Boswellia

Boswellia is a genus of pachycaul, drought-adapted trees and shrubs in the torchwood family Burseraceae, native to the dry country of Arabia, tropical Africa, Socotra and the Indian subcontinent. The genus is famous as the source of frankincense — the fragrant resin tapped from the bark of several species — and increasingly popular with succulent enthusiasts, who grow young plants as swollen-based caudiciform curiosities on the windowsill and in the greenhouse.

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Description

Boswellia are woody plants rather than true stem succulents, but they share the water-storing habit that draws collectors to caudiciforms. Most species develop a thick, tapering trunk and fat, often flaking or papery bark that peels in translucent sheets, and many cling to bare rock with buttressed, disc-like bases that spread across the substrate. The wood and bark are richly aromatic, and any wound weeps a milky-to-amber resin that hardens into the fragrant tears sold as frankincense.

Leaves are pinnate (divided into leaflets), frequently clustered at the branch tips, and are shed during the dry season so the plant spends much of the year leafless — a normal, healthy deciduous rhythm rather than a sign of distress. Small five-petalled flowers in white, cream, pink or greenish tones are carried in short sprays, followed by small capsular fruits. In habitat the largest species become substantial trees; in cultivation, grown hard and pot-bound, they stay compact and show off their bark and swollen bases to good effect.

Distribution

The genus is centred on the arid tropics of the Old World. Species occur across the Horn of Africa (notably Somalia and Ethiopia), the southern Arabian Peninsula (Oman and Yemen), the island of Socotra — a hotspot of endemic, swollen-based forms — and southward into East Africa, with a separate group in India. Plants typically grow in hot, seasonally dry scrub and on rocky slopes and limestone cliffs, rooting into thin, sharply drained ground and enduring long rainless periods.

Notable species

Boswellia sacra — the classic frankincense tree of Oman and Yemen; the archetypal species of the genus.
Boswellia carteri — an African frankincense tree, often treated as closely allied to (or synonymous with) B. sacra.
Boswellia serrata — the Indian frankincense, a larger tree long used in traditional practice.
Boswellia neglecta — an East African species valued by growers for its bark and manageable size.
Boswellia socotrana — a Socotran endemic with a swollen, caudex-like base, prized as a caudiciform.
Boswellia elongata — another Socotran, growing into a stout, thick-trunked small tree.

Cultivation

Boswellia reward patience and a hands-off approach to water. Give them the brightest position you can — full sun in the growing season suits them, and strong light keeps growth stocky and the trunk swollen. Grow in a very free-draining, mostly mineral mix in a pot that drains freely, and let the medium dry out completely between soakings; see Watering for general technique.

The critical point is dormancy. These are drought-deciduous plants: as the days shorten and the leaves yellow and drop, taper off and then withhold water almost entirely, keeping them warm and dry through their leafless rest. Watering a dormant, leafless Boswellia is the surest way to rot the roots and caudex. Warmth matters year-round — they resent cold, wet conditions and should be kept well above freezing. Resume watering cautiously in spring once new leaves begin to push. Repot infrequently, handling the brittle roots gently; see Repotting.

Hobby and cultivar notes

There is little in the way of named cultivars in Boswellia; the appeal lies in the natural forms — especially the fat-based Socotran species — and in the character that individual plants develop with age. Most collector plants are grown from seed, which gives the best root system and the most attractive, tapered caudex over time. Grafting is not part of the usual hobby practice here, unlike in many stem succulents. Because wild-collected material of the rarer island endemics raises real conservation and legal concerns, seed-grown, nursery-propagated plants are the responsible choice.

Propagation

Seed is the primary and most satisfying method: sow fresh seed onto a warm, gritty, well-drained surface and keep it lightly moist until germination, then grow the seedlings on hard and bright. Fresh seed germinates far better than old, so source it as fresh as you can; see Propagation - seed. Cuttings can be attempted with some species but tend to produce a poorer, less swollen root base than seedlings — see Propagation - cuttings if you wish to experiment.

Common problems

Rot — overwhelmingly caused by watering during dormancy or using a mix that holds moisture; the caudex or roots soften and blacken.
Leaf drop out of season — usually normal deciduous behaviour, but sudden collapse of leaves paired with a soft base points to root rot.
Etiolation — too little light produces weak, drawn-out growth and a poorly developed trunk.
Pests — mealybugs and scale can shelter in the flaking bark and leaf axils; see Pests and diseases.

References

Horticultural information for growing these plants as ornamentals. Always confirm plant identification and any handling, grafting, or safety advice against authoritative sources before acting.