Houstonia caerulea (azure bluet)

If you go botanizing during growing season, year after year, you notice trends. For example, there’s almost always something in the Asteraceae (daisy family) blooming, but the family really gets going in high summer and into autumn. Plants in the Violaceae (violet family) bloom in early spring and are done by the time the earliest Lamiaceae (mint family) start. And so on.

Cephalanthus occidentalis (buttonbush)

This is the time of year for the Rubiaceae, or madder family. Worldwide, it is the fourth largest flowering plant family in terms of number of species (Asteraceae, Orchidaceae (orchid family), and Fabaceae (pea family) are first, second, and third). Depending on which authority you consult, there are about 13,500 species in more than 600 genera.

Galium concinnum (shining bedstraw); the flowers are about 3/16″ wide

The Rubiaceae is a cosmopolitan family, with species found worldwide except in the polar regions, but most are in the tropics or subtropics. The plants can be trees, shrubs, lianas, or herbs. Some are well-known garden ornamentals (pentas, ixora, gardenia), and some are economically important, producing dyes (like madder), or drugs (like quinine), or beverages (coffee!).

Here in North America, Rubiaceae ranks 18th, with 264 native species. (Asteraceae is first, Fabaceae second, and Orchidaceae ranks 13th.)

Diodia virginiana (Virginia buttonweed)

In Maryland, Rubiaceae is tied with Onagraceae for 13th place, with 39 species. (Asteraceae is first, Fabaceae second, and Orchidaceae is tied with Plantaginaceae for 9th place.)  I gleaned this information from the Maryland Biodiversity Project but did not weed out the aliens, so if you count only natives the rank may change. But you get the idea.

Geez, this is starting to read like baseball stats.

Anyway, the Maryland species fall into eight genera:

• Cephalanthus (one species, C. occidentalis, aka buttonbush, found almost statewide)
• Diodia (two species, D. teres and D. virginiana, both found primarily in the Coastal Plain and Piedmont)
• Gallium (as many as 24 species depending on taxonomy; distribution varies)
• Houstonia (five species; see previous post)
• Mitchella (one species, M. repens, aka partridgeberry, found statewide except Washington County)
• Oldenlandia (one species, O. uniflora, aka clustered mille-grains; coastal plain)
• Sherardia (one species, O. arvensis, aka field madder, alien found in the coastal plain)
• Spermacoce (one species, S. glabra, aka smooth false buttonweed, listed S1/endangered, found in Montgomery County)

Houstonia purpurea (purple bluet)

Looking at just the sampling of species here in Maryland, you can see traits that characterize the family: flowers consist of four petals, often pointed, partly fused into a tube; there are two leaves or more per node on the stem (opposite or whorled arrangement), the leaves usually entire, and often there are stipules. Most are herbaceous, except partridgeberry, which hugs the ground and never gets more than an inch tall yet has a woody stem, and buttonbush, which is a shrub. I should note, though, that there are many exceptions to these traits in the worldwide accounting of species.

Mitchella repens (partridgeberry)