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.¹²³

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.⁴

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 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.

SL28 and SL34 are not the only survivors of the Scott Laboratories programme. WCR’s catalogue also carries SL14, selected at the same station in 1936 from a single tree labelled “Drought Resistant II (D.R. II)” — the same drought-hardy line of thinking that produced SL28, and economically important today in Kenya and Uganda. It is a plainer plant: WCR rates its quality potential “Good” rather than SL28’s “Exceptional,” with medium yield and the same total lack of disease resistance.⁵ That contrast is the point — the Scott programme produced several usable trees, and only two of them became a global selling point.

To verify

The selection year is disputed. World Coffee Research says 1935, and Volcana Coffee agrees.⁶ Trabocca states 1931. Perfect Daily Grind hedges to “the 1930s”.⁷ 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.”

One partial reconciliation is worth recording: Inheritance Coffee says SL28 was “formally cataloged in 1935”,⁸ which would make 1931 the year the tree was picked and 1935 the year it was filed. That reading fits every source, but no source actually states it — it is inference, not documentation.

To verify

A longer list of growing countries appears in one source only. Inheritance Coffee lists SL28 as grown in Kenya, Tanzania, Uganda, Malawi, Rwanda, Burundi, Colombia, Panama and Costa Rica. WCR corroborates only Kenya, Uganda, Malawi, Zimbabwe and an unspecified spread “to Latin America” — it names no Latin American country, and does not mention Tanzania, Rwanda or Burundi at all. Treat the specific countries beyond WCR’s list as a single grower-facing claim.

Separately, some roaster write-ups trace SL28’s ultimate ancestry to Yemen or Ethiopia rather than to the Tanganyika Drought Resistant population. The research report behind this note attributes that hedge to Singular Coffee; that page could not be retrieved (the server returned HTTP 429), so it is recorded here as an unverified variant narrative and is deliberately not listed as a source below. Every source that was read points to Tanganyika.

To verify

Scott Agricultural Laboratories’ founding and successor institution. Trabocca states the British colonial government established the labs in 1922⁹ — a single source, still unconfirmed. A later research pass attributed the same 1922 date to Morning Wines as a second source; reading that page directly, it gives no founding year at all — only that the station was established under British colonial rule and that the selections were made in the 1935–1939 period.¹⁰ So the apparent corroboration is not real, and 1922 rests on Trabocca alone. 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. Those may be two different institutional lines. Do not state either succession as fact. Both disputes — the founding date and the succession — are now laid out in full on the Scott Agricultural Laboratories note, which adds a third founding claim (1903, from Sweet Maria’s) to the two recorded here.

Update — the founding date is now largely settled, and 1922 was measuring the wrong thing. An excerpt from an FAO report on colonial research systems in Kenya, read directly for the Scott Agricultural Laboratories note, records that the colonial government established Kenya’s first experimental station at a government farm in Kabete in 1903, and that “in 1924 the staff was transferred to a new facility called Scott Agricultural Laboratories.”¹¹ So 1903 dates the station and 1924 dates the name — the two claims were never really in conflict, and Trabocca’s 1922 is two years off the documented naming date. The same source has coffee research leaving Scott Labs for Ruiru in 1949, which is what reconciles the NARL and KALRO succession stories. Safe statement here: SL28 was selected at a station that had carried the Scott name since 1924.

What the plant is like

From the World Coffee Research record:¹²

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”.

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°. Growers aiming at the cup rather than the crop set the bar higher: Inheritance Coffee says SL28 needs to be above 1,500 m for the flavour to fully express, and Volcana Coffee puts the optimal band at 1,400–2,100 m — confirmed by reading that page directly.⁶ That range sits comfortably inside the roughly 1,400–2,100 m span the Kenya note treats as the honest figure for the country as a whole, so nothing here is specific to the variety rather than to the origin.

Two further traits come from the grower-facing description at Inheritance Coffee, and both are consistent with what WCR records. The root system is deep, which is the physical mechanism behind the drought tolerance — the tree reaches water that shallower-rooted plants cannot, so a dry season shortens the crop instead of killing the plant. And ripening is medium-late, meaning SL28 cherries come in towards the back of the harvest rather than with the early varieties, which matters for how a farm schedules picking labour.

To verify

Sources disagree on the colour of SL28’s new leaf tips — and this one matters. World Coffee Research records the leaf tip as green, and Perfect Daily Grind calls green-versus-bronze the “only major visual difference” between SL28 and SL34. But two other sources describe SL28 itself as bronze-tipped: Inheritance Coffee has “broad, bronze-tipped leaves” and My Coffee Explorer has “bronze-tipped new growth”.¹³

This is not a cosmetic detail — the leaf-tip colour is the field test used to tell the two varieties apart, and it is the tell recorded in the trait table above, in the comparison table further down this note, and in the SL34 note. WCR is the more authoritative source and is followed here, but a reader should know that widely-read secondary descriptions say the opposite, and that a bronze tip on a Kenyan tree therefore does not reliably identify it as SL34.

Added: the test is leaky from the other direction too. WCR’s own SL34 record notes “a few green-tipped strains” of SL34,¹⁴ so a green tip does not reliably identify a tree as SL28 either. Treat leaf-tip colour as a hint, not an identification.

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; Inheritance Coffee says “moderate to low (especially compared to modern hybrids)”. 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. Inheritance Coffee’s parenthetical — “especially compared to modern hybrids” — points the same way, which is mild support for that reading, but no source states the comparison baseline outright.

A later research pass added a further grower-facing source, Xliii Coffee, said to describe SL28 as high-yield with low nutritional requirements. That page could not be retrieved to check the wording (the server returned HTTP 403), so it is recorded here as an unverified restatement of the same grower-side view and is deliberately not listed as a source below. It changes nothing: the contradiction with WCR stands.

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.

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.

Sold as a pair, they are also usually harvested and milled as a pair, which is why almost nothing in the trade record separates them. A buyer asking for “SL28 and SL34” is in practice asking for high-grown washed Kenyan coffee, not for two identified plants.

To verify

“SL varieties are as much as 80% of exported Kenyan coffee.” Morning Wines writes that “it is estimated that SL-series varieties comprise as much as 80% of all exported Kenyan coffee” — confirmed by reading the page.¹⁰ MTPak Coffee carries a near-identical ~80% figure.¹ But both are roaster-facing write-ups with no Kenyan government or research source behind them, and this wiki’s Kenya note already shows why the number looks derived rather than measured. Two sources repeating the same estimate is not two measurements. “SL varieties dominate Kenyan production” is the safe statement; the 80% is not verified.

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:¹⁵¹⁶

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. Two further sources add the same shape in different words — a lively citric acidity, a juicy, syrupy body, mixed fruit across tropical, red-berry and stone-fruit registers, a brown-sugar or caramel sweetness, and “wine-like structure and vibrant clarity”; and blackcurrant, grapefruit, tomato and tropical fruit over “a full, juicy body and a clean, lingering finish” — both 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.

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.

The one score figure that turned up is unusable as stated. My Coffee Explorer writes that well-grown SL28 “has earned it consistent scores above 90 points in international competitions” — but names no competition, no year, no lot and no cupping panel, so there is nothing to check it against. No Cup of Excellence, Alliance for Coffee Excellence or Specialty Coffee Association record isolating SL28 was located. There is a structural reason variety-level scores are hard to source at all: Kenya has no Cup of Excellence programme — the Alliance for Coffee Excellence’s own competition list covers 17 countries and Kenya is not one of them¹⁷ — and Kenyan lots are sold at the Nairobi Coffee Exchange by screen-size grade such as “Kenya AA” rather than by variety, so the paperwork that would carry a variety-level score mostly does not exist.

Related

• Kenya — the origin it was selected in and still defines
• Bourbon — the family SL28 belongs to
• SL34 — its constant stablemate, which genetic testing puts on the Typica side instead
• SL14 — the third Scott Labs selection in WCR’s catalogue, drought-tolerant like SL28 but rated only “Good” in the cup
• 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, and the reason its lots are recorded by grade rather than by variety
• Cup of Excellence · Alliance for Coffee Excellence — the competition circuit Kenya sits outside of

Official links

• Official site — World Coffee Research variety record: SL28

Footnotes

1. MTPak Coffee — SL28 and SL34: a coffee roaster’s guide ↩ ↩²

2. Podium Coffee Club — The history of the SL28 coffee varietal ↩

3. Premium Roast — SL28 coffee variety: origins, flavor & roasters ↩

4. Trabocca — SL28 variety in a nutshell ↩

5. World Coffee Research — SL14 ↩

6. Volcana Coffee — SL28 coffee variety: origins, agronomics & cup profile ↩ ↩²

7. Perfect Daily Grind — Exploring popular Kenyan coffee varieties: SL-28 & SL-34 ↩

8. Inheritance Coffee — SL-28 cultivar ↩

9. Trabocca — Kenyan coffee varieties: a comprehensive overview ↩

10. Morning Wines — SL28 & SL34: Kenya’s legendary cultivars ↩ ↩²

11. Excerpt on colonial research systems in Kenya, from FAO (2002), Report on the Impact of Foreign Assistance — confirmed via WebFetch for the Scott Agricultural Laboratories note; the source of the 1903 Kabete station, the 1924 Scott Agricultural Laboratories naming and the 1949 transfer of coffee research to Ruiru ↩

12. World Coffee Research — SL28 ↩

13. My Coffee Explorer — SL28 glossary entry ↩

14. World Coffee Research — SL34 ↩

15. World Coffee Research — Ruiru 11 ↩

16. World Coffee Research — Batian ↩

17. Alliance for Coffee Excellence — Cup of Excellence competitions ↩