Flower:

Numerous tiny, stalked flowers along branching stems, the whole cluster often more than half the height of the plant. Flowers are rarely open, usually appearing bud-like, and are arranged singly all along a branch, often on just one side of the branch (secund).

Flowers are less than 1/8 inch (to 2 mm) long with 5 sepals in 2 series. The 3 inner sepals are broadly egg-shaped, green to red with a darker green midrib; the 2 outer sepals are green, linear, and usually longer than the inner sepals. Hidden by the sepals are 3 shorter, dark red petals and up to 20 red stamens. Flower stalks and sepals are all sparsely to moderately covered in appressed hairs.

Leaves and stems:

Leaves are alternate to nearly opposite, mostly linear, up to about ¾ inch long, about 1 mm wide (often less), toothless, blunt to pointed at the tip, and stalkless. Lower leaves are largest but drop off early; leaves on the flowering branches are persistent but tend to be smaller.

The upper leaf surface is hairless, the lower has sparse, appressed hairs along the midvein and edges. Stems are erect to ascending, multiple from the base, usually brown near the base and green into the branches, and sparsely covered in appressed hairs.

Fruit:

Fruit is a globular, 3-sided capsule up to 1.7 mm long and slightly shorter than the inner sepals, tuning reddish brown as it matures. Inside are 2 to 5 seeds that are yellowish to reddish brown and up to 1 mm long.

Notes:

Narrowleaf Pinweed is very rare in Minnesota, with only 4 locations ever recorded in the state. According to the DNR, in Minnesota it's only been found in high-quality sand prairie or savanna, habitat that is also rare, not to mention vulnerable to woody encroachment as well as incompatible land uses. It was listed as an Endangered species in 1996.

Of the three Lechea species known to be in Minnesota, Narrowleaf Pinweed is the easiest to identify from the outer sepals on the flowers usually being longer than the inner. It tends to be a smaller, more slender plant than either Prairie Pinweed (Lechea stricta) or Intermediate Pinweed (Lechea intermedia); the leaves of the other two are also more consistently at least 1.5 mm wide and capsules more than 1.7 mm long. Narrowleaf Pinweed flowers are often all on one side of a branch (secund) but not always. While it is uncommon to see the flowers open on any of these three, rumor has it that one might get lucky in the early morning hours of sunny days.

Some references list two vars of L. tenuifolia, the distinguishing characteristic apparently being whether the upper leaf surface is hairy (var. occidentalis) or not (var. tenuifolia). These vars are not universally accepted but the consensus is var. tenuifolia would be present in Minnesota. Its longer outer sepals is a trait shared with Thyme-leaf Pinweed (Lechea minor), which isn't known to be in Minnesota but is present in Wisconsin; it has much broader leaves (4+ mm), tends to have more compact flower clusters, and capsules are slightly longer than the sepals.