Aloe marlothii is one of the most imposing and widespread arborescent species in the genus Aloe, forming a dominant feature of the southern African bushveld landscape from the highveld plateau of Gauteng to the coastal lowlands of KwaZulu-Natal and Mozambique. With its massive rosette of fiercely armed grey-green leaves perched atop a trunk sheathed in a skirt of withered foliage, and its spectacular winter inflorescence of up to 30 near-horizontal racemes, it is instantly recognizable — at least when in flower. Out of bloom, it is routinely confused with Aloe ferox and Aloe excelsa, two closely related tree aloes whose ranges partly overlap. Named after the German-born South African botanist Rudolf Marloth (1855–1931), the species has proven surprisingly adaptable in cultivation well beyond its native range, tolerating harder frosts than most tree aloes and thriving in Mediterranean, subtropical, and even warm-temperate gardens worldwide. This article examines its distribution, ecology, morphology, and taxonomy, compares it with its two most commonly confused relatives, and draws on grower reports — including Brian Kemble’s documented hardiness data from the Ruth Bancroft Garden — to establish the precise temperature thresholds for cultivation under temperate climates.
Family: Asphodelaceae Subfamily: Asphodeloideae Genus: Aloe Accepted name (POWO): Aloe marlothii A.Berger Subspecies: Aloe marlothii subsp. marlothii (inland form); Aloe marlothii subsp. orientalis (coastal form, sometimes clumping, fewer leaf spines) Common names: Mountain Aloe, Flat-Flowered Aloe, Spiny Aloe, Transvaal Aloe, Tree Aloe; bergaalwyn (Afrikaans)
Taxonomy and Nomenclature
Aloe marlothii was described by Alwin Berger in 1905 from material collected in Botswana by Rudolf Marloth. POWO recognizes two subspecies:
- Subsp. marlothii: the widespread inland form, with a tall unbranched stem, densely spiny leaves, and racemes borne horizontally or slightly angled. This is the form most commonly encountered in the wild and in cultivation.
- Subsp. orientalis (previously treated as a variety): a coastal form found primarily in KwaZulu-Natal, characterized by a shorter stem, fewer leaf surface spines, a tendency to form clumps, and racemes held at an oblique angle rather than horizontally. It also tolerates sandy soils, including beach sand — a habitat unusual for this species.
The species grades through intermediate forms into Aloe spectabilis Reynolds in KwaZulu-Natal, and natural hybridization with over 30 other Aloe species has been documented, reflecting both the wide distribution of Aloe marlothii and its ecological overlap with many congeners. Flower color varies geographically: the typical form produces golden-yellow to orange-red flowers, but a bicolored pinkish-red and cream variant occurs near Barberton (Mpumalanga), and a deep scarlet form is found near Utrecht (KwaZulu-Natal). A rare white-flowering form has also been reported and is prized by collectors.
The epithet marlothii is sometimes misspelled “marlothii” or “marlothi” in older literature. The species sits within a clade of large, single-stemmed southern African aloes alongside Aloe ferox, Aloe excelsa, Aloe africana, Aloe candelabrum, and Aloe thraskii.
Distribution and Ecology
Native Range
Aloe marlothii has one of the widest distributions of any tree aloe, spanning five countries in southern Africa:
- South Africa: North-West, Gauteng (including the Klipriviersberg hills in southern Johannesburg and areas around Pretoria), Limpopo, Mpumalanga, and KwaZulu-Natal north of Durban.
- Eswatini (Swaziland)
- Eastern Botswana
- Mozambique (southernmost provinces)
- Zimbabwe (southern and eastern regions)
- The species extends northward over the Soutpansberg and has been reported as far north as Malawi, though populations thin out considerably at the extremes of its range.
Elevational range is broad: from sea level (particularly subsp. orientalis along the KwaZulu-Natal coast) to approximately 1,600 m, with some records up to 2,000 m on rocky mountain slopes. The species is listed as Least Concern on the South African Red List (Mtshali & von Staden, 2018) and is considered common throughout most of its range. Like all Aloe species except Aloe vera, it is listed on CITES Appendix II, meaning international trade requires permits.
Habitat and Ecology
Aloe marlothii is a characteristic component of bushveld (savanna woodland) vegetation across the summer-rainfall regions of southern Africa. It favors rocky terrain — hillslopes, cliff margins, boulder-strewn grasslands, and koppies (rocky outcrops) — where thin, well-drained soils and exposed rock faces provide excellent drainage and protection from grass fires. The species also colonizes open flat country and degraded savanna, and has been shown to establish readily on abandoned settlement sites: dense populations near former Ndebele village sites in Mpumalanga are estimated to have established 20 to 30 years after abandonment, with the largest individuals potentially exceeding 100 to 200 years of age.
The ecology of Aloe marlothii is shaped by three key environmental pressures:
Fire. Unlike many aloes, Aloe marlothii has an effective fire-survival strategy. The persistent skirt of withered dead leaves insulates the trunk against the radiant heat of grass fires, and the plant’s preferred rocky habitats further reduce fire intensity. However, seedlings and small juveniles remain vulnerable, which explains why the species concentrates in rocky microsites where fire frequency is lower.
Frost. Within its native range, Aloe marlothii experiences regular winter frosts on the Highveld (Johannesburg averages 20 to 30 frost days per year, with minima occasionally reaching –8 °C). The species copes well with dry frost, though not with the sustained wet cold that characterizes maritime temperate climates. Critically, it avoids the coldest high-altitude grasslands of the Drakensberg escarpment; its upper elevational limit appears to correlate with the frequency of severe frost events. Brian Kemble’s habitat hardiness record of 22 °F (–5.6 °C) is consistent with Johannesburg winter minima.
Drought and summer rainfall. Aloe marlothii is adapted to a summer-rainfall regime with a distinct dry winter. Annual precipitation across its range varies from approximately 500 to 900 mm, concentrated between October and March. The species stores substantial water reserves in its thick succulent leaves and broad trunk, enabling it to survive dry winters of five to six months without rainfall. In cultivation, this means the species tolerates — and indeed benefits from — reduced winter watering.
Pollination ecology. The massive winter inflorescence is a keystone nectar resource for sunbirds, including the malachite sunbird (Nectarinia famosa) and the greater double-collared sunbird (Cinnyris afer). In regions where Aloe marlothii is abundant, it sustains pollinator populations through the resource-scarce winter months. The horizontal orientation of the racemes facilitates landing by non-hovering bird pollinators — a trait shared with several other large southern African aloes but strikingly different from the erect racemes of Aloe ferox and Aloe vera.
Morphological Description
Aloe marlothii is a large, single-stemmed (rarely branching), evergreen arborescent succulent, typically reaching 2 to 4 m in height at maturity but occasionally exceeding 6 m, with exceptional specimens reportedly approaching 10 m.
Stem. The trunk is erect, unbranched (except when the growing point is damaged, which induces offsetting), up to 25 cm in diameter, and densely covered with persistent dried leaves that form a characteristic “petticoat” or skirt. This feature is one of the most reliable field characters: the dead leaf skirt is retained far more consistently in Aloe marlothii than in Aloe ferox, which tends to shed its old leaves and expose a smooth trunk.
Rosette and leaves. The crown consists of a single, dense, spreading rosette of 20 to 30 leaves. Individual leaves are 75 to 150 cm long and 7.5 to 25 cm wide, broadly lanceolate, deeply channeled on the upper surface, and incurved to spreading or slightly recurved with age. Leaf color is dull greyish-green to glaucous, often with a reddish or bronze tint under drought stress or in full sun. Both leaf surfaces — especially the convex lower (abaxial) surface — are armed with scattered, short, reddish-brown prickles, a trait that is most pronounced in juvenile plants and diminishes as the plant grows taller and escapes the reach of browsing animals. Leaf margins bear stout, deltoid, reddish-brown teeth with orange tips, spaced approximately 10 to 15 mm apart.
Juvenile Aloe marlothii plants are remarkably spiny compared to adults: the dense armature on leaf surfaces is an ontogenetic anti-herbivory defense that becomes progressively reduced with height.
Inflorescence and flowers. The inflorescence is the species’ most diagnostic feature: a single, much-branched panicle up to 80 to 90 cm long, arising from the center of the rosette and bearing 20 to 30 (exceptionally up to 50) racemes. The racemes are 20 to 25 cm long and critically — unlike those of nearly all other tree aloes — are held horizontally or at a slight downward angle, never erect. This horizontal orientation of the racemes is the most reliable character for distinguishing Aloe marlothii from all its look-alikes and gives the plant its alternative common name, “flat-flowered aloe.”
Individual flowers are tubular, 3 to 3.5 cm long, densely packed, and vary in color from golden-yellow through orange to reddish-orange or bright red, depending on geographic provenance. Anthers and style protrude from the mouth of the corolla. A single mature plant can produce up to 400 individual flowers on a single inflorescence.
Flowering period in the Southern Hemisphere is May to September (autumn to spring), with peak flowering typically in June to August — the heart of the dry winter. In the Northern Hemisphere under cultivation, flowering shifts to November to March, triggered by the shortening days and cooler nights of autumn.
Fruit and seeds. The fruit is a dry, dehiscent capsule containing flat, winged seeds approximately 5 to 7 mm in diameter. Seeds are orthodox (tolerate drying and freezing) and can be stored in the Millennium Seed Bank under standard protocols. Germination rates are high (up to 92% under laboratory conditions at 20 °C) and seedling emergence typically occurs within 7 to 14 days.
Root system. The root system is relatively shallow and spreading, adapted to capture moisture from surface runoff on rocky slopes. This architecture makes the species sensitive to waterlogging but efficient at utilizing the brief, intense summer thunderstorms characteristic of its native bushveld habitat.
Comparison with Two Commonly Confused Species
Aloe marlothii vs. Aloe ferox Mill. (Cape Aloe)
Aloe ferox is the species most frequently confused with Aloe marlothii in both the wild and the horticultural trade. Their ranges overlap marginally in KwaZulu-Natal, and both produce large, single-stemmed plants with fiercely armed leaves and branched inflorescences. However, several characters reliably distinguish them:
| Character | Aloe marlothii | Aloe ferox |
|---|---|---|
| Distribution | Summer-rainfall bushveld (Gauteng, Limpopo, Mpumalanga, KwaZulu-Natal north of Durban, Zimbabwe, Botswana, Mozambique) | Winter- and year-round-rainfall regions (Eastern Cape, Western Cape, KwaZulu-Natal south of Durban) |
| Dead leaf skirt | Persistent; trunk densely covered with withered leaves | Usually shed; mature trunk often smooth or only partially skirted |
| Leaf surface spines | Abundant on both surfaces, especially lower (abaxial) | Typically confined to leaf margins and keel; upper surface usually smooth |
| Raceme orientation | Horizontal to slightly angled (diagnostic) | Erect (vertical) |
| Flower color | Variable: yellow, orange, red, bicolored | Typically bright orange-red; occasionally white or yellow |
| Number of racemes | 20–30 (up to 50) | 5–12 |
| Inflorescence shape | Very broadly spreading, candelabra-like | More compact, narrower |
| Typical height | 2–4 m (up to 6 m) | 2–3 m (up to 5 m) |
| Cold hardiness (Kemble) | 20 °F (–6.7 °C), leaf damage | 20 °F (–6.7 °C), flowers damaged, leaves OK |
The practical distinction for gardeners: Aloe ferox generally tolerates wetter winters better than Aloe marlothii (reflecting its adaptation to the year-round and winter-rainfall zones of the Cape), but both species share similar frost tolerance. The inflorescence orientation is the single most reliable identification character.
Aloe marlothii vs. Aloe excelsa A.Berger (Zimbabwe Aloe)
Aloe excelsa replaces Aloe marlothii in the drier, hotter lowveld and midriver valleys of Zimbabwe and northern Mozambique, and the two are virtually indistinguishable when not in flower. Both are tall, single-stemmed, densely skirted tree aloes with spiny leaves.
| Character | Aloe marlothii | Aloe excelsa |
|---|---|---|
| Distribution | Broad: South Africa, Eswatini, Botswana, Mozambique, Zimbabwe | Primarily Zimbabwe, northern Mozambique, marginally into Limpopo |
| Typical height | 2–4 m | Often taller: 3–6 m (up to 10 m in exceptional cases) |
| Leaf surface spines | Dense on both surfaces | Fewer surface spines; leaves often smoother |
| Raceme orientation | Horizontal | Erect (like Aloe ferox) |
| Flower color | Yellow to orange-red | Typically scarlet to deep red, occasionally orange |
| Number of racemes | 20–30 | 4–8 (fewer, more compact) |
| Cold hardiness (Kemble) | 20 °F (–6.7 °C) | Low 20s °F (~–5 to –6 °C), leaf damage |
For identification in the field or in a collection, the rule is simple: if the racemes are horizontal, it is Aloe marlothii; if they are erect, it is either Aloe ferox or Aloe excelsa. Beyond inflorescence, the geographic origin provides the best clue: a large tree aloe from the Johannesburg or Mpumalanga bushveld is almost certainly Aloe marlothii; one from the Eastern Cape is Aloe ferox; one from the Zimbabwe lowveld is likely Aloe excelsa.
Optimal Growing Conditions
Light
Aloe marlothii demands full sun — at least six to eight hours of direct sunlight per day. In its native habitat, it grows fully exposed on rocky hillslopes. Unlike Aloe vera, it does not benefit from afternoon shade except during the initial acclimatization of plants recently moved from shade to full exposure. Insufficient light results in etiolated, pale, floppy leaves and failure to flower.
Temperature
Optimal growth occurs between 18 and 32 °C (65 to 90 °F). The species tolerates heat very well — temperatures above 40 °C are routine in the Limpopo lowveld during summer. Cold tolerance is substantially greater than Aloe vera but less robust than the hardiest dwarf aloes:
- Growth threshold: Below approximately 10 °C (50 °F), metabolic activity slows and the plant enters semi-dormancy.
- Frost tolerance (documented): Brian Kemble’s records from the Ruth Bancroft Garden (Walnut Creek, California, USDA zone 9b) document survival at 20 °F (–6.7 °C), consistent with the habitat minimum of 22 °F (–5.6 °C) recorded across its South African range.
- Lethal threshold: Sustained exposure below approximately 15 °F (–9 °C) is likely fatal, though some growers have reported survival at slightly lower temperatures for very brief periods with excellent drainage and dry conditions.
The key variable is moisture: Aloe marlothii tolerates dry frost far better than cold combined with wet soil. In its native Highveld habitat, winter frosts occur during the dry season when soils are desiccated and the plant’s leaves are partially dehydrated, reducing ice crystal damage.
Substrate
The species thrives in a wide range of soil types — sandy loam, clay loam, rocky rubble — provided drainage is excellent. It naturally grows in thin, mineral soils over rock, and in cultivation performs best in a mix of approximately 60% mineral aggregate (crushed stone, pumice, coarse river sand) and 40% organic compost. pH tolerance is broad: slightly acidic to alkaline (6.0 to 8.5). In regions with heavy clay soils, raised beds or rockeries are essential.
Watering
Aloe marlothii is adapted to a summer-rainfall pattern: deep, infrequent watering during the warm growing season (spring through autumn) and minimal to no supplemental irrigation in winter. In Mediterranean climates, this means the natural rainfall pattern is reversed relative to the species’ native cycle, which can cause problems. Growers in winter-rainfall regions (southern California, Mediterranean Europe) should provide rain protection or ensure extremely fast-draining substrates to prevent winter rot.
During summer, water deeply once every 10 to 14 days. A sign of overwatering is over-inflated, pale, glossy leaves; excessive overwatering will lead to root and stem rot.
Fertilization
Minimal fertilization is required. In the wild, the species grows in nutrient-poor soils. An annual application of slow-release balanced fertilizer (NPK 14-14-14) or bone meal at the base of the plant in early spring is sufficient. Avoid high-nitrogen feeds, which promote soft, leggy growth vulnerable to cold damage.
Hardiness Zone
USDA zones 9a to 11b for reliable year-round outdoor cultivation. In zone 9a (average annual minimum –6.7 to –3.9 °C), the species survives with some leaf damage during the coldest winters but recovers well. In zone 8b (–9.4 to –6.7 °C), outdoor cultivation is marginal and requires protected microclimates, excellent drainage, and ideally overhead rain shelter in winter.
Success and Failure Under Temperate Climates: What the Growers Report
The Brian Kemble Dataset
Brian Kemble, curator of the Ruth Bancroft Garden in Walnut Creek, California (USDA zone 9b), has compiled one of the most rigorous documented datasets on aloe cold hardiness. His records show Aloe marlothii surviving 20 °F (–6.7 °C) in cultivation, with a habitat minimum of 22 °F (–5.6 °C). This places it in the upper tier of cold-tolerant tree aloes — comparable to Aloe ferox, Aloe arborescens, and Aloe striata, and significantly hardier than Aloe vera (minimum approximately 40 °F / 4 °C).
Forum Reports and Grower Experiences
Agaveville forum — Sonoma, California (zone 9b). A grower reported significant leaf damage and near-total loss of Aloe marlothii after a mild winter with lows breaching 30 °F (–1 °C) only a few times. The critical factor was not the absolute temperature but the combination of cold and moisture: the plants were outdoors with inadequate rain protection. The same grower noted that Aloe ferox, Aloe pilansii, Aloe plicatilis, Aloe rupestris, and Aloe ‘Hercules’ in comparable or less protected situations fared much better. Another Agaveville member responded that their Aloe marlothii survived mid-20s °F (approximately –4 °C) undamaged, but their plants were in extremely well-drained raised beds against a south-facing wall. This discrepancy is instructive: it confirms that drainage and winter dryness are as important as absolute temperature for this species.
Southern California (zones 10a–10b). Aloe marlothii is widely grown in the ground in the greater Los Angeles, San Diego, and Central Coast areas, where it performs spectacularly, reaching full arborescent stature (3 to 5 m) within 15 to 20 years. It is promoted as a flagship species for low-water, fire-resistant landscaping, alongside agaves and other succulents. Winter lows in these zones rarely drop below 30 °F (–1 °C), and the species has no issues.
Mediterranean Europe (southern Spain, southern Portugal, Côte d’Azur, Ligurian Riviera, Sicily). Aloe marlothii is increasingly grown in Mediterranean gardens where winter minima typically range from –2 to +5 °C (zones 9b–10a). Success is good in these regions, provided drainage is excellent. The main challenge is not cold but winter rain: the combination of cool temperatures (5 to 10 °C) and persistent soil moisture promotes root rot far more than a brief dry frost at –5 °C. Growers along the French Riviera report that positioning plants on sloping ground with a southern exposure and mineral substrate effectively eliminates this risk.
Northern Europe (UK, northern France, Germany, Netherlands). Outdoor cultivation year-round is not feasible in these regions, where winter minima regularly reach –8 to –15 °C and winter rainfall is persistent. Container culture with winter shelter (unheated greenhouse, frost-free conservatory) is the standard approach. UK growers on Hardy Tropicals UK forums note that Aloe marlothii can survive brief excursions to –5 °C under temporary fleece protection, but only if the rootball is bone-dry. Prolonged exposure below –3 °C with any soil moisture is fatal.
Eastern Australia (Sydney, Brisbane, Perth). Aloe marlothii thrives in Australian gardens in USDA zones 9b–11, where the summer-rainfall pattern of eastern Australia closely mirrors its native South African climate. It is widely available in Australian nurseries and grows rapidly in full sun with minimal care.
Practical Recommendations for Temperate Growers
- Zones 9b–11: Plant in the ground in full sun, in well-drained mineral substrate, ideally on a south-facing slope or against a thermal mass (stone wall). No winter protection needed unless exceptional cold events (below –7 °C) are forecast, in which case temporary fleece wrapping of the rosette is sufficient.
- Zone 9a: Possible in the ground with careful site selection: south-facing, elevated, excellent drainage, winter rain protection (eaves, overhang, or temporary plastic sheeting). Expect some leaf tip damage in hard winters.
- Zone 8b and colder: Container culture only. Use a large terracotta or concrete pot with mineral substrate. Overwinter in an unheated but frost-free greenhouse or bright conservatory (minimum 2 to 5 °C). Water sparingly, once monthly at most.
- Winter-rainfall climates (Mediterranean type): The reversed rainfall pattern is the main risk. Ensure substrates contain at least 60% mineral aggregate and elevate containers on feet to improve basal drainage. Avoid irrigating from November to March.
- Growth rate expectations: Aloe marlothii is a slow to moderate grower. Expect a plant to reach 1 m stem height in approximately 8 to 12 years from seed under favorable outdoor conditions (zones 10–11). Trunk formation begins once the plant produces a rosette of approximately 60 to 80 cm diameter.
Ethnobotanical and Medicinal Uses
Aloe marlothii has a long history of ethnomedicinal use among indigenous southern African communities. The Zulu used leaf decoctions as a vermifuge against roundworm and tapeworm. Fresh leaf sap was applied to women’s breasts as a weaning aid. The spiny leaves were used by the Ndebele to scrape animal hides in the preparation of traditional garments, and ash from dried leaves served as an additive to snuff.
Pharmacologically, the leaf exudate contains aloin and related anthraquinone glycosides, similar to (but chemically distinct from) those in Aloe ferox — the commercial source of “Cape aloes” drug. Research has explored Aloe marlothii extracts for dermal permeation enhancement, anti-erythema activity, and anti-tick treatments for livestock, with promising results in some studies.
The species is non-toxic to humans but may be mildly to moderately toxic to pets (cats and dogs) if leaf material is ingested, as with most Aloe species containing aloin.
Bibliography
Berger, A. (1905). “Aloe marlothii.” In Engler, A. (ed.), Botanische Jahrbücher für Systematik 36: 62.
Carter, S., Lavranos, J.J., Newton, L.E. & Walker, C.C. (2011). Aloes. The Definitive Guide. Kew Publishing, Royal Botanic Gardens, Kew. 720 pp.
Grace, O.M., Klopper, R.R., Figueiredo, E. & Smith, G.F. (2011). The Aloe Names Book. Strelitzia 28. South African National Biodiversity Institute, Pretoria. 232 pp.
Grace, O.M., Buerki, S., Symonds, M.R.E. et al. (2015). “Evolutionary history and leaf succulence as explanations for medicinal use in aloes and the global popularity of Aloe vera.” BMC Evolutionary Biology 15: 29.
Kemble, B. (undated). “Brian Kemble’s List of Hardy Aloes.” Ruth Bancroft Garden / San Marcos Growers. Published online: smgrowers.com/info/brian_aloe.pdf
Mtshali, H. & von Staden, L. (2018). “Aloe marlothii A.Berger subsp. marlothii.” National Assessment: Red List of South African Plants.
Reynolds, G.W. (1950). The Aloes of South Africa. Aloes of South Africa Book Fund, Johannesburg. 520 pp.
Reynolds, G.W. (1966). The Aloes of Tropical Africa and Madagascar. The Aloes Book Fund, Mbabane. 537 pp.
Van Wyk, B.-E. & Smith, G.F. (2014). Guide to the Aloes of South Africa. 3rd ed. Briza Publications, Pretoria. 376 pp.
Cousins, S.R. & Witkowski, E.T.F. (2012). “African aloe ecology: A review.” Journal of Arid Environments 85: 1–17.
Klopper, R.R., Crouch, N.R. et al. (2020). “A synoptic review of the aloes (Asphodelaceae, Alooideae) of KwaZulu-Natal.” PhytoKeys 142: 1–88.
Authoritative Online Resources
- POWO — Plants of the World Online (Kew): Aloe marlothii
- PlantZAfrica (SANBI): Aloe marlothii species profile
- TreeSA: Aloe marlothii tree profile
- GBIF — Global Biodiversity Information Facility: Aloe marlothii distribution data
- CITES Checklist: Aloe trade regulations
- Brian Kemble’s Hardy Aloe List (San Marcos Growers / Ruth Bancroft Garden): PDF
- Kew Science — Aloe research portal: Royal Botanic Gardens, Kew
- Gardenia.net — Cold-Hardy Aloes: Landscaping guide