Thaumatococcus daniellii is a rhizomatous monocotyledonous, perennial herb species of the Marantaceae family. The fruit aril contains thaumatin, a sweet protein with sweetness intensity that is about 1,600-3,000 times that of sucrose and is extensively employed as a natural sweetener. The species has potential as an imperative economic crop if cultivated under proper agronomic practices. Phenological development and thermal time requirements are critical to ensuring a high yield. The aim of this study was to determine the phenological stages according to the BBCH scale and growing degree days (GDDs) under different irradiance and fertiliser levels. Seedlings were raised from rhizomes and grown on the field using a randomised complete block design with four treatments and three replications. The study was carried out at the Botanical Garden, Shah Alam, Malaysia, from July 2014 to February 2017. The treatments consisted of combinations of two irradiance and two fertiliser levels, which were designated as high irradiance with either low (T1) or high fertiliser (T2) and low irradiance with either low (T3) or high fertiliser (T4). A detailed phenological development using two-and three-digit coding systems was conducted on 12 clumps (one clump/replicate/treatment). The species has eight principal stages, which include bud and leaf development, formation of side tillering, petiole elongation, emergence of synflorescence, flowering, fruit development and fruit maturity. Both irradiance and fertiliser levels affected certain phenological stages; with leaf, tiller, synflorescence and fruit strongly influenced by irradiance levels. Leaf growth was more vigorous under high irradiances compared with low irradiances and took about 36 to 42 days to attain full maturity. The clump growth or size is a function of tiller numbers. The tiller number increased with increase in clump age. The onset of certain phenological stages was associated with clump age. The clumps commenced production of synflorescence when tiller number was in the range of 30 to 40, and started fruit formation when tiller number reached 70. Both events occurred under a combination treatment of high irradiance with high fertiliser. The GDD requirement of any phenological stage or development of any plant part is contingent upon the duration of a particular stage or process, as measured in days after emergence or transplanting.The longer the duration, the higher the GDD and it varies with treatments. The total GDD required for the completion of the single leaf growth ranged from 852 to 1,022 C, while the emergence of synflorescence under high irradiance occurred with