This species is characterized by its slender perennial habit, well developed underground parts, and rather small leaves, inflorescences, flowers, and fruits. Declieuxia fruticosa is widespread and commmonly collected, and found throughout the range of the genus. It is remarkably consistent morphologically compared to other species of this genus, and Kirkbride (1976) did not separate any infraspecific taxa. Plants of Central America and Mexico are generally similar morphologically, but plants of South America show variation in particular in leaf size and proportional width.
Kirkbride (1976: 23) outlined seven somewhat well marked "minor variants" and their geographic distributions, and noted some of the published names that correspond to those forms. Some of these forms are quite similar to other species, such as Declieuxia leiophylla. One of these variants, his # 3, is particularly well marked with densely pilosulous to hirtellous pubescence and subsessile, ovate to obovate, proportionally broad leaves with subpalmate venation and thinly revolute margins (e.g., Anderson 6687). These plants are found in east-central Brazil in the Espinhaço massif, and corresponds the type of Declieuxia origanoides. Similar plants that are glabrous were regarded by him as variants #5 and #7, which are found in the same area and correspond to the types of Declieuxia calophylla (e.g., Irwin et al. 20155) and Declieuxia rubioides (e.g., Irwin 28234), respectively. Kirkbride additionally (1976) noted that Declieuxia passerina was found to hybridize with Declieuxia fruticosa, forming a suite of variants or hybrids.
The names Houstonia fruticosa and Declieuxia chiococcoides were based on the same collection of specimens made by Humboldt & Bonpland, but these were described independently in 1818 and 1819, respectively, based on different duplicates from that set. The holotype of Houstonia fruticosa was deposited in the Willdenow collection in Berlin, while the intended holotype of Declieuxia chiococcoides was deposited in the Kunth herbarium in Paris. Names published this way would normally be independent, but these particular two are linked nomenclaturally because Kunth cited the earlier name Houstonia fruticosa as a synonym in his protologue of Declieuxia chiococcoides. This means that Kunth's later name is superfluous (Art. 52.2) and therefore illegitimate (but valid). This species is then correctly called by its earliest epithet, and takes the name Declieuxia fruticosa.