SL-28 and SL-34 are well-known Kenyan coffee varieties. They were bred by Scott Agricultural Laboratories (SAL). SAL was founded in 1903 by the Kenyan Colonial government to function as a research institution studying agricultural products.
SL-28 and SL-34 quickly became the varieties of choice for most growers. Their deep root structures helped them acquire water in the dry environments present throughout much of Kenyan, even without irrigation. These varieties also had higher yields than the traditional French Bourbon rootstock and were considered somewhat more disease resistant.
Though both SL varieties spread across Kenya extremely quickly, the release of Ruiru-11 in 1985 by the Kenya Coffee Research Institute (CRI) brought a new kid to the block. Many farmers planted the new Ruiru-11 variety because it was far more resistant to Coffee Berry Disease (CBD), a fungal disease attacking ripening coffee cherry, and Coffee Leaf Rust (CLR), a fungal disease that targets the leaves of coffee trees. It could also be planted at a higher density than the SL varieties, allowing farmers to maximize yields on small plots of land.
One downside to Ruiru-11 was that its shallower root structure made is more susceptible to drought and required more fertilizer. Farmers found that that by grafting Ruiru-11 to SL variety trees, they could have the best of both worlds. Trees where Ruiru-11 was grafted onto an SL variety plant had deeper root structures for drought-times (thanks to the SL variety) and higher immunity to disease and larger yields (thanks to the Ruiru-11).
Other farmers are experimenting with Batian, as well, a relatively new variety introduced by Coffee Research Institute (CRI) in 2010. Batian is named after the highest peak on Mt. Kenya and is resistant to both CBD and CLR. The variety has the added benefit of early maturity and begins bearing fruit after only two years. Some challenges (such as vegetative structure) have prevented it from becoming widespread so far, but its popularity is certainly growing.
While most farms in Kenya still have the traditional SL varieties, most also have Ruiru-11 and, increasingly, Batian. Most farms are far too small to be able to handle lot separation by variety. This means that most lots coming out of Kenya—whether single estate or smallholder group—are a blend of SL, Ruiru-11 and (sometimes) Batian.