Three in one

Bluntly stated, popcorn cassia (Senna didymobotrya syn. Cassia didymobotrya) offers amazing, impactful flower contributions, textural foliage and a beguiling fragrance out in the garden! I remember my first exposure to this ornamental plant more than 15 years ago. A local nursery donated some small, potted specimens at Rotary Botanical Gardens in Janesville, Wisconsin. These plants grew quickly and were not only impressive in stature, but also in bloom. To say that these plants were an instant hit for all the features mentioned above would be an understatement and I’ve been encouraged in seeing increasing availability and use of this plant particularly over the last couple of years.

This member of the Fabaceae family grows quickly and in its native range of tropical central and eastern Africa, this legume is commonly used as a cover crop or green manure. This semi-deciduous, multi-branching shrub or small tree can get as large as 25 feet tall in its native range but is much more compact and refined in cooler locales. Although widely used as a medicinal plant in native locations, it is important to mention that all parts of this plant are poisonous which should be considered in relation to both small children and pets. This plant has also begun to show signs of being significantly aggressive or invasive in tropical climates and select locations outside of natural habitats which would encourage a population balance. Popcorn plant has naturalized in some parts of Australia and is becoming invasive in parts of Kenya, Uganda and Tanzania.

In colder areas, this plant is not of concern in terms of spreading and is appreciated for a quick growth rate, beautiful flowers, textural foliage and scent. Hardy in zones 9-11, popcorn cassia is primarily used as an annual which is how I’ve utilized it in the Midwest. Myriad common names exist for this plant with popcorn cassia likely being most recognized in the nursery trade. However, it is also called African senna, popcorn senna, candelabra tree and peanut butter cassia. (More about the fragrance later!)

The yellow flower spikes are about 12 inches tall with 20 to 30 black buds, opening from the bottom of the spike to the top. Even small plants will bloom. I’ve typically observed two strong waves of blooms in early summer, then again in late summer, although flower color is evident throughout the warmer portion of the growing season. Popcorn cassia loves temperatures over 80° F and the quickest gain in size is seen over the hottest summer months. The flower spikes are quite showy when hovering above the foliage and the contrast between the bright, golden-yellow open flowers and the “yet to open” plump, black buds is quite nice. In longer summers, you may see 5 inch long brown pods form. These pods will typically contain 16 smooth, flattened, “bean-like” seeds.

While most popcorn cassia are purchased as rooted cuttings or more established containerized plants, the seeds will germinate readily if soaked in water for 24 hours or scarified. I should mention that I personally never had success with starting the seeds! There are various ways of trying to overwinter this plant inside although I’ve been aware of only mixed results and frequent disappointments.

This plant’s pinnately compound foliage (up to 18 inches in length) is a showy, blue-green and you’ll see very few insect or disease issues. I’ve observed the foliage remaining almost entirely intact throughout the entire growing season. This leathery foliage acts as a perfect foil for the bright yellow flowers hovering in close proximity. The entire plant is left alone by deer.

While good air circulation around the plant is recommended, , consider some sheltering of the plant or extra staking as needed in areas of more significant wind. The texture of this plant combines very well with large-leaf tropicals like elephant ears (Colocasia, Alocasia, Xanthosoma), bananas and cannas.

The foliage and flowers combine so well with other plants that the popcorn plant can play an effective supplemental or starring role in the garden bed, border or container.

Full sun is required for the sturdiest plants in terms of growing conditions, as too much shade will adversely affect flowering and create a floppy, unsightly plant. The popcorn cassia thrives in deep, well-drained soil high in organic matter. Don’t plant too early as this plant won’t thrive until the soils have started to warm up sufficiently. A good rule of thumb is to plant your popcorn cassia at the same time as you would tomatoes. Supplemental fertilization will stimulate additional growth and create very robust specimens in a short amount of time. I used a balanced fertilizer (20-20-20) every three weeks over the course of the summer and was rewarded with plants (in the Midwest) that grew from 1 foot tall in late May to over 7 feet tall by September! It’s very important not to overwater this plant. While it is drought tolerant once established, popcorn cassia likes ample moisture but in well-drained situations. Heavier soils and/or heavy irrigation can lead to a quick and noticeable decline in plant health.

The fragrance of this plant will continue to be discussed and debated eternally. Scent, of course, being subjective will be perceived differently by the recipient of the scent. While popcorn cassia emits the faintest of scents, rubbing the foliage and flower buds most certainly produces a scent. The common names of popcorn cassia and peanut butter cassia have already hinted at some opinions on the scent. Personally, I felt that rubbing the un-opened flower buds and smelling your fingers produced a “buttered popcorn” scent while the foliage (also being rubbed) produced the smell of peanut butter. Opinions from visitors at the botanic garden varied, but those who engaged the plant were certainly intrigued by the scent ... I’m aware that some describe the scent as “wet dog,” which can be a unique addition to the olfactory garden experience.

Keeping in mind the poisonous nature of the plant, it has found a common use in children’s gardens, sensory gardens and gardens with a focus on scent. At Rotary Botanical Gardens, this plant has a permanent role in offering interest and color. For many years it was an important part of the “Smelly Garden” theme of the children’s garden. We considered this plant a “gateway” plant for kids to engage and hopefully connect with enjoying and possibly growing plants in the future. Placement of the plant, despite the size, was frequently along bed edges or pathways so visitors of all sizes could enjoy the plant. Other specimens were used in more traditional combinations and container situations. Signage would also help direct visitors to engage the plant on a more sensory level and the popularity of this plant at the garden encouraged our acquisition and display of over 200 of these plants on an annual basis. Additionally, the garden even donated popcorn cassia plants to youth summer gardening programs in the area for the same purpose of education, enjoyment and inspiration.

Popcorn cassia in the summer garden will certainly be a conversation piece for its beauty but bringing guests closer to experience the scent is what really makes this plant special, noteworthy and memorable in the garden.

The author was the director of horticulture at Rotary Botanical Gardens for 21 years. He now operates Landscape Prescriptions by MD. landscapeprescriptionsmd.com