SL28
SL28 is the coffee plant most people are actually tasting when they say a cup tastes “Kenyan” — the blackcurrant, the grapefruit sharpness, the almost wine-like weight. It was picked out of a Kenyan research field in the 1930s, one single tree out of a whole population, and nearly a century later it is still what buyers fly to Kenya for.
The name is an inventory code, not a word. SL is short for Scott Agricultural Laboratories, the Kenyan research station that did the selecting, and 28 is simply the serial number that tree was filed under (Trabocca, accessed 2026-07-23, confirmed via WebFetch).
It is a plant of extremes. It shrugs off drought, survives neglect for decades, and cups exceptionally — and it catches every major coffee disease going. Kenyan farms have spent forty years breeding replacements that are tougher, and every one of those replacements still has SL28 inside it, because nobody could match the flavour any other way.
Where it came from
Everything in this section comes from the World Coffee Research variety record (accessed 2026-07-23, confirmed via WebFetch) unless otherwise noted — WCR’s catalogue is the specialty industry’s reference database for coffee varieties, maintained by World Coffee Research.
- SL28 was selected from a single tree in a population called Tanganyika Drought Resistant — material originally from what is now Tanzania, chosen for its ability to survive dry seasons.
- The selection was made by Scott Agricultural Laboratories in Kenya.
- Genetically it sits in the Bourbon side of the Bourbon–Typica family. WCR states that “recent genetic tests have confirmed that SL28 is related to the Bourbon genetic group” — so its parentage lines up with Bourbon, the sweet-cupping, fragile founding variety that also produced Caturra, Catuaí and Mundo Novo.
From Kenya it travelled. WCR records SL28 as important in Uganda, commonly found in Malawi and Zimbabwe, and spread into Latin America. Growers outside Kenya generally report that the plant travels better than the flavour does — the full Kenyan profile is widely held to depend on Kenyan soil and Kenyan processing as much as on the variety itself.
To verify
The selection year is disputed. World Coffee Research says 1935, and Volcana Coffee agrees (both confirmed via WebFetch). Trabocca states 1931 (confirmed via WebFetch). Perfect Daily Grind hedges to “the 1930s” (confirmed via WebFetch). No primary Scott Laboratories document was located to settle it. Note that the existing Bourbon note here already records 1935 on WCR’s authority; safest phrasing is “early-to-mid 1930s.”
To verify
Scott Agricultural Laboratories’ founding and successor institution. Trabocca states the British colonial government established the labs in 1922 (confirmed via WebFetch) — a single source, unconfirmed elsewhere here. Separately, the research report behind this note claimed Scott Labs is today the National Agricultural Laboratories (NARL); that claim could not be confirmed, and it is not what WCR says. WCR names the Coffee Research Foundation, now the Kenya Agricultural and Livestock Research Organization (KALRO), as the body behind Kenya’s later varieties (confirmed via WebFetch). Those may be two different institutional lines. Do not state either succession as fact.
What the plant is like
From the World Coffee Research record (accessed 2026-07-23, confirmed via WebFetch):
| Trait | SL28 |
|---|---|
| Stature | Tall |
| Leaf tip colour | Green |
| Bean size | Large |
| Yield potential | Low |
| Quality potential at high altitude | Exceptional |
| Coffee leaf rust | Low resistance / susceptible |
| Coffee berry disease | Susceptible |
| Nematodes (root-attacking soil worms) | Susceptible |
| Drought | Tolerant |
Two things stand out. The first is rusticity — WCR’s word for a plant that “can be left untended for years or even decades at a time, and then return to successful production.” That is unusual, and it is the direct inheritance of the drought-resistant Tanganyikan population SL28 came out of. Trabocca puts the same trait in plainer terms: it “can go without water for a long time and still bear fruit” (confirmed via WebFetch).
The second is the disease list. SL28 is susceptible to all three of the diseases that matter most in East Africa: Coffee leaf rust (the fungus Hemileia vastatrix), coffee berry disease (a fungal rot that blackens the fruit on the branch), and root nematodes. Trabocca adds bacterial blight of coffee to that list. Every source consulted agrees on this point without exception — it is the variety’s defining weakness and the reason Kenya spent decades breeding around it.
Optimal altitude depends on how far the farm sits from the equator, because latitude and elevation trade off against each other. WCR’s thresholds: above 1,200 m between 5°N and 5°S, above 900 m at 5–15°, above 700 m beyond 15°.
To verify
Sources flatly contradict each other on yield. World Coffee Research rates yield potential “Low.” Perfect Daily Grind says SL28 trees have “very high yields,” quoting a grower that “getting above 10 kg per plant does not require too much from the producer.” Trabocca says “high-yield”; Volcana says moderate (all confirmed via WebFetch). This is not a small gap — it is the difference between a commercially marginal plant and a productive one. The likeliest reconciliation is that WCR is scoring SL28 against modern high-yield hybrids while growers are comparing it to older heirloom material on well-managed Kenyan farms, but that is inference, not a documented explanation.
To verify
Sources also disagree on how long the tree takes to bear fruit: Trabocca says two years, Perfect Daily Grind says three. Neither cites a trial.
SL28 and SL34
The two are almost always named in the same breath — “SL28 and SL34” is how Kenyan lots get described on bags worldwide — but they are not siblings.
| SL28 | SL34 | |
|---|---|---|
| Leaf tip colour | Green | Dark bronze |
| Genetic group (WCR testing) | Bourbon-related | Typica-related |
| Historical origin story | Tanganyika Drought Resistant population | A “French Mission” (Bourbon) population |
| Relative demands | Less demanding | Needs more nutrition, yields slightly less |
The visual tell is the leaf tips: green on SL28, dark bronze on SL34, and that is the “only major visual difference” between two structurally similar trees (Perfect Daily Grind, confirmed via WebFetch).
The genetic tell is bigger. As recorded in this wiki’s Bourbon and Typica notes, World Coffee Research’s testing places SL34 in the Typica group despite the Bourbon origin story attached to it, and says outright that the historical record “could be incorrect.” SL28 is the one that still tests as Bourbon-related. Two famous stablemates, two different halves of the Arabica family tree.
The varieties bred to replace it
Because SL28 catches everything, Kenya’s coffee researchers spent decades building tougher plants — and they kept feeding SL28 back in to preserve the flavour. Both of the following are verified against their own WCR records (accessed 2026-07-23, confirmed via WebFetch):
| Variety | Released | Bred by | SL28 in its pedigree? | Rust | Coffee berry disease |
|---|---|---|---|---|---|
| Ruiru 11 | 1985, Kenya | Coffee Research Foundation (now KALRO) | Yes — via the male parent, alongside K7, N39 and Rume Sudan | Highly resistant | Resistant |
| Batian | 2010, Kenya | Coffee Research Foundation (now KALRO) | Yes — alongside SL34, Rume Sudan, N39, K7, SL4 and Timor Hybrid | Intermediate resistance | Resistant |
Ruiru 11 is compact and very high-yielding, built for intensive planting; WCR rates its cup quality “Good.” Batian is tall and high-yielding with “Very good” quality at altitude — a rare combination of disease resistance in a tall plant, which WCR notes makes it well suited to smallholders. Neither is rated as highly on the cup as SL28’s “Exceptional.” That gap is why SL28 blocks are still standing on Kenyan farms that could have replanted years ago.
What it tastes like
World Coffee Research’s formal verdict is one word: at high altitude, quality potential is “Exceptional” — the top band in its catalogue, and a rating shared by very few varieties.
The trade’s shared description is more specific: blackcurrant above all, plus grapefruit, a savoury tomato-vine brightness, and a heavy, wine-like body (Volcana Coffee, confirmed via WebFetch). Perfect Daily Grind describes SL varieties grown at suitable altitude as producing “high-quality, full-bodied coffees with good acidity and nutty, fruity flavours,” with a floral and spicy fragrance in the dry grounds (confirmed via WebFetch).
Read those descriptors as the trade’s shared impression rather than a measurement — they come from roaster and industry write-ups, not a controlled sensory study. See Cupping and Coffee scoring for how cup quality is formally assessed.
Kenyan SL28 is conventionally washed, and typically double-washed — fermented, rinsed, then soaked again in clean water before drying, a step largely specific to Kenya. See Washed process.
To verify
Volcana Coffee explains the blackcurrant character as SL28’s chemistry plus Kenya’s high-potassium volcanic soils and double-washed processing, producing “high concentrations of the fruit acids and thiols cuppers read as blackcurrant.” This is a plausible and widely repeated mechanism, but it appears here in a single roaster-facing source with no cited study. Treat the soil-chemistry explanation as a hypothesis, not established science.
To verify
No numeric cupping scores, auction prices, or competition placements for specific SL28 lots could be verified. The consulted sources describe SL28 as consistently rewarded at Kenyan auction, but only qualitatively — none give figures, dates, ranks or named buyers. Anything specific would need to come from Nairobi Coffee Exchange auction records or competition databases directly. Do not state SL28 price or score records as fact on the strength of the sources listed below.
Related
- Bourbon — the family SL28 belongs to
- SL34 — its constant stablemate, which genetic testing puts on the Typica side instead
- Ruiru 11 · Batian — the disease-resistant Kenyan varieties bred to replace it, both carrying its genes
- Scott Agricultural Laboratories — the research station that selected it
- World Coffee Research — keeper of the variety catalogue used throughout this note
- Coffee leaf rust — the disease SL28 has no defence against
- Caturra · Catuaí · Mundo Novo — fellow Bourbon descendants
- Geisha — the other variety with a reputation built almost entirely on cup quality
- Cupping · Coffee scoring — how that cup quality gets judged
- Washed process — the processing style Kenyan SL28 is built around
- Nairobi Coffee Exchange — the auction system that rewards it
Official links
Sources
- World Coffee Research — SL28 — accessed 2026-07-23
- World Coffee Research — Ruiru 11 — accessed 2026-07-23
- World Coffee Research — Batian — accessed 2026-07-23
- Perfect Daily Grind — Exploring popular Kenyan coffee varieties: SL-28 & SL-34 — accessed 2026-07-23
- Trabocca — SL28 variety in a nutshell — accessed 2026-07-23
- Trabocca — Kenyan coffee varieties: a comprehensive overview — accessed 2026-07-23
- Volcana Coffee — SL28 coffee variety: origins, agronomics & cup profile — accessed 2026-07-23
- MTPak Coffee — SL28 and SL34: a coffee roaster’s guide — accessed 2026-07-23
- Podium Coffee Club — The history of the SL28 coffee varietal — accessed 2026-07-23
- Premium Roast — SL28 coffee variety: origins, flavor & roasters — accessed 2026-07-23