SL28 vs. SL34
Two selections made at the same Kenyan laboratory in the 1930s, usually named together and sold together. What separates them is where they suit and where they came from.
In short
SL28 and SL34 appear together on so many bags of Kenyan coffee that they read as a pair, or as two grades of one thing. They are two separate selections from two different trees, made at the same laboratory in the same few years.
The letters stand for Scott Laboratories, the colonial government's agricultural research station at Kabete in Kenya. Individual tree selections made there between 1935 and 1939 were given the prefix SL and a number.
Two trees with two stories
Sourced
SL28 was selected in 1935 from a single tree in a population called Tanganyika Drought Resistant. The catalogue traces that population to 1931, when the laboratory's senior coffee officer toured what is now Tanzania and noticed a variety that appeared to tolerate drought, disease and pests. Seed was brought back, its drought resistance was confirmed, and SL28 was picked out of its progeny. The catalogue calls it the prize selection of that period.
SL34 was selected in the late 1930s from a single tree on a private estate at Kabete, with which the laboratory worked. The tree was labelled French Mission, the name given to coffee descended from Bourbon seed that missionaries had brought from La Réunion to Kenya in the 1890s.
The label on the tree was probably wrong
Accounts differ
Because of that label, SL34 was long understood to be a Bourbon. The catalogue reports that recent genetic tests indicate it is related to the Typica group, and says it is possible that the original story of its selection from a French Mission population is incorrect.
SL28 went the other way round. Its origin in a Tanzanian population says nothing about family, and tests have confirmed it is related to the Bourbon group. So the two selections that share a name, a laboratory and a decade sit on opposite sides of arabica's oldest division. The catalogue files each by genetic background and names no parent variety for either.
What each was valued for
Sourced
- SL28
- Drought tolerant and suited to medium to high altitudes. The catalogue singles out its rusticity: it can be left untended for years and brought back into production, and it notes trees in Kenya that are sixty to eighty years old and still bearing. Its yield is rated low.
- SL34
- Described as adapted to high-altitude areas with good rainfall. Its yield is rated a band higher than SL28's. Its young leaves are mostly dark bronze, where SL28's are mainly green, which is one way the two are told apart in the field.
Sourced
Both are rated exceptional for quality potential at high altitude, the catalogue's top category. Both are rated susceptible to leaf rust, to coffee berry disease and to nematodes.
That is the problem Kenya's later breeding set out to solve. Ruiru 11 and Batian both count SL28 and SL34 among their ancestors, chosen for cup quality, alongside other parents chosen for resistance. The catalogue also records SL28 spreading well beyond Kenya, to Uganda and other parts of Africa and now to Latin America.
Why a bag rarely tells you which one you are drinking
CoffeeHQ explanation
Where a lot pools cherries from many farms, a label that reads 'SL28, SL34' is a list of what those farms grow and not a recipe, and it says nothing about proportions. CoffeeHQ has read no source on how often Kenyan lots are sold that way.
The catalogue gives the two the same quality category, and nothing read for this page separates them by flavour. Anyone who tells you what SL28 tastes like against SL34 is describing particular coffees, and CoffeeHQ has not tasted the two apart.
What to do next
- SL28 — The drought-tolerant selection and its full catalogue entry.
- SL34 — The high-rainfall selection, and the question over its origin.
- Ruiru 11 vs Batian — The two disease-resistant varieties Kenya bred with these as ancestors.
- Typica vs Bourbon — The two families these selections turned out to belong to.
- Kenya — The cooperative system that pools these varieties into one lot.
At a glance
| Aspect | SL28 | SL34 |
|---|---|---|
| What it is | Selection | Traditional variety |
| Genetic group | Bourbon and Typica group, Bourbon-related | Bourbon and Typica group, Typica-related |
| The catalogue's lineage field | Selection of "Tanganika Drought Resistant." Bourbon-like genetic background. | Typica-like genetic background. |
| Stature | Tall | Tall |
| Bean size | Large | Large |
| Yield potential | Low | Medium |
| Quality potential at high altitude | Exceptional | Exceptional |
| Coffee leaf rust | Susceptible | Susceptible |
| Coffee berry disease | Susceptible | Susceptible |
| Nematodes | Susceptible | Susceptible |