Bluets Illustrate a Lesson in Plant Identification

Houstonia species
Rubiaceae

20150416-20150416-_DSC0359From mid-April to mid-May, the dryer, rocky soils around Carderock will be carpeted in rattlesnake weed and this adorable plant [left].  Standing only a few inches high, each stem bearing a single, terminal flower and few or no leaves, it’s very easy to identify as Houstonia caerulea (azure bluets).

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Half a mile or so west on the C&O Canal towpath, come early June, you might see just a dozen or so of these plants [right] on a certain dry hillside.  Standing about a foot to a foot and a half tall, with a small terminal cluster of flowers and three-nerved, oval leaves, it’s pretty easy to tell that this one is Houstonia purpurea, variously known as large bluets, woodland bluets, and Venus’ pride.

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Here’s when I get into trouble. Satisfied with the identification, I fail to examine every single plant and start concentrating on taking lovely photos, until I get one that I really like.

<—– Like this one.

Yesterday, I posted that photo to an internet discussion group and was quickly challenged: isn’t this Houstonia longifolia (long-leaf bluets)?  Let’s consider it, with descriptions from my three favorite ID books.*

H. purpurea: stem leaves egg-shaped or broadly lance-shaped, with 3-5 veins (Newcomb); plants erect, 6-20″, leaves rounded or notched at the base, stalked or stalkless (C&G); leaves oval, paired, sessile, entire, 3-veined (Peterson).

H. longifolia: stem leaves lance-shaped or oblong, 1-veined (Newcomb); plants erect, 4-10″, leaves narrowly oval or narrowly oblong, 1-nerved, tapering to base, stalkless (C&G); a small plant, slender paired leaves (Peterson).

Note that I didn’t describe the flowers; in all three sources the descriptions pretty much match, but more importantly, there can be a lot of morphological variation in flowers.  Color is probably the worst clue, because it can vary so much.  And look at the previous picture again: can you see that the topmost flower has 5 petals, while the others have 4?  These things happen.  To distinguish these two species, you need to examine the leaves.

Back to the question: which species is pictured?  There isn’t enough information to say for sure. Only the uppermost stem leaves are shown clearly, and while they appear to be narrow and 1-veined, the same can be said of the uppermost stem leaves on the plants ID’d as H. purpurea (in other pictures; I’ll spare you). Is it possible there are two different species in a single stand of plants?  Sure. Is it likely?  I don’t know enough to say.

These things bother me.  As I finish my third cup of coffee, I’m going to hit “publish immediately”, get my camera out, and go to the canal.  If I’m lucky these plants will still be blooming and I’ll get a good look.  If they aren’t, I probably won’t be able to pick them out from all the other foliage in the area.

I’ll post an update later today.

———–
*Newcomb’s Wildflower Guide; Wildflowers in the Field and Forest (Clemants and Gracie); Peterson’s Field Guide Wildflowers Northeastern/North-central North America

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