Plietesials

Plietesials are plants that grow for a number of years, flower gregariously (synchronously), set seed and then die. The length of the cycle can vary between 8 and 16 years. For example, the neelakurinji plant flowers every 12 years and bloomed as expected in 2006 and 2018 in the Munnar region of Kerala, India.

Certain species of unrelated families of flowering plants (including Poaceae, Arecaceae, Scrophulariaceae, Fabaceae, Apocynaceae, and Acanthaceae) are plietesial. The term plietesial has been used in reference to perennial monocarpic plants "of the kind most often met with in the Strobilanthinae" (a subtribe of Acanthaceae containing Strobilanthes and allied genera) that usually grow gregariously, flower simultaneously following a long interval, set seed, and die. Other commonly used expressions or terms describing a plietesial life history include gregarious flowering, mast seeding, and supra-annual synchronized semelparity (semelparity = monocarpy). It is not clear why gregarious flowering after long vegetative intervals would be associated with death after flowering, although both are associated with higher reproductive outputs.

Description
A good description of this natural history aspect of a plant's life cycle can be found in the following report:

"A plietesial life history has long been noted for certain species among unrelated families of flowering plants including Poaceae, Arecaceae, Scrophulariaceae, Fabaceae, Apocynaceae, and Acanthaceae. Bremekamp used the term plietesial in reference to perennial monocarpic plants "of the kind most often met with in the Strobilanthinae" (a subtribe of Acanthaceae containing Strobilanthes and allied genera) that usually grow gregariously, flower simultaneously following an interval of several years, set seed, and die. A similar life history is especially well known and documented among certain bamboos. In plietesials, the seed crop typically germinates simultaneously shortly following the mass death of the parental plants and initiates a new cycle with the same periodicity. Other commonly used expressions that apply to part or all of the plietesial life history include: gregarious flowering, mast seeding, and supra-annual synchronized semelparity (semelparity = monocarpy).

There is considerable variation in life history for Strobilanthes. Most known plietesial Strobilanthes take between 10 and 15 years (usually 12; although reports of 5 to 9 year cycles also have been made ) to flower gregariously, set seed, and die. The flowering periodicity in all individuals is rarely 100%, with the result that flowering of rare individuals in non-mass-flowering years is not uncommon. In some species, mass flowering occurs over a wide area on a species-specific cycle; in other species, populations in different regions follow their own cycles. Some species flower gregariously in certain years but do not die following the mass flowering, and are therefore not plietesial. At least one species exhibits different flowering patterns in different portions of its range. The perennial Strobilanthes wallichii flowers annually in the eastern Himalayan portion of its range and plietesially in the western Himalayan portion (Wood 1994). Literature reports of life history for some taxa are ambiguous. For example, Robinson (1935) noted a 12 year plietesial cycle for S. consanguineus C.B. Clarke whereas Bowden (1950) indicated that this species flowers every year. Such discrepancies likely result either from misidentifications of or life history variations within taxa."