Senna bicapsularis – Senna Bicapsularis
Senna bicapsularis (L.) Roxb., formerly described as Cassia bicapsularis L., is a fast-growing, deciduous shrub or small tree in the legume family (Fabaceae), subfamily Caesalpinioideae. The species was described by Linnaeus in 1753 in the genus Cassia, and in 1832 William Roxburgh transferred it to the genus Senna; this combination is now accepted by Plants of the World Online and most taxonomic databases, although the older name Cassia bicapsularis remains widely used in the horticultural trade. Other synonyms include Adipera bicapsularis, Cathartocarpus bicapsularis and Diallobus bicapsularis.
The plant is valued in subtropical horticulture primarily for its abundant, golden-yellow flowering in autumn and winter, hence the English names winter cassia, Christmas bush, Christmas senna, money bush and yellow candlewood. Because its leaves serve as food for caterpillars of butterflies in the genus Phoebis, in North America it is also known as butterfly cassia or butterfly bush. The genus name Senna comes from the Arabic word sanā, meaning plants with laxative properties, while the epithet bicapsularis (Latin for “two-capsuled”) refers to the structure of the pod, whose interior is divided by a partition into two rows of seed chambers.
Botanical description and growth habit
Senna bicapsularis usually branches just above ground level and forms a broad, low crown with a loose, irregular habit. The shoots may be upright, but are often slender and arch downward or lean on neighboring shrubs, so in the wild the plant may resemble a climbing shrub. Mature specimens most often reach 2.5–4 m in height, although the literature gives a range of 2–12 m; the upper values refer to specimens growing in forests with support from other trees. The crown typically spreads 2–3 m in cultivation. The wood is hard, with dark brown heartwood, and crushed leaves give off a fairly strong, unpleasant odor.
The leaves are alternate, paripinnate, 2.5–9 cm long, and composed of three to five pairs of leaflets (most often three or four). The leaflets are obovate, rounded at the apex, 1.6–4.5 cm long and 1.1–2.3 cm wide, dark green on the upper surface and slightly paler beneath. As in many other legumes, the leaflets exhibit sleep movements: in the evening they fold upward in pairs, then spread open again in the morning. A distinctive gland is located between the lowest pair of leaflets; it is an important diagnostic feature distinguishing this species from the related and frequently confused Senna pendula.
Growth rate and development
This species is characterized by exceptionally rapid growth, typical of pioneer plants in disturbed habitats. In warm climates, young plants reach a height of 1.8–2.5 m within 6–10 months, and the trunk diameter at the base may exceed 3–4 cm after the first season. The shoots become woody quickly, but the crown retains a loose, spreading structure for many years and, without pruning, tends to collapse under the weight of the inflorescences and pods.
Senna bicapsularis is relatively short-lived for a woody shrub, but it has remarkable regenerative capacity: after pruning, and even after the entire above-ground part has been killed by frost, it regrows from the base and blooms again as early as the following season. In cooler subtropical regions, gardeners sometimes treat it almost like a perennial, cutting frost-damaged shoots back to the ground in spring. When grown in containers in Europe, its growth rate is significantly lower, and the plant's size can conveniently be limited to 1–2 m.
Flowering and propagation
The flowers are borne in short racemes that emerge from the leaf axils at the ends of the shoots, making the flowering plant look as though it is covered in gold. Each flower is 12–16 mm long, with five unequal, bright yellow petals and prominent, long, curved stamens that give it a somewhat orchid-like appearance. The flowering season begins in autumn and lasts until the beginning of winter; in subtropical North America, it occurs from October to January, while in the tropics the plant may flower in several flushes throughout the year.
The flowers are pollinated by insects, mainly bees, and are readily visited by butterflies. In the Americas, Senna bicapsularis is a host plant for caterpillars of butterflies in the genus Phoebis, including the common Phoebis sennae, whose caterpillars feed on both the leaves and the flowers. The fruit is a narrow, cylindrical, browning pod up to about 15 cm long, divided internally by transverse partitions; the seeds are embedded in sweetish pulp with a flavor reminiscent of tamarind. In warm climates, the plant self-seeds, and gardeners also propagate it from semi-hardwood stem cuttings.
Environmental requirements and tolerance
The species' natural range covers tropical America: the Caribbean, Central America from Mexico to Panama, and the northern part of South America, from Colombia and Venezuela through the Guianas to Ecuador, Peru, and Bolivia. There it grows in moist thickets, along forest edges, in secondary forests, along streams and rivers, on hillsides, and in disturbed habitats, from lowlands up to approximately 1500 m above sea level. According to POWO, the species is associated primarily with the seasonally dry tropical forest biome, which explains its good tolerance of periodic drought.
Senna bicapsularis requires full sun and flowers poorly or not at all when light is insufficient. It tolerates a wide range of soils, from clay through sandy to humus-rich, provided they are well drained; it prefers acidic to neutral soil. Unlike many leguminous plants, it does not form a symbiosis with root nodule bacteria and does not fix atmospheric nitrogen. Its frost hardiness is limited: leaves and young shoots are damaged by temperatures below approximately -2 °C, while the root system survives in the ground in USDA zones 9–11; in zone 8, the plant is sometimes grown with some risk, regrowing from the base after winter.
Cultivation in a temperate climate
In cooler regions of Europe, Senna bicapsularis is grown as a container plant. In summer, the pot should be placed in the warmest and sunniest spot on the terrace or in the garden, preferably against a south-facing wall. Prepare the growing medium from universal potting soil mixed with coarse sand or perlite, and place a drainage layer at the bottom of the container. Water moderately, allowing the top layer to dry out between waterings; on hot days, water requirements increase, but water left standing in the saucer is more harmful than brief drying out. From spring to the end of summer, fertilize every 3–4 weeks with a balanced fertilizer at half the recommended dose.
Before the first frosts, move the plant to a bright, cool room with a temperature of 5–12 °C, such as a conservatory, bright greenhouse or unheated stairwell. Flowering often occurs during this period, so provide as much light as possible. In winter, reduce watering to a minimum, and the loss of some leaves is natural. In spring, after the last frosts, gradually harden off the plant and move it outdoors. Prune after flowering: shortening the shoots by one third maintains a compact habit and encourages branching, while any damaged sections should be removed back to healthy wood.
Uses in ornamental gardening
In regions with mild winters, from Florida and Texas to Australia and southern Europe, Senna bicapsularis is planted as a specimen, in informal flowering hedges and in butterfly gardens. Its greatest advantage is its flowering period: golden flowers appear when most shrubs have already finished flowering, providing a valuable autumn source of pollen and nectar. In a temperate climate, it serves as a terrace and conservatory plant, combining well with other species from warm regions, such as oleanders, lantanas and citrus trees. Thanks to its rapid growth, an impressive specimen can be obtained within one or two seasons.
It is worth remembering that in many tropical and subtropical areas, the species has become naturalized and is considered invasive, including in eastern Africa, the Galápagos Islands, New Caledonia and parts of Australia. In the temperate climate of Europe, this risk does not occur because the plant does not survive the winter in the ground; however, in Mediterranean regions, planting it near natural areas should be avoided.
Cultural significance, history and uses
The species has been used in the folk medicine of Latin America since pre-Columbian times; the Nahua people used it to treat skin diseases. An infusion of the leaves was used to wash rashes, wounds, bites and fungal lesions, while the leaves, like those of other species in the genus, have a laxative effect. In local cuisine, young leaves were sometimes cooked as a vegetable, the sweet pulp of the pods was eaten raw, and roasted seeds were used as a coffee substitute. The hard wood was used as fuel and for small items, while the shrub itself was planted as a hedge and screen. In Europe, the plant has been known since the 18th century as an orangery curiosity, and for several decades it has increasingly appeared in nursery offerings as a patio shrub.
Who this species is for
Senna bicapsularis is a good choice for people looking for a fast-growing, spectacularly flowering container plant and who have a bright, cool place for overwintering. It will appeal to exotic plant enthusiasts, gardeners with a winter garden or greenhouse, and anyone wanting to enrich their patio with flowers that appear in autumn. Forgiving and tolerant of drought and pruning, it is also suitable for beginners, provided it is given full sun and protection from frost. However, it is not a plant for gardeners expecting a shrub that can overwinter outdoors in the ground in a temperate climate.
Summary
Senna bicapsularis, better known as Cassia bicapsularis, combines tropical exuberance with modest requirements: it grows quickly, tolerates drought and pruning, and in autumn becomes covered with golden flowers that attract bees and butterflies. In the temperate climate of Europe, it is grown as a potted plant overwintered in a cool, bright room. The species is not listed as threatened in the IUCN Red List; in databases such as NatureServe and Useful Tropical Plants, it is classified as secure / least concern, while in many tropical regions it is even considered an invasive plant.
