Rattlebox (Sesbania punicea)

Although widely used in landscaping for its pleasing flowers, leaves, and fruit, rattlebox (Sesbania punicea) is a Category 1 invasive plant in South Africa, and considered a pest species in other countries as well. Its tendency to form dense, unpassable thickets has a damaging environmental impact, especially near waterways, where it decreases water flow, character, and recreational activities.

I. Appearance and Characteristics 

Sesbania punicea (Spanish gold, rattlebox, or scarlet sesban) is an ornamental shrub with reddish-orange flowers.

The species is native to Brazil, Argentina, Paraguay and Uruguay. It has spread to other parts of South America and many coastal southern United States as well as to parts of Africa. Due to its high demand for water, this species is often found on marshy shorelines. It also forms dense thickets and thrives in disturbed areas.

The deciduous leaves are alternate and compound. Between five and 20 pairs of elliptical leaflets occur on a single stalk. The leaf margins are commonly entire, with little or no serration. Each leaflet is oblong and ends in a pointy tip. The leaves contain stipules that are usually inconspicuous. The flowers and fruits droop at the tips of the stalks. The branches of this shrub are rather thin, and green, but turn a darker red brown when they mature. The bark varies from gray brown to red brown with obvious horizontal lenticels.

The flowers are shaped like pea flowers, 2–3 cm long, and are commonly red-orange or red-purple. Flowers often form a raceme. Characteristic of the family Fabaceae, this species has five fused sepals and five free petals. The flower always contains ten stamens, sometimes with various combinations of fused filaments. The ovary is superior and the style is often curved.

Characteristic of the subfamily Faboideae, these flowers are zygomorphic and have a specialized structure. The upper petal is referred to as the banner, and encapsulates the petals when they are in the bud. The two adjacent flowers, called the wings, overlap the bottom two petals. The bottom petals are often fused at the apex, forming a keel. The flowers appear “showy” because they are most commonly pollinated by insects, who are attracted by these features.

The large pea pods are divided into four compartments and appear as if they have shrunk slightly due to drying These fruits are dehiscent and dry out as they become mature. Each fruit can contain between five and ten seeds, which are dispersed when the pod dries out and opens up. On average, a single plant can produce 100-300 pods. The immature pods are yellow or green. As the season progresses, they turn a darker green, eventually becoming dark brown. Seedpods often stay on the plant far into the winter, and the seeds rattle when shaken by the wind, giving the plant its common name of “rattlebox”.

Sesbania puniceas contains toxic saponic glycosides. The poison can be dangerous to local birds and mammals. Humans who ingest this plant or its seeds may become very sick and may suffer vomiting, diarrhea, respiratory failure, and death.

II. How to Grow and Care

Rattlebox requires a warm climate and well-drained soils to thrive. It’s particularly sensitive to frost and needs protection in cooler climates. A key care point for rattlebox includes ensuring adequate moisture without overwatering, as it is prone to root rot. Regular pruning is also essential to maintain shape and promote healthy growth, as the plant can become leggy if not managed properly.

Sunlight

Rattlebox thrives best in full sun conditions. Optimal growth, health, and flowering are promoted by long hours of direct sunlight, enhancing photosynthesis efficiency. Although rattlebox is adaptable, insufficient light may result in stunted growth and fewer blooms. To maintain vitality, outdoor cultivation should prioritize open spaces with clear exposure to sunlight. When planted in environments with less than ideal light exposure, rattlebox might exhibit reduced vigor and diminished decorative appeal. Placement decisions should consider maximizing light availability to ensure rattlebox’s robust performance.

Watering

Hailing from moist, subtropical areas, rattlebox has adapted to environments where regular hydration supports its growth. This species thrives with consistent moisture and displays a moderate tolerance for drought, preferring a stable balance of water intake. Accordingly, it benefits from watering every week to maintain its health and vigor. Given that rattlebox is predominantly an outdoor plant, it’s crucial to consider the impact of seasonal rainfall patterns on its watering schedule, especially during its active growing season when water demand peaks.

Fertilizing

For rattlebox, optimal growth requires fertilization balancing nitrogen, phosphorus, and potassium, particularly high-phosphorus during blooming for vibrant flowers. Annual spring application is typically sufficient, with adjusted quantities for young or established plants. Avoid over-fertilization to prevent growth at the expense of flowering. Seasonal growth stages dictate fertilization frequency; less in fall and winter. Use slow-release fertilizers for gradual nutrient absorption and root development. Over-application can lead to nutrient runoff harming the environment. Follow the manufacturer’s instructions for concentration to protect the rattlebox’s health.

Propagation

Rattlebox can be effectively propagated using the sowing of seeds. For successful germination, treat the seeds with a pre-sowing soak to soften the hard outer coat. Directly sow the soaked seeds in well-draining soil and ensure they are kept moist. Consistency with soil moisture and warmth is imperative for optimal growth. When seedlings are hardy enough, they should be transplanted to their permanent location, taking care to minimize root disturbance which rattlebox can be sensitive to.

Repotting

Repotting rattlebox every 2-3 years, typically in the spring to support its rapid growing season. As a sizable shrub, it requires a large, sturdy container to accommodate robust roots. Post-repotting, ensure the rattlebox’s soil is consistently moist but well-draining to prevent root rot. Select a pot one size larger, and include humus-rich soil for best results. Careful watering is key to rattlebox’s recovery and sustained health.

III. Uses and Benefits 

These plants are used as ornamentals due to their graceful shape and beautiful red flowers.

IV. How to control

The root systems of young S. punicea plants are not very extensive and the soil is loose under waterlogged conditions, so these plants can be removed by hand or simple garden tools. Trees with larger trunks can be cut and treated with glyphosate and/or triclopyr. If they are found in standing water, the stem can be cut to below the water level. Simply flooding the area will not kill them.

Biological controls have also been used to prevent this species from spreading. The South American apionid weevil, Trichapion lativentre, was introduced to South Africa in the late 1970s. This weevil has now dispersed over most of the range of S. punicea. The adult weevils feed on the leaves and lay single eggs in premature flower buds. The larvae then feed on the stamens and carpels of the flower and pupate in the hollow husks of the buds. The weevil has prevented the rapid spread of S. punicea in South Africa.