Annuals were traditionally defined as plants that proceed from seed, to flower and finally back to seed before perishing with the autumn frosts. Over the past 20 years, tropical plants have suddenly become the new ‘annual’! ‘Tropicals’ are a perennial in their native provenance, but function as an annual in colder garden regions unless brought indoors. Popcorn Plant, botanically known as Senna (Cassia) didymobotrya is one Tropical that has recently been receiving much attention from our garden visitors and students alike! Interestingly, students will actually seek out this plant and share their interest of it with their friends – a true indicator that this is one cool plant!
Popcorn Plant is a member of the Fabaceae or Pea Family and is native to tropical regions of Eastern Africa. Senna has between 260 and 350 species, with most of the members scattered pantropic, although several species are winter hardy in northern Gardens. The genus was originally authored in 1753 by Carl Linnaeus (1707-1778) as Cassia. Cassia is from the Hebrew Kidday or Qiddah and is a reference to peelable bark or a strip of bark. In 1754, the English Botanist Philip Miller (1691-1771) reclassified this group of plants under the genus Senna. Senna comes from the Arabic sanā, meaning brilliant and most likely refers to the brilliant and glowing yellow flowers that typify this genus. Unfortunately for Philip Miller, this name change was not accepted until 1982 when the genus was finally reclassified by Howard Irwin and Robert Barneby. To be a successful botanist requires great patience! In 1839, this particular species was described by the German physician and botanist Johann Baptist Georg Wolfgang Fresenius (1808-1866) as Cassia didymobotrya, with the genus reclassified in 1982 to Senna. The species epithet of didymobotrya is from the Greek Didymus referring to pairs or clusters and botry, a grape-like structures. It is in reference to the 1” diameter flower buds that are protected by a dark purple bract, giving them a distinct resemblance to grapes clustered along the flower stalk! To its detriment, the original genus name of Cassia is often preferred in catalogues and literature.
Despite the confusion in botanical names, there is no confusing the value of this plant for the Garden! Popcorn Plant thrives in the hot, humid African tropics where it is a long lived shrub or small tree, approaching a stature of 25’ in height! As an annual, rapid growth is restricted to the heat of July into early September and it reaches more modest heights of 6-8’. The flowers begin to appear in early to mid-July and extend through the first frosts of autumn. The 12” long upright flower stalks, called racemes, are clothed with golden yellow flowers that open sequentially from the bottom upwards over several weeks. The contrast between the nearly black, grape-like flower buds and the golden yellow flowers is very dramatic. The five petals of each flower are initially curved slightly inward, but slowly expand outwards, developing individual flowers upwards of 2” in diameter. Before the flowers senescence, a tiny legume fruit or ‘pea pod’ can be seen expanding from within the flower, which continues to expand to 4-5” long following petal drop.
In addition to sheer size and floral display, the Popcorn Plant also provides lush and dramatic foliage for the Garden. The foliage is pinnately compound, with 8-18 oppositely paired leaflets along the central rachis. Each leaflet can reach 3” long while the leaf itself can reach upwards of 18” in length. The tropical appearance of the leaf is not its only benefit – it also gives rise to the common name! When you rub your hand along the foliage, it releases a fragrance much like freshly popped and buttered popcorn. Since deer clearly dislike buttered popcorn, this fragrance also makes it resistant to deer browse!
Senna is best grown in full sun and in well-drained soils. It can be grown from seed and has been used as a nitrogen-fixing cover crop. However, it grows far larger in Northern Gardens when grown from cuttings. Larger is certainly better as it not only provides a better floral presentation, but enhances the temptation of passing your hands through the foliage and bringing forth that sweet smell of buttered popcorn for all enjoy. And, as we all know, if this attribute is appreciated by students, than every garden should have a little buttered popcorn for their visitors to enjoy!