Commelina diffusa is typically an annual herb, though it may be perennial in the tropics. It spreads diffusely, creeping along the ground, branching heavily and rooting at the nodes, obtaining stem lengths up to 1 metre. Pubescence on the stem is variable and ranges from glabrous to hispidulous, which can occur either in a line or throughout. The leaf blades are relatively variable, ranging from lanceolate to ovate, with proximal leaves tending to be more oblong. They measure 3 to 12 cm in length by 0.8 to 3 cm in width. North American populations tend to have smaller leaf size, typically measuring 1.5 to 5 cm, by 0.5 by 1.8 cm. The leaf apex is acute to acuminate. The leaf surface can be either glabrous (i.e. hairless) or hispid (i.e. bristly). The leaves are subsessile (i.e. having a very small petiole) with a leaf sheath striped with red and covered with hispid pubescence. The flowers are arranged into cincinni (singular: cincinnus), or scorpioid cymes. This is a form of a monochasium where the lateral branches arise alternately on opposite sides of the false axis. There are typically two cincinni present, with the lower cincinnus bearing 2 to 4 flowers, while the upper cincinnus has one to several flowers. The upper cincinnus is generally exerted on specimens with larger spathes, but it may be included in specimens with smaller spathes. The upper cincinnus bears only male flowers and has a longer peduncle, while the lower cincinnus bears bisexual flowers on a shorter peduncle. The pedicels supporting single flowers, and later the fruits, are thick and curved and measure about 3 to 5 mm. The membranous sepals are inconspicuous at only 3 to 4 mm in length.The petals are blue, though may be lavender in rare cases. The upper two petals measure 4.2 to 6 mm. The anther connective (i.e. the tissue connecting the two halves of the anther) of the centre-most stamen has a broad transverse band of violet. The spathes are solitary, borne on a peduncle and typically falcate (i.e. sickle-shaped) with a cordate (i.e. heart-shaped) to rounded base, acuminate apices and can be either glabrous or hispidulous (i.e. minutely hispid) beneath. They usually measure 0.8 to 2.5 cm long, but may be as short as 0.5 cm and as long as 4 cm. They are typically 0.4 by 1.2 cm wide, but may be up to 1.4 cm long. Their peduncles are usually 0.5 to 2 cm long and rarely up to 2.9 cm. Flowering occurs from May to November. Pollens are elongated with bilateral symmetry, approximate size is 73 microns.