Flower:

Elongating clusters of stalked flowers at the top of the plant, arising from the upper leaf axils, and at the tips of branching stems, with a densely packed, rounded cluster of open flowers at the tip and fruit forming below.

Flowers are tiny, less than 1/8 inch across, with 4 oblong-elliptic sepals that are light green with thin, whitish edging and have a few hairs on the outer surface. Alternating with the sepals are 4 obscure white petals that are shorter than the sepals but more often absent altogether. In the center are 2 yellow-tipped stamens and a stubby, white style at the tip of a green ovary. Flower stalks are minutely hairy, the hairs cylindric to club-shaped and mostly straight.

Leaves and stems:

Leaves are in a basal rosette as well as alternate all along the stem. Basal leaves are long stalked, spatula shaped to pinnately lobed and wilt away early. Lower stem leaves are irregularly toothed or lobed, 1 to 3 inches long, up to ½ inch wide, widest above the middle, tapering to a stalk at the base.

Leaves become more linear, less toothy, and stalkless as they ascend the stem. Surfaces are minutely hairy. Stems are erect, often much branched and also minutely hairy. Stem and leaf hairs are cylindric to club-shaped and mostly straight.

Fruit:

Fruit is a flat pod, oval to round in outline, usually slightly longer than wide, widest near or above the middle, about 1/8 inch (to 3.5mm) long with a small notch at the tip. It dries to a papery brown shell and splits down the middle when mature. Each side of the pod contains a single seed. Inside the seed, the pair of seed leaves (cotyledons) and embryo are all parallel to each other, in a single row when viewed in cross-section.

Notes:

The origin of this species is not entirely certain, some believing it is a European or Eurasian introduction, some that it is native to western North America and adventive east of the Mississippi, some believing it is native in the east. In Minnesota, the general consensus is it is native here and we're going with that. Some references note multiple varieties but they are not recognized in Minnesota. Needless to say, it can be quite variable in amount of branching, leaf size and shape, and other characteristics, likely depending at least in part on soil type, moisture, and other environmental conditions, and ranging from sparsely branched, scrawny plants to robust, bushy plants.

Green-flowered Peppergrass is the most common Lepidium species in the state, found in virtually every county. It is very similar to the less common Virginia Peppergrass (Lepidium virginicum) and often confused with it, but there are 3 key differences: L. virginicum flower petals are typically about twice as long as the sepals, rarely obscure; stem and stalk hairs are never club-shaped and are more or less curved (magnification required to see); inside the seed, the pair of seed leaves (cotyledons) are parallel to each other but perpendicular to the embryo (magnification also required). While most references note the shape of the fruit is also diagnostic, specifically whether it is widest at or above the middle, this is too subtle a difference to be consistently reliable.

There are several non-native Lepidium species that are similar to L. densiflorum, but they are either not present in Minnesota, though may be in neighboring states, or are very rarely encountered here. Lepidium campestre and L. chalepense stem leaves are lobed (auricled) at the base, L. sativum stem leaves are stalked and shaped like their basal leaves, L. perfoliatum stem leaves are heart-shaped at the base and clasping. The more western native L. ramosissimum is also very similar, but the flower clusters do not elongate nearly as much and it has curved hairs on stems and stalks where L. densiflorum hairs are mostly straight.