SL28 and SL34 Coffee Varieties

SL stands for Scott Labs, short for Scott Agricultural Laboratories, a British colonial research station founded in 1922 at Kabete, outside Nairobi. Under agronomist A.D. Trench, the lab ran a systematic varietal selection program through the 1930s: evaluate Kenya’s existing coffee trees, propagate the best individual performers, and release the winners to farmers under an SL number. Podium Coffee Club’s history of the program puts the catalog at SL01 through SL45, with SL28 and SL34 among the selections officially released to growers in the late 1930s and 1940s.

The two varieties come from different trees and, it turns out, different genetic families, despite decades of roasters lumping them together as “Kenya’s two great varieties.” SL28 was selected in 1935 from a single standout tree in a population literally named Tanganyika Drought Resistant, imported from what is now Tanzania; World Coffee Research classifies its background as Bourbon-like. SL34 came a few years later from one tree on the Loresho Estate in Kabete, a tree labeled “French Mission” for its descent from Bourbon seed that French missionaries had carried to Kenya decades earlier, and for years it was assumed to share SL28’s Bourbon lineage. Homegrounds’ SL34 profile reports that more recent genetic testing overturned that assumption: SL34 actually groups with Typica, not Bourbon, making the two most-planted Kenyan varieties genetically distinct cousins rather than siblings.

SL28’s drought-tolerant reputation is the part of the story that gets repeated as settled fact, and the origin story is real: it was selected, by name, from a population bred for exactly that trait. But the direct evidence is thinner than the lore suggests. A 2014 peer-reviewed seedling trial in Zimbabwe put SL28 through 21 and 28 days of induced soil moisture deficit alongside three other varieties, including CR95, and found SL28 accumulated the least total dry biomass of the four under sustained drought stress, the opposite of a clear win. That is one trial, at the seedling stage, outside Kenya, and it says nothing about how a mature SL28 tree’s root system performs in its home highlands, so it does not overturn the origin story so much as complicate it: a coffee variety literally selected for drought resistance in 1935 did not automatically prove itself the most drought-tolerant option in a controlled modern trial. Both varieties also carry a real agronomic cost for that heritage cup quality: World Coffee Research rates both susceptible to coffee leaf rust, coffee berry disease, and nematodes, part of why disease-resistant hybrids like Ruiru 11 and Batian exist alongside them in Kenya today.

None of that has slowed the varieties’ spread. SL28 and SL34 are no longer just Kenyan: growers in Colombia, Hawaii, and Taiwan now plant both, chasing the same fruit-forward, structured cup. Colombian producer Wilton Benitez’s SL-28, put through an anaerobic and ultrasound process, scored 96 points from Coffee Review with passionfruit and orange blossom notes, while Hula Daddy’s Kona-grown Laura’s Reserve SL34 scored 98, among the highest marks Coffee Review has given any coffee, on black cherry, dark chocolate, and gardenia. That is the variety, not the place, carrying the cup.