Houstonia L. (Rubiaceae)

Houstonia had been represented by nine taxa in North Carolina, until recent work found persistent populations of H. micrantha (native to the Deep South) and H. procumbens (State SR-P, S1 G5; apparently spreading north from South Carolina; neither yet keyed below). Both would fall under lead 1. below. Both exhibit white corollas. Houstonia micrantha is an annual most similar to H. pusilla, except for the white (and smaller) corollas. Houstonia procumbens is most similar to H. serpyllifolia (both are creepers), but the latter usually exhibits blue flowers (all of which are chasmogamous and above ground; H. procumbens exhibit chasmagamous and cleistogomous flowers, the latter borne underground). Houstonia montana is federally listed as a variety of H. purpurea (Fed E, S2 G2). No other taxa are listed or proposed for listing, but H. lanceolata (State SR-P, S1 G5) is also of conservation concern and tracked by NHP.

Federally listed taxon—
Houstonia montana (Fed E, State E | S2 G2)

Habitat. Crevices in high elevation rock outcrops or in adjacent balds (ca. 4200 ft and above).

Range. Endemic to the southern Appalachians of North Carolina and Tennessee.

Additional resources. NHP | Recovery plan | 5-yr review

Key to Houstonia in North Carolina

Key adapted from Radford et al. (1968) and Weakley (2008). Photos by Krings, unless otherwise indicated. Line drawings from Britton & Brown (1913), unless otherwise indicated. Maps courtesy of USDA PLANTS and the North Carolina Natural Heritage Program.

1. Inflorescence appearing very open, diffuse; pedicels 4.5–30x as long as the sepals (usually >6x), flowers terminal (rarely axillary); corolla salverform...2.

2. Stems prostrate, creeping...H. serpyllifolia

2'. Stems erect or spreading...3.

3. Plants perennial, basal rosette persistent, well-developed; corolla 5.8–16 (–21) mm long, tube (2–) 4–11 (–12) mm long, lobes white to light blue, each base with yellow markings...H. caerulea

3'. Plants annual, basal leaves few (if any), short-lived; corolla 2–5.5 mm long, tube 0.8–2.8 mm long, lobes lavender, each base with red markings...H. pusilla

1'. Inflorescence appearing compact, dense; pedicels 1–5x as long as the sepals (usually 1–3x), flowers terminal and axillary; corolla funnelform...4.

4. Leaves ovate or lanceolate (widest toward the base or at the middle), 1–6× as long as wide, 4–34 mm wide; calyx lobes 1–7 mm long...5.

5. Calyx lobes 4–7 mm long; leaves mostly lanceolate (varying from narrowly lanceolate to broadly ovate), 1.7–3.3 x 0.4–1 cm, 3.3–6× as long as wide...H. lanceolata

5'. Calyx lobes 1–4 mm long; leaves mostly ovate (varying from broadly ovate to ovate-lanceolate), 0.8–6.3 x 0.6–3.4 cm, 1–3.2× as long as wide...6.

6. Stems glabrous (or slightly shortpubescent on the lower nodes only); adaxial leaf surface glabrous, blades 3–8 (–13) mm wide; corollas purple, 8–12 mm long; high elevation rocky summits and adjacent grassy balds...H. montana*

6’. Stems sparsely to densely pubescent; adaxial leaf surface sparsely pubescent, blades (6–) 12–30 (–34) mm wide; corollas light purple to white, (4–) 5–8 (–10) mm long; habitats various, widely distr...H. purpurea

4'. Leaves linear to narrowly elliptic (widest at the middle or toward the apex or nearly equally wide for most of their lengths), 4–20× as long as wide, 0.5–6 mm wide; calyx lobes 0.5–3 mm long...7.

7. Stipules usually broader than long; leaves 1.3–4.7 cm long, 0.5–4.0 mm wide (mostly < 2.5 mm wide), 7–20× as long as wide; inflorescence very diffuse and open, to 20 cm long, the branches ascending, spreading, or deflexed, slender and often ultimately filiform, with 1–4 remote nodes bearing reduced leaves, the pedicels to 14 mm long; internodes mostly 4–9; mature capsules mostly 1.5–2.5 mm long and wide; stem densely cinereous-puberulent, especially at the nodes...H. tenuifolia

7'. Stipules usually longer than broad or as long as broad; leaves 1.6–4.0 cm long, 1.5–6.0 mm wide (mostly > 2.5 mm wide), 4–11× as long as wide; inflorescence rather open to rather compact, < 12 cm long, the branches ascending or spreading, slender, pedicels to 8 mm long; internodes mostly 7–11; mature capsules mostly 1.8–3.0 mm long and wide; stem densely cinereous-puberulent, glabrate, or glabrous...8.

8. Gap between longitudinal ridges subtending stipules narrow, at least as much stem face visible outside ridges as within; stems glabrous or glabrate (sometimes puberulent on the nodes only); internodes 7–10, the median internodes (1.1–) 2.0–4.5 (–6.0) cm long; high elevation granitic domes...H. longifolia var. glabra

8'. Gap between longitudinal ridges subtending stipules broad, ridges usually at edge of stem face or sometimes a fraction of as much face outside as within visible; stems densely cinereous-puberulent; internodes (6–) 7–11 (–13), the median internodes (1.0–) 1.5–3.5 (–4.4) cm long; widely distributed...H. longifolia var. compacta