Variation in plant size
Leptocereus septentrionalis includes shrubs 2–3 m tall and tree-sized specimens reaching 8 m. In large trees the trunk is 3–4 m long and about 200 mm in diameter. Terminal segments measure 0.5–2 m in length and 20–25(–30) mm in diameter. There are 5–6(–7) ribs with undulate margins. Areole felt is white. In the described specimens, usually 15–22 spines measured 12–25 mm and retained a golden-yellow colour before darkening later. [Hoxey et al. 2023b, pp. 67–70]
Northern sites
The authors described small colonies near Monte Cristí in the Dominican Republic and a more extensive area in northwestern Haiti, from Port-de-Paix to Môle Saint Nicolas, with an additional observation near Baie de Henne. The two main areas are about 150 km apart. Plants generally occurred within 5 km of the coast. Large Monte Cristí trees grew in intact woodland, while smaller Haitian shrubs more often occupied exposed coral limestone where surrounding vegetation had been modified. [Hoxey et al. 2023b, pp. 67–70]
Fruits and seeds
Elongated fruits measure 60–80 × 40–50 mm and have a thick wall of 10–12 mm. They are green with a yellowish hue. Their areoles retain 12–20 spines measuring 6–15 mm. Seeds measure 2.3–2.6 × 1.4 × 0.6 mm. Raised cells with pointed apices form the seed coat surface. Flowering short shoots are initially covered with white felt and are spineless, later becoming woody. The authors did not observe fresh flowers and described the floral tube spines from dried remnants. [Hoxey et al. 2023b, pp. 67–70]