February 16, 2021

Exploring popular Kenyan coffee varieties: SL-28 & SL-34


Kenya is the 16th largest coffee producer in the world by volume, and is recognised today as a prominent specialty coffee origin. Growing regions in the country (especially its central highlands) offer rich, acidic soil, and optimal conditions for specialty coffee production.

Kenyan coffee is often associated with bright acidity, a rich, full body, and a distinctive flavour in the cup. Some of the most popular varieties in the country include K7, Blue Mountain, Batian, Ruiru 11, and the SL series.

The SL series in particular has been cultivated in Kenya for almost 90 years. Although it covers a number of varieties, two of the most popular are SL-28 and SL-34.

Today, despite much discussion about newer, disease-resistant varieties and hybrids, the SL varieties remain popular among Kenyan producers. They offer good longevity, high yields, and excellent cup quality. To learn more about these varieties, I spoke with a few professionals who work in the Kenyan coffee value chain. Read on to find out what they said.

You may also like our brief guide to Kenyan coffee.

What are the SL-28 & SL-34 varieties?

If you have ever tried Kenyan specialty coffee, there’s a good chance that it came from an SL-series plant.

SL stands for Scott Laboratories, which is the name of the Kenyan research centre that first developed these varieties in the 1930s. While World Coffee Research recognises three “official” SL varieties (and there are supposedly many more), SL-28 and SL-34 are the most popular in specialty coffee. 

While both varieties can be found outside the country in small concentrations (SL-28 has spread to parts of Latin America, for instance), they are most common by far in Kenya. As such, they are now almost synonymous with Kenyan specialty coffee production.

Francis Mwangi is a self-professed “SL expert”, who has farmed the varieties for more than 30 years. He tells me that he thinks they will “always be the best”. 

“If well looked after, the SL varieties give back in abundance,” he says. “The yield is just unbelievable.

“Getting above 10kg per plant does not require too much from the producer, unlike Ruiru 11 (another variety) which constantly demands attention.”

SL-28 trees are tall, with green tipped leaves, very high yields, and good cup quality. They require little nutrition, and are resistant to drought, but susceptible to coffee leaf rust, coffee berry disease (CBD), and nematodes in the soil.

SL-28 trees produce large cherries after three years, and if looked after properly, they have a very low “milling loss” (meaning a favourable parchment to green coffee ratio).

In comparison, SL-34 trees are structurally similar to SL-28. The only major visual difference is that the leaf tips of SL-34 trees are a dark bronze colour, rather than green.

SL-34 trees are also more demanding in terms of nutrition, and have slightly lower yields. Much like SL-28, they are also highly susceptible to CBD, leaf rust, and pests. 

The SL varieties have been cultivated in Kenya for decades. They have been around for much longer than other popular coffee varieties in the country, and are naturally suited to the country’s climate. Today, it is estimated that SL-series varieties comprise as much as 80% of all exported Kenyan coffee.

“An advantage of the SL varieties over other, newer coffees is that even after years of neglect, the coffee still continues producing (although at lower volumes),” Francis says. “Furthermore, once you get back to caring for the SL, it can return to producing as though it was never forgotten.

“The trees do not demand a lot of attention or fertilisers, but still still give good returns,” he adds. “However, it is a favourite for pests, especially berry borers.”

He adds that the trees are incredibly resilient and hardy, noting that he has seen some in Kenya that have been there for “many decades”. 

Cup profile

When grown at medium to high altitudes (SL varieties grow best at 700 m.a.s.l. and above) these plants ripen slowly and fully, creating high-quality, full-bodied coffees with good acidity and nutty, fruity flavours. 

David Mathenge is a Kenyan coffee grader (known locally as a “liquorer”). He tells me that there is often a “floral and spicy” fragrance when SL-variety beans are ground.

“The cup is so full-bodied, you can feel it right away,” he continues. “If we would have come up with a more resistant SL cultivar, [the Kenyan coffee sector] would have been on a completely different level.”

While there are debates over the quality of SL varieties when compared to others, they are still largely recognised as delivering a consistently good cup profile. David tells me that he thinks they have a superior cup quality to new varieties like Batian and Ruiru 11. 

“This is probably one of the reasons why they have been around for so long,” he says. “They have a really complex cup characteristic.

“Newer varieties are still adapting to the soil and the weather in Kenya, even though they are of high quality. The SL varieties just have that ‘X factor’ that a lot of tasters look for.” 

David adds: “Most of the coffees we [see in Kenya] are SL varieties, and they are [generally] all rated highly. The only defects we tend to see come as a result of insect damage and disease… apart from that, the SL varieties’ large cherries mean good prices for the farmer.”

Despite being common across the country, the SL varieties do bring a host of challenges for farmers.

As mentioned previously, both varieties are extremely prone to coffee leaf rust, CBD, and nematodes in the soil. Some farmers have lost whole seasons as a result of the crop damage caused by these conditions. Understandably, all three are feared by Kenyan farmers. 

Joseph Wambugu is a major coffee producer in Kenya. “CBD is very dangerous for farmers of the SL varieties,” he says. “It caused me to lose around half of my crop just two years ago… most farmers lost everything.”

Thankfully, Joseph’s Ruiru 11 plants were resilient enough to support him in the aftermath, but other farmers were not so lucky.

To effectively control these problems, it is a question of cost. Joseph says: “These varieties may have high yields, but most of the money [from the harvest] goes back into the farm for maintenance.

“We have a [seasonal] programme that farmers must strictly follow to control coffee leaf rust and CBD, which keeps them safe,” he explains.

However, following these programmes to the letter is difficult, and alongside pest control, it is a costly and labour-intensive process.

Naturally, this means there is competition among varieties. Newer, more resistant varieties like Batian and Ruiru 11 are resistant to most major diseases and some pests, making them cheaper and easier to maintain.

As such, Joseph says he’s seeing more and more farms turn to these mewer varieties. Some existing farms are even carrying out top-working to “convert” SL varieties to these more resistant alternatives.

However, Joseph says that despite a move away from the SL series, he doesn’t think they will vanish overnight.

“[Even though] these new coffees are much better in terms of disease resistance, I believe [existing widespread popularity] means the SLs will be on farms for a long time to come,” he says.

Despite these issues and challenges, the SL varieties remain popular across Kenya today. Additionally, even where they aren’t planted outright, they are commonly used as a base to breed new varieties, or even to provide rootstock for grafting.

As such, if new cultivars continue to be derived from the SL varieties (whether through breeding or grafting) they will still be around in some capacity for decades to come.

In time, this could mean that we see a “descendant” of the SL family with increased resistance to pests and diseases, improving their viability for the producer. 

At present, it’s clear that the most attractive features of the SL varieties are their high yields and good cup quality. No matter how difficult they might be to grow, this means they will always be a viable (if risky) option.

Fans of Kenyan specialty coffee around the world still know what to expect from SL-28 and SL-34, and these varieties are still sought-after by both consumers and roasters alike. It doesn’t seem like this is set to change any time soon.

Enjoyed this? Then read our article on how researchers are combating the impact of climate change with hybrid coffee varieties.

Photo credits: Peter Gakuo

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