
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, but is highly susceptible to Coffee Berry Disease and is also vulnerable to coffee leaf rust.
That's part of the reason it has remained primarily associated with high-altitude specialty production rather than becoming a universally useful commercial variety.
WCR considers its quality potential at high altitude to be exceptional. It also recommends higher elevations around the equator, taking both cup quality and the variety's disease susceptibility into account.
For producers, there is therefore a familiar trade-off. SL34 can produce exceptional coffee, but achieving that quality requires a suitable environment and careful management.
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. When the harvest is well picked and processed, the result can be extremely distinctive.
That cup quality is the main reason SL34 has been maintained despite its agronomic weaknesses. World Coffee Research rates its quality potential at high altitude as exceptional, while its yield potential is only medium.
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.
SL28 is classified by World Coffee Research as Bourbon related, while SL34 is Typica related. Both have exceptional quality potential at high altitude, but their genetic histories took different paths.
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.
The disease problem
The main weakness of SL34 is disease susceptibility.
World Coffee Research describes the variety as highly susceptible to Coffee Berry Disease, and coffee leaf rust is another significant concern. That makes SL34 more demanding than modern disease-resistant varieties such as Ruiru 11 and Batian.
It also helps explain why SL34 remains concentrated in particular environments. In Kenya's highlands, the combination of altitude, climate and established production knowledge can make the variety worthwhile. Elsewhere, the balance may look very different.
Why has SL34 lasted?
The simplest answer is cup quality.
SL34 can produce exceptional coffee under the right conditions, and that has given farmers and buyers a reason to continue growing it despite the disease pressure and relatively moderate productivity.
Its history also makes it a useful reminder that coffee genetics aren't always as tidy as the labels suggest. SL34 spent decades being described as Bourbon because of its association with French Mission. DNA testing gave researchers a different answer, but hasn't yet explained exactly how the mismatch happened.
That mystery is still open.
SL34 today
SL34 remains widely grown in Kenya and is an established part of the country's coffee landscape. It is also found in other East African coffee-producing countries, although Kenya remains its main home.
Its continued use reflects a fairly simple calculation. The plant can be difficult to grow, but where the environment suits it and the coffee is well managed, the quality potential is unusually high.
That's enough to keep SL34 interesting, both in the field and in the cup.
Quick varietal facts
Varietal: SL34
Species: Coffea arabica
Type: Single-tree selection
Genetic group: Typica related
Origin: Kenya
Selection: Loresho Estate, Kabete, during the 1935–1939 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
Quality potential at high altitude: Exceptional
Disease resistance: Highly susceptible to Coffee Berry Disease; also susceptible to coffee leaf rust
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
- Scientific Reports - Genetic Diversity of Coffea arabica Cultivars in Kenya
- World Coffee Research - SL28 Variety Profile
Interested in SL34's family history? Read our guides to SL28, Typica, and Bourbon.