Linum intercursum E. P. Bicknell, a primarily Coastal Plain species, is disjunct to moist, sandy meadows in northern Indiana and may occur in areas of Michigan known for Coastal Plain disjuncts. It has the outer sepals entire, like our three perennial yellow-flowered species, but with a distinctive conical fruit, clearly longer than wide. Our similar species have the fruits wider than long.

1. Petals blue or white (yellow-based); pedicels mostly (4-) 6-15 (-20) mm long (at least in fruit).

2. Cauline leaves opposite (except uppermost bracts); petals ca. 3-4 mm long, white with yellow claw.

L. catharticum

2. Cauline leaves all alternate; petals ca. (8-) 9-15 mm long, blue (except in albinos).

3. Inner sepals ciliate; annuals, homostylous.

L. usitatissimum

3. Inner sepals not ciliate; perennials, heterostylous.

L. perenne

1. Petals yellow throughout; pedicels (even in fruit) mostly less than 3.5 mm long.

4. Outer sepals glandular-ciliate (as are the inner ones), (3.5-) 4-4.5 (-5.5) mm long; leaves with a pair of dark stipular glands at the base; annuals.

L. sulcatum

4. Outer sepals (and often inner ones) entire, less than 3.5 mm long; leaves without stipular glands; perennials.

5. Inner sepals glandular-ciliate; leaves all alternate (except sometimes at the lowermost 1-2 nodes); inflorescence with stiffly ascending branches.

L. medium

5. Inner sepals glandless (or with a few glands on the distal half); leaves usually opposite at several or more of the lower nodes; inflorescence with branches usually ± divaricate.

6. Upper part of stem distinctly angled (narrowly winged) with 3 ridges decurrent from each leaf base for 1-2 internodes.

L. striatum

6. Upper part of stem (below the inflorescence) terete or nearly so (at most, some slightly raised lines).

L. virginianum