Houstonia

1. Flowers solitary, much shorter than their tiny-bracted scapes.

H. caerulea

1. Flowers in cymes, on short pedicels.

2. Cauline leaves mostly lance-ovate (broadest below the middle), with 3 definite longitudinal veins (the midrib most prominent).

H. purpurea

2. Cauline leaves linear, narrowly oblong-elliptic, or oblanceolate (± parallel-sided or broadest beyond the middle), with only the midrib evident beneath.

3. Basal leaves forming conspicuous rosettes, pubescent above (especially toward the edges) and with minutely ciliate margins; main stems with 4–6 internodes (including short lower ones but not branches of the cyme); median leaves oblanceolate (or some narrowly elliptic).

H. canadensis

3. Basal leaves none or if present usually glabrous; main stems with (5–) 6–9 (–10) internodes; median leaves linear (or some narrowly elliptic).

H. longifolia

All species found in Houstonia

Houstonia caeruleaBLUETS 
Houstonia canadensisBLUETS 
Houstonia longifoliaLONG-LEAVED BLUETS 
Houstonia purpureaLARGE HOUSTONIA 

Citation:

MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. July 20, 2017. http://michiganflora.net/genus.aspx?id=Houstonia.