Nepenthes flava is a climbing plant growing to a height of 6 m. It only remains in the rosette stage for a short time before transitioning into a scrambling vine.
The stem, which may be branched, is around 3 mm in diameter and has roughly cylindrical internodes measuring up to 14 cm in length.
The stem ranges in colour from green to dark red.

The lamina (leaf blade) is linear, oblong, or narrowly obovate, and measures up to 9 cm in length by 2.5 cm in width.
Its apex is usually acute, but may also be obtuse.
The base of the lamina is attenuate and clasps the stem for around half to three-quarters of its circumference.
Three prominent longitudinal veins are present on either side of the midrib.
Indistinct reticulate veins are also present.
The lamina is yellowish-green in colour and the midrib may be reddish.
Tendrils reach 24 cm in length and may be green to red.

Rosette and lower pitchers are either infundibular throughout or ovate and variably inflated.
They are small, measuring only up to 7 cm in height by 4 cm in width.
A pair of wings (≤3 mm wide) often runs down the upper third of the pitcher's ventral surface, bearing fringe elements around 5 mm long.
Occasionally, the wings may be absent altogether.
The peristome varies from cylindrical to slightly expanded and has a wavy outer margin.
It is up to 12 mm wide and bears fine ribs up to 0.6 mm high and spaced up to 1 mm apart.
On the inner margin of the peristome, these ribs terminate in teeth up to 1.5 mm long, with the largest located towards the rear.
The pitcher mouth is round and slightly elongated towards the rear, although it rarely exhibits a neck.
The pitcher lid or operculum is narrowly ovate or elliptic and measures up to 3.5 cm in length by 2 cm in width.
On its lower surface, the lid bears a basal glandular crest or no appendages at all, as well as numerous circular nectar glands (0.3 mm in diameter), which are concentrated around the midline and crest.
An unbranched, filiform spur (≤4 mm long) is inserted near the base of the lid.
Unlike their aerial counterparts, lower pitchers vary greatly in pigmentation.
The pitcher cup may be yellow, orange, red, or even purple, whereas the inner surface may be light yellow, light orange, or whitish.
The peristome is usually darker than the rest of the pitcher, typically being dark orange to purple, although it may be creamy white and occasionally exhibits red stripes.
The lid has a similar colour to the pitcher cup, usually having a darker upper surface with dark red speckles.

Upper pitchers are tubular to narrowly infundibular in the lower two-thirds, becoming broadly infundibular above.
Characteristically, the hollow pitcher tube often continues past the curved basal portion and for some distance up the tendril.
Upper pitchers are similar in size to their terrestrial counterparts, reaching 6 cm in height by 3.5 cm in width.
The peristome is flattened and expanded, reaching 15 mm in width.
As in lower pitchers, its outer margin is undulate.
The peristome ribs are up to 0.2 mm high and spaced up to 0.5 mm apart.
The lid is ovate or elliptic and has a somewhat truncate apex.
It measures up to 4 cm in length by 2.5 cm in width.
A reduced basal crest may be present on the underside of the lid or it may be absent entirely.
Other parts are similar to those found in terrestrial traps.
Upper pitchers are typically yellow throughout, but some specimens may have a completely red or red-striped peristome and mature traps may be orange or reddish on the upper surface of the lid.

Nepenthes flava has a racemose inflorescence up to 15 cm long.
Male inflorescences are more floriferous, bearing 15–40 flowers, compared to 15–25 in females.
The peduncle measures up to 4 cm in males and up to 8 cm in females.
Flowers are borne solitarily on pedicels that reach 6 mm in males and 11 mm in females.
A basal bract is often present (≤4 mm long).
The nectariferous tepals are ovate and measure up to 3 mm in length by 1 mm in width.
The androphore is around 3 mm long and terminates in an anther head measuring 1–1.5 mm in diameter.
The ovary
Nepenthes flava is a climbing plant growing to a height of 6 m. It only remains in the rosette stage for a short time before transitioning into a scrambling vine. The stem, which may be branched, is around 3 mm in diameter and has roughly cylindrical internodes measuring up to 14 cm in length. The stem ranges in colour from green to dark red.
The lamina (leaf blade) is linear, oblong, or narrowly obovate, and measures up to 9 cm in length by 2.5 cm in width. Its apex is usually acute, but may also be obtuse. The base of the lamina is attenuate and clasps the stem for around half to three-quarters of its circumference. Three prominent longitudinal veins are present on either side of the midrib. Indistinct reticulate veins are also present. The lamina is yellowish-green in colour and the midrib may be reddish. Tendrils reach 24 cm in length and may be green to red.
Rosette and lower pitchers are either infundibular throughout or ovate and variably inflated. They are small, measuring only up to 7 cm in height by 4 cm in width. A pair of wings (≤3 mm wide) often runs down the upper third of the pitcher's ventral surface, bearing fringe elements around 5 mm long. Occasionally, the wings may be absent altogether. The peristome varies from cylindrical to slightly expanded and has a wavy outer margin. It is up to 12 mm wide and bears fine ribs up to 0.6 mm high and spaced up to 1 mm apart. On the inner margin of the peristome, these ribs terminate in teeth up to 1.5 mm long, with the largest located towards the rear. The pitcher mouth is round and slightly elongated towards the rear, although it rarely exhibits a neck. The pitcher lid or operculum is narrowly ovate or elliptic and measures up to 3.5 cm in length by 2 cm in width. On its lower surface, the lid bears a basal glandular crest or no appendages at all, as well as numerous circular nectar glands (0.3 mm in diameter), which are concentrated around the midline and crest. An unbranched, filiform spur (≤4 mm long) is inserted near the base of the lid. Unlike their aerial counterparts, lower pitchers vary greatly in pigmentation. The pitcher cup may be yellow, orange, red, or even purple, whereas the inner surface may be light yellow, light orange, or whitish. The peristome is usually darker than the rest of the pitcher, typically being dark orange to purple, although it may be creamy white and occasionally exhibits red stripes. The lid has a similar colour to the pitcher cup, usually having a darker upper surface with dark red speckles.
Upper pitchers are tubular to narrowly infundibular in the lower two-thirds, becoming broadly infundibular above. Characteristically, the hollow pitcher tube often continues past the curved basal portion and for some distance up the tendril. Upper pitchers are similar in size to their terrestrial counterparts, reaching 6 cm in height by 3.5 cm in width. The peristome is flattened and expanded, reaching 15 mm in width. As in lower pitchers, its outer margin is undulate. The peristome ribs are up to 0.2 mm high and spaced up to 0.5 mm apart. The lid is ovate or elliptic and has a somewhat truncate apex. It measures up to 4 cm in length by 2.5 cm in width. A reduced basal crest may be present on the underside of the lid or it may be absent entirely. Other parts are similar to those found in terrestrial traps. Upper pitchers are typically yellow throughout, but some specimens may have a completely red or red-striped peristome and mature traps may be orange or reddish on the upper surface of the lid.
Nepenthes flava has a racemose inflorescence up to 15 cm long. Male inflorescences are more floriferous, bearing 15–40 flowers, compared to 15–25 in females. The peduncle measures up to 4 cm in males and up to 8 cm in females. Flowers are borne solitarily on pedicels that reach 6 mm in males and 11 mm in females. A basal bract is often present (≤4 mm long). The nectariferous tepals are ovate and measure up to 3 mm in length by 1 mm in width. The androphore is around 3 mm long and terminates in an anther head measuring 1–1.5 mm in diameter. The ovary
Nepenthes flava