Aloe arborescens is arguably the most widely cultivated aloe in the world after Aloe vera, yet it could hardly be more different from its famous cousin. Where Aloe vera is a solitary, stemless rosette adapted to Arabian desert heat, Aloe arborescens is a multi-stemmed, sprawling, densely branching shrub that can grow into impenetrable thickets three meters high across the cliff faces and rocky ridges of southeastern Africa. Its common name — krantz aloe, from the Afrikaans krans (cliff) — describes its quintessential habitat: the species thrives on exposed rock outcrops, from sea level to alpine summits, across one of the broadest ecological and elevational ranges of any species in the genus Aloe.
For gardeners, Aloe arborescens offers a unique combination of qualities: vigorous growth, exceptional tolerance of both wet and dry conditions, winter-blooming scarlet flowers that attract hummingbirds and sunbirds, easy vegetative propagation from cuttings, and a cold hardiness that places it among the toughest aloes — though not quite as invulnerable as its reputation sometimes suggests. Its geographic variability has given rise to markedly different forms, from compact cliff-dwellers with bicolored flowers to massive garden specimens with pure red torches, and its readiness to hybridize has produced some of the most spectacular aloe crosses in cultivation. This article explores the full spectrum of that variation, examines the species’ taxonomy and ecology, and provides detailed cultivation guidance based on documented grower experience.
Family: Asphodelaceae Subfamily: Asphodeloideae Genus: Aloe Accepted name (POWO): Aloe arborescens Mill., The Gardeners Dictionary, ed. 8 (1768) Accepted subspecies (POWO): Aloe arborescens subsp. arborescens; Aloe arborescens subsp. mzimnyati van Jaarsv. & A.E.van Wyk Selected synonyms: Aloe arborea Medik. (1784), Aloe frutescens Salm-Dyck, Aloe fruticosa Lam., Aloe mutabilis Pillans, Aloe natalensis J.M.Wood & M.S.Evans Common names: Krantz Aloe, Torch Aloe, Candelabra Aloe, Tree Aloe; kransaalwyn (Afrikaans); ikalene (Zulu); inkalane (Xhosa)
Taxonomy and Nomenclature
Aloe arborescens was described by Philip Miller in 1768. The epithet arborescens (Latin: “becoming tree-like”) is somewhat misleading — the species is not truly arborescent in the manner of Aloe ferox or Aloe marlothii but rather a large, multi-stemmed, freely branching shrub. Despite this, it has been assigned a national tree number (28.1) in the South African Trees List.
Taxonomically, Aloe arborescens forms part of the Arborescentes series of very closely related species, alongside Aloe pluridens Haw. (French Aloe) and, until recently, Aloe mutabilis Pillans. The species is remarkably variable across its vast range, and several infraspecific taxa were described in the early 20th century by Alwin Berger, none of which are currently recognized by POWO:
- Aloe arborescens var. frutescens (Salm-Dyck) Link (1821)
- Aloe arborescens var. milleri A.Berger (1908)
- Aloe arborescens var. natalensis (J.M.Wood & M.S.Evans) A.Berger (1908)
- Aloe arborescens var. pachystyrsa A.Berger (1908)
- Aloe arborescens var. viridifolia A.Berger (1908)
These names reflect genuine morphological variation across the species’ range but are currently treated as part of the normal variability of the typical subspecies.
The Two Accepted Subspecies
POWO recognizes two subspecies:
Subsp. arborescens is the widespread, common form distributed from the Cape Peninsula to Malawi. It is a large, freely branching shrub up to 2 to 3 m tall (occasionally higher), with dense rosettes of grey-green, recurved, toothed leaves and erect, conical racemes of scarlet to orange-red flowers. This is the plant known to millions of gardeners worldwide.
Subsp. mzimnyati van Jaarsv. & A.E.van Wyk is a geographically restricted endemic of KwaZulu-Natal, described from the Mzimnyati (Buffalo) River valley. It differs from the typical subspecies in its smaller stature, fewer branches, and localized distribution. It is rarely encountered in cultivation.
The Aloe mutabilis Question
The species formerly known as Aloe mutabilis Pillans — a cliff-dwelling form with smaller, less branched rosettes and distinctive bicolored (red-and-yellow) flower spikes — is now included within the synonymy of Aloe arborescens. This form is most evident on the high inland plateau of the northern provinces of South Africa (Limpopo, Gauteng, North-West) and can be seen dramatically draped over cliffs at sites such as the Walter Sisulu National Botanical Garden near Johannesburg and along the Waterberg escarpment. In the nursery trade, it is still sometimes offered under the name Aloe mutabilis or as Aloe arborescens ‘Mutabilis’. Horticulturally, this form is notable for its more compact habit, somewhat greater cold tolerance (it thrives on the frost-prone Highveld), and its striking bicolored inflorescence — a combination of qualities that makes it arguably more garden-worthy than some of the larger coastal forms.
Geographic Variation and Range
Native Distribution
Aloe arborescens has the third widest distribution of any aloe species, exceeded only by Aloe maculata and Aloe greatheadii. Its native range spans five countries in southeastern Africa:
- South Africa: from the Cape Peninsula (its southwestern limit) along the entire eastern seaboard through the Eastern Cape, KwaZulu-Natal, Mpumalanga, Limpopo, and Gauteng, and marginally into the Free State and North-West.
- Eswatini (Swaziland)
- Mozambique (southern and central coastal areas)
- Zimbabwe (eastern highlands)
- Malawi (southern mountains, including Mount Mulanje — the northernmost natural limit)
- Botswana (marginally, eastern border)
The species occurs from sea level to over 2,500 m — an extraordinary elevational amplitude that few other aloes match. Coastal populations grow in salt-sprayed, subtropical conditions; montane populations endure regular frost and occasional snow. This ecological breadth is the source of the species’ remarkable variability and its exceptional adaptability in cultivation.
Naturalization Worldwide
Aloe arborescens has become naturalized far beyond its native range, often persisting in areas where it was planted as a living fence or ornamental and subsequently spreading by vegetative fragmentation. Naturalized populations are documented in the western Mediterranean (southern France, Italy, Spain, Portugal, Sicilia, Baleares), the Macaronesian islands (Canary Islands, Madeira, Azores), North Africa (Algeria, Morocco, Tunisia), Australia (New South Wales, South Australia, New Zealand), California, Mexico, South Korea, and Japan. In Japan, where the species was first introduced in the 17th century, it has become fully naturalized along the southern coast and is widely cultivated both as an ornamental and as a medicinal and food plant — a cultural significance it holds nowhere else in the world.
Key Geographic Forms
Although POWO does not currently recognize formal varieties within subsp. arborescens, the following geographic forms are horticulturally distinct and may be encountered under informal cultivar names:
Coastal Eastern Cape form. Large, vigorously branching plants with bright green leaves, forming massive thickets in frost-free coastal habitats. Flowers are typically uniform scarlet. This is the form most commonly grown in Mediterranean gardens and the one from which most nursery stock derives.
Montane Highveld form (ex-Aloe mutabilis). Compact, less branching, with glaucous blue-green leaves and bicolored (red-and-yellow or red-and-cream) racemes. More frost-tolerant than coastal forms owing to its adaptation to the cold, dry Highveld winters of Johannesburg and Pretoria (where regular frosts of –5 to –8 °C occur). This form is increasingly valued by growers in marginal climates.
Yellow-flowering form. Scattered populations and individual clones with pure yellow or golden-yellow flowers occur across the range. These are not formally named but are propagated and sold by collectors. The yellow form tends to have more narrow, elegant rosettes than the typical red-flowering plant.
KwaZulu-Natal midlands form. Tall, somewhat open plants with large rosettes and long, arching leaves, intermediate between the coastal and montane forms. Often found in association with mist-belt grasslands and forest margins at 800 to 1,500 m.
Hybrids: Natural and Horticultural
Aloe arborescens is one of the most prolific hybridizers in the genus. It is not self-fertile (a characteristic shared by nearly all aloes), meaning a single isolated plant will not set seed; two genetically distinct plants must cross-pollinate. When other Aloe species flower simultaneously in proximity — which is common in both the wild and in multi-species garden collections — hybridization is virtually inevitable. Sunbirds, honeybees, and hummingbirds (in the Americas) are all effective at transferring pollen between species.
Notable Natural Hybrids
Aloe × principis (Haw.) Stearn — the most celebrated natural aloe hybrid. This cross between Aloe arborescens and Aloe ferox occurs wherever the two parent species coexist, particularly in the Eastern Cape. It produces a vigorous, semi-shrubby plant intermediate in habit: multi-stemmed like Aloe arborescens but with larger, more fiercely armed rosettes and taller inflorescences than either parent. Flowers are brilliant scarlet to orange, densely packed on erect racemes. Aloe × principis displays classic hybrid vigor (heterosis): it grows faster, flowers more prolifically, and may be slightly hardier than either parent. It is widely cultivated in Mediterranean and California gardens and is sometimes mislabeled as Aloe arborescens in the nursery trade. Note that the parentage is variable: each cross produces a unique individual, so the name Aloe × principis encompasses a range of forms.
Aloe arborescens × Aloe maculata: natural hybrids between these two widespread, sympatric species are common in the Eastern Cape and KwaZulu-Natal. The resulting plants combine the shrubby habit of Aloe arborescens with the spotted leaves and capitate inflorescences of Aloe maculata, producing attractive but taxonomically confusing intermediates.
Aloe arborescens × Aloe marlothii: documented in areas of overlap in northern KwaZulu-Natal and Mpumalanga. These hybrids tend to be taller and more arborescent than typical Aloe arborescens, with sparser branching and larger rosettes.
Selected Horticultural Hybrids and Cultivars
‘Variegata’ (syn. Aloe arborescens f. variegata) — the most widely grown cultivar, bearing green leaves with creamy yellow margins or, in some clones, predominantly cream leaves with green stripes. Awarded the Royal Horticultural Society’s Award of Garden Merit (AGM). Slightly less vigorous than the wild type and more sensitive to strong sun (the variegated tissue lacks chlorophyll and scorches easily). Prefers light afternoon shade in hot climates.
‘Gold Rush’ — a variegated cultivar similar to ‘Variegata’ but with broader cream-yellow sectors. Less commonly available.
‘Tangerine’ — a named hybrid between Aloe arborescens and Aloe ferox, selected for its true orange (rather than scarlet) flowers. More compact and upright than typical Aloe × principis forms.
Yellow-flowering selections — sold under various informal names or simply as “Aloe arborescens yellow.” These reproduce true from cuttings but not necessarily from seed (since open-pollinated seed may revert to red-flowered offspring or produce hybrids).
Aloe nobilis — a plant of uncertain origin, possibly a man-made hybrid between Aloe perfoliata and Aloe arborescens, that has been treated at various times as a species, a hybrid, and a cultivar. It forms compact, golden-toothed rosettes and is very commonly sold in garden centers.
Morphological Description
Aloe arborescens is a large, multi-stemmed, freely branching, evergreen succulent shrub, typically reaching 2 to 3 m in height and spreading by basal suckering and branch rooting to form dense, impenetrable clumps several meters across.
Stems. Multiple erect to sprawling stems arise from the base, branching freely above and below. Stems are woody, smooth when old (dead leaves are generally shed, unlike the persistent skirt of Aloe ferox or Aloe marlothii), and can become procumbent, rooting where they contact soil — a trait that facilitates natural spread and makes vegetative propagation exceptionally easy.
Rosette and leaves. Each branch terminates in a compact rosette of 20 to 30 leaves. Leaves are lanceolate, 50 to 60 cm long and approximately 5 to 7 cm wide, narrower and more strongly recurved than those of Aloe vera. Leaf color ranges from bright green (coastal forms, well-watered) to grey-green or blue-green (montane forms, drought-stressed), sometimes with a yellowish tinge. Margins are armed with small, pale, triangular teeth spaced approximately 3 to 5 mm apart — much finer and less formidable than the teeth of Aloe ferox. Leaf surfaces are smooth and unspotted.
Inflorescence and flowers. Each rosette produces a single (rarely branched) erect inflorescence bearing a dense, conical raceme 20 to 30 cm long. In well-established clumps, dozens of rosettes flower simultaneously, producing a spectacular massed display. Flowers are tubular, 3.5 to 4 cm long, and typically brilliant scarlet to orange-red. Stamens and style protrude modestly from the corolla mouth. The bicolored form (ex-Aloe mutabilis) displays flowers that open yellow from red buds, creating a two-toned effect on the same raceme.
Flowering period in the Southern Hemisphere is May to August (late autumn to winter). In the Northern Hemisphere, flowering shifts to November to February, with peak bloom in December and January — making Aloe arborescens one of the most valuable winter-flowering ornamental plants for mild climates.
Fruit and seeds. Dry, dehiscent capsules containing flat, winged seeds. Germination is straightforward but slow compared to vegetative propagation; stem cuttings root in weeks, while seed-grown plants require 3 to 5 years to reach flowering size.
Comparison with Two Related Species
Aloe arborescens vs. Aloe vera (L.) Burm.f.
The general public frequently confuses these two species, both of which are sold as “medicinal aloes.” In reality, they differ in almost every respect:
| Character | Aloe arborescens | Aloe vera |
|---|---|---|
| Growth habit | Multi-stemmed branching shrub, up to 3 m | Stemless solitary rosette, 0.6–1 m |
| Leaf shape | Narrow, strongly recurved, 50–60 cm | Broad, erect, 30–60 cm |
| Leaf color | Bright green to grey-green, never spotted | Grey-green, often with white flecks |
| Flower color | Scarlet to orange-red (rarely yellow) | Yellow (never red) |
| Inflorescence | Simple conical raceme | Simple raceme |
| Propagation | Very easy from stem cuttings; prolific suckering | Offsets (pups) from base; no stem cuttings |
| Cold hardiness (Kemble) | 19 °F (–7.2 °C); killed below this | ~40 °F (+4 °C); killed at 32 °F (0 °C) |
| Wet tolerance | Excellent — one of the most wet-tolerant aloes | Poor — highly sensitive to wet cold |
| Gel content | Lower gel volume per leaf; higher aloin concentration | Higher gel volume; lower aloin concentration |
| Commercial use | Medicinal (Japan, Italy, Russia); ornamental | Cosmetics and health products globally |
The critical distinction for gardeners: Aloe arborescens is dramatically hardier, more vigorous, and more tolerant of adverse conditions, but it lacks the thick gel that makes Aloe vera commercially dominant.
Aloe arborescens vs. Aloe pluridens Haw. (French Aloe)
Aloe pluridens is the closest taxonomic relative and the species most likely to be confused by knowledgeable collectors. Both are multi-stemmed, branching aloes with recurved leaves, but they differ in several key respects:
| Character | Aloe arborescens | Aloe pluridens |
|---|---|---|
| Branching pattern | Freely branching from base and above; dense shrub | Less branching; tends toward a single or few tall stems |
| Mature height | 2–3 m (shrub) | 3–5 m (more arborescent) |
| Leaf width | 5–7 cm | 3–5 cm (narrower, more grass-like) |
| Leaf teeth | Small, pale, closely spaced | Very fine, almost hair-like (hence pluridens = “many teeth”) |
| Flower color | Scarlet to orange-red | Salmon-pink to dull orange |
| Distribution | Very wide (Cape to Malawi) | Eastern Cape coastal belt only |
| Cold hardiness | Good | Moderate (slightly less hardy) |
In the garden, the two species can be distinguished primarily by the shrubby, dense, sprawling habit of Aloe arborescens versus the taller, more upright, tree-like form of Aloe pluridens.
Optimal Growing Conditions
Light
Aloe arborescens is unusually flexible regarding light. It performs best in full sun, where leaf color is compact and grey-green and flowering is prolific. However, it tolerates semi-shade (light woodland, north side of buildings in the Southern Hemisphere, or east-facing exposures) far better than most aloes — coastal populations in its native range sometimes grow in the understory of coastal forest. In deep shade, plants become etiolated and rarely flower, but survive. Variegated cultivars (‘Variegata’, ‘Gold Rush’) benefit from afternoon shade in hot climates to prevent scorching of the cream-colored leaf tissue.
Temperature
Optimal growth occurs between 18 and 26 °C (65 to 80 °F). The species tolerates short-term heat spikes to 38 °C without difficulty. Cold tolerance is a nuanced subject:
Brian Kemble’s data (Ruth Bancroft Garden): survival at 19 °F (–7.2 °C), with the plant killed outright below this threshold. Kemble adds the critical note: “fairly hardy and very wet tolerant” — distinguishing Aloe arborescens from nearly all other aloes of comparable size, which require dry conditions to survive frost.
Agaveville forum (southern California): An experienced grower offers a more cautious assessment: “Cold hardiness is NOT its strong suit as is often advertised.” Plants subjected to annual lows of 20 °F (–6.7 °C) showed “severe damage” with flowers destroyed below 26 °F (–3.3 °C), but the plants consistently resprouted from the base by midsummer. This regenerative capacity — the ability to “come back” after being cut to the ground by frost — is a key trait of Aloe arborescens that distinguishes it from single-stemmed tree aloes, which are killed outright when the sole growing point is destroyed. A frost-damaged Aloe ferox dies; a frost-damaged Aloe arborescens resprouts.
PFAF database: killed at –1 °C if wet; tolerates occasional –4 °C if dry.
Practical synthesis: Aloe arborescens reliably survives USDA zone 9b (–3.9 to –1.1 °C) in the ground with minimal protection. In zone 9a (–6.7 to –3.9 °C), it survives most winters but may be cut to the ground during hard frosts, resprouting in spring. Below zone 9a, container culture with winter shelter is recommended. The montane Highveld form (ex-Aloe mutabilis) appears somewhat more cold-tolerant than coastal forms, though this has not been rigorously quantified.
Substrate
One of the most substrate-tolerant aloes in cultivation. Aloe arborescens grows well in sandy soils, clay soils, rocky rubble, and standard garden loam — “almost no soil is a problem for this species” (Agaveville). The only requirement is that waterlogging should not persist for weeks at a time. Unlike most aloes, it tolerates moderately wet conditions, including winter rain, without rotting — a trait linked to its natural occurrence in year-round and winter-rainfall zones in the southwestern Cape.
For optimal performance, a well-drained garden soil amended with 20 to 30% mineral aggregate (perlite, pumice, coarse sand) is ideal. In containers, a standard cactus/succulent mix works well.
Watering
Aloe arborescens accepts a wide range of watering regimes: from near-zero supplemental irrigation (relying on natural rainfall) to generous summer watering. It is drought-tolerant once established but looks better and grows faster with regular summer irrigation. Unlike Aloe marlothii or Aloe excelsa, it does not require a dry winter rest and tolerates — even thrives in — winter-wet Mediterranean climates.
Fertilization
Minimal requirements. An annual spring application of balanced fertilizer promotes vigorous growth but is not essential. Plants grown in enriched garden soil and regularly mulched can persist for decades without any supplemental feeding.
Hardiness Zone
USDA zones 9b to 11b for reliable year-round outdoor cultivation with minimal leaf damage. In zone 9a, plants may be cut to the ground by hard frosts but typically resprout from the root crown in spring. In zone 8b and below, container culture is recommended.
Propagation
Aloe arborescens is one of the easiest succulents to propagate. Three methods are commonly used:
Stem cuttings are the fastest and most reliable method. Cut a branch 20 to 40 cm long with a clean, sharp blade. Allow the cut end to dry (callus) for 3 to 7 days in shade. Plant the cutting in a free-draining mix of sand and perlite, water lightly, and keep in bright indirect light. Roots form within 2 to 4 weeks. Cuttings can be taken at any time of year but establish fastest in spring and early summer.
Basal offsets (pups) develop readily around the base of mature plants. Separate pups with roots attached using a sharp knife, allow the cut surface to callus for 48 hours, and pot up in standard succulent mix.
Seed is a viable but slow method. Sow fresh seed on the surface of a moist vermiculite or perlite mix, cover with a thin layer of fine grit, and maintain at 22 to 28 °C. Germination occurs within 1 to 3 weeks. Seedling growth is slow, and plants take 3 to 5 years to reach flowering size. Remember that open-pollinated seed from garden plants is very likely to produce hybrids rather than true Aloe arborescens, especially if other aloe species are flowering in the vicinity.
Medicinal and Cultural Significance
Aloe arborescens has a rich history of medicinal use that predates and, in some regions, rivals that of Aloe vera. Research has shown that its leaf gel and exudate contain higher concentrations of several active compounds (aloin, aloctin A and B, acemannan-like polysaccharides) than Aloe vera, although in lower total volume due to the narrower leaves.
In South Africa, the Zulu traditionally use dried, powdered leaves as a storm charm and leaf decoctions to ease childbirth. In the Transkei, it is employed against stomach ache and as a poultry remedy. Throughout rural southern Africa, it is planted around kraals (livestock enclosures) as a living fence — abandoned homesteads more than a century old can still be traced by the persisting aloe hedges.
In Japan, Aloe arborescens (known as kidachi aloe) is cultivated commercially as both a medicinal plant and a food ingredient. The leaves are sold fresh as a vegetable and processed into candies, yogurt flavoring, and over-the-counter health supplements. Japan is probably the largest producer of Aloe arborescens, though production is for the domestic market. The species attracted global attention when its gel was used to treat radiation burn victims after the atomic bombings of Hiroshima and Nagasaki — an episode that catalyzed Western interest in aloe therapeutics.
In Italy, commercial cultivation for medicinal and cosmetic use has been established in recent decades, and the species is widely naturalized along the Ligurian and Tyrrhenian coasts.
In Russia, Aloe arborescens (known as stolietnik, literally “centenarian”) is one of the most popular medicinal houseplants, grown on windowsills across the country and used for a wide range of folk remedies.
Pests and Diseases
Aloe arborescens is generally robust but susceptible to several issues:
Aloe mite (Aceria aloinis) is the most serious pest. This microscopic eriophyid mite causes cancerous, bumpy galls (aloe cancer) on leaves, stems, and especially flower buds. Infestations are difficult to control because the mites feed within the plant tissue, protected from surface-applied pesticides. Affected parts should be cut out and disposed of immediately (not composted). The species appears particularly prone to aloe mite — collections at several Los Angeles arboreta show severe, widespread infestation.
Mealybugs and scale insects colonize leaf bases and stem axils. Treat with horticultural oil, neem oil, or systemic insecticide (imidacloprid) for severe infestations. Ants farming mealybug colonies should be controlled at the source.
Aloe rust (Phakopsora pachyrhizi or related fungi) causes brown, corky leaf lesions, particularly in humid, poorly ventilated conditions. The Algarve (Portugal) grower report on PlantZAfrica describes persistent rust-like brown blotching on older leaves, with some branches dying back completely. Improve air circulation, reduce overhead watering, and remove severely affected branches.
Root rot is uncommon in this species (due to its superior wet tolerance) but can occur in persistently waterlogged, heavy clay soils with poor drainage.
Bibliography
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.
Kemble, B. (undated). “Brian Kemble’s List of Hardy Aloes.” Ruth Bancroft Garden / San Marcos Growers. Published online: smgrowers.com/info/brian_aloe.pdf
Miller, P. (1768). The Gardeners Dictionary, ed. 8.
Reynolds, G.W. (1950). The Aloes of South Africa. Aloes of South Africa Book Fund, Johannesburg. 520 pp.
Smith, G.F., Klopper, R.R. & Crouch, N.R. (2008). “Aloe arborescens (Asphodelaceae: Alooideae) and CITES.” Haseltonia 14: 189–198.
Van Jaarsveldt, E.J. & Van Wyk, A.E. (2006). “Aloe arborescens subsp. mzimnyati.” Bothalia 36(1): 52–55.
Van Wyk, B.-E. & Smith, G.F. (2014). Guide to the Aloes of South Africa. 3rd ed. Briza Publications, Pretoria. 376 pp.
Authoritative Online Resources
- POWO — Plants of the World Online (Kew): Aloe arborescens subsp. arborescens
- POWO — Aloe arborescens subsp. mzimnyati: Kew
- PlantZAfrica (SANBI): Aloe arborescens species profile
- GBIF — Global Biodiversity Information Facility: Aloe arborescens distribution data
- CITES Checklist: Aloe trade regulations
- Brian Kemble’s Hardy Aloe List (San Marcos Growers / Ruth Bancroft Garden): PDF
- Gardenia.net: Aloe × principis profile
- Gardenia.net — Cold-Hardy Aloes: Landscaping guide
- Agaveville — Aloe arborescens discussion thread: Forum
- PROTA (PlantUse): Aloe arborescens full profile
- Deep Green Permaculture: Growing the Medicinal Aloe Arborescens