Bog willowherb is a perennial herb that thrives in moist areas across the eastern and northern parts of North America, commonly inhabiting bogs, fens, marshes, wet meadows, and other wetland habitats. Growing up to 40 inches tall, this species has alternate leaves that are linear to narrowly elliptic in shape measuring ¾ to 3 inches long, and about ¼ inch wide. The leaves are blunt to pointed at the tip, stalkless or supported by very short stalks, often with small leaf clumps in the axils. Leaves are mostly smooth, with sparse hairs, especially along the midvein beneath. The leaf edges are smooth and are slightly to strongly rolled under.

This boggy plant bears small white or pinkish flowers arranged in raceme‑like clusters, blooming either at the top of the plant or from the leaf axils in the upper half of the stem. Each delicate flower is supported by four lance‑triangular sepals that cradle the notched petals. The elongated ovary, about an inch long, extends above the stem at the base of the blossom. Sepals, pedicels, and ovaries are covered with short curved hairs mixed with gland‑tipped hairs. The stems, usually single but occasionally branched, also bear hairs that are denser in the upper plant and sparser toward the base, sometimes arranged in lines.

E. leptophyllum spreads both vegetatively and by seed. It produces stolons—creeping stems that root and form new shoots the following spring. Seed dispersal occurs from slender capsules about 1 inch long, which split lengthwise when dry to release numerous tiny elliptic brown seeds, each about 2 mm long and tipped with silky pale brown hairs that aid in wind dispersal.

Although bog willowherb is considered secure due to its wide distribution and abundance, local populations may be threatened by wetland drainage and hydrological alteration. Conservation efforts help ensure its persistence in natural habitats.