Flower:

Numerous tiny, stalked flowers along branching stems, the whole cluster a third to half the height of the plant and the branches all ascending to erect. Flowers are not often seen open, usually closed and appearing bud-like. Open flowers have 3 short, reddish petals, a pompom-like cluster of stigmas, and 5 or more stamens.

The calyx surrounding the flower is less than 1/8 inch (1.6 to 1.8 mm) long with 5 sepals in 2 series. The 3 inner sepals are broadly egg-shaped and dark red; the 2 outer sepals are green, linear, and usually shorter than the inner sepals. Flower stalks and sepals are all moderately to densely covered in straight, appressed white hairs.

Leaves and stems:

Leaves are alternate to nearly opposite, mostly linear, up to about ¾ inch (13 to 20 mm) long, up to 1/8 inch (1.5 to 3 mm) wide, toothless, pointed at the tip, and stalkless. Most leaves below the flower cluster drop off early; leaves on the flowering branches are persistent but tend to be smaller.

The upper leaf surface is hairless or has a few straight appressed hairs, the lower surface is sparsely to densely covered in appressed hairs with the densest hairs along the midvein and edges. Stems are multiple from the base, often abruptly erect from near the base, unbranched except in the flower clusters, and covered in appressed hairs, more sparsely so at the base and denser into the flower clusters. In late summer, several short, leafy shoots radiate from the base.

Fruit:

Fruit is a round capsule less than 2 mm wide and about as long as the inner sepals, containing 3 or 4 smooth, brown seeds.

Notes:

Prairie Pinweed is the most common of the 3 Lechea species known to be in Minnesota, though distinguishing this from Intermediate Pinweed (Lechea intermedia) can be quite the challenge.

Nearly every reference notes the hairiness can distinguish one from the other: L. stricta is more densely hairy, more gray-green, and the lower leaf surface densely hairy, while L. intermedia is less hairy, more green, and the lower leaf surface hairless or hairy only on the midvein and edges. However, Flora of North America notes: "Some individuals of Lechea stricta can be difficult to determine with certainty; the extremes of variation of L. stricta often grade into that of L. intermedia". Concerning hairiness, Michigan Flora also notes: "differences defy any quantitative statement". Indeed.

Having inspected dozens of populations around the state, we have yet to see any plant with dense hairs across the lower leaf surface. All have had hairs along the midvein and edges, but the surface between those zones rarely has more than a few sparse hairs, and overall plants may look green, grayish, or any shade in between. It appears both may have flowering stems that abruptly become erect from near the base. The more telling difference is likely mature fruit, where L. stricta has 3 or 4 brown seeds per capsule and L. intermedia 4 to 6 light to medium brown seeds that are irregularly covered in a thin, grayish, veiny coating.

Going just by the number of seeds, I can't say with any confidence we've encountered L. intermedia anywhere in the state. Revisiting sites described in herbarium records has not yielded the desired results and none of the reports on iNaturalist from Minnesota or Wisconsin that I've looked at show what I would consider distinctive characteristics for a positive ID, but we are still looking.

Prairie Pinweed is more easily distinguished from Narrowleaf Pinweed (Lechea tenuifolia), a rare species which has the 2 outer sepals much longer than the inner 3. Fruiting Frostweeds (Crocanthemum [syn. Helianthemum] spp.) may bear some resemblance to Lechea, but have stems and leaves covered in short, star-shaped hairs, not straight, appressed hairs.