Leaves and stems:

The leafy frond, called a trophophore, is single near the top of the stem, oblong in outline, usually with 2 or 3 pairs of leaflets (pinnae), occasionally 4. The trophophore is 1 to 4 cm (to ~1½ inches) long, held erect to ascending, its stalk 0 to 1.5 cm (to ~½ inch) long. The stem below the tropophore is 1 to 4 cm long. Pinnae are irregularly shaped, often rectangular (wider than long) and may be overlapping, especially in robust plants, and may or may not fold in towards the stalk. The tip edge is smooth to scalloped.

Spores:

At the top of the stem is the fertile frond, called a sporophore, 2 to 8 cm (to ~3 inches) long, rising above the tropophore, the stalk portion .5 to 3 cm (to ~1 inch) long. Forest-grown plants tend to be spindly with very short branches, each with a few round capsules of spores (sporangia), or branches absent altogether with sporangia developing directly on the stalk. Open-grown plants tend to have longer branches with more numerous sporangia, and commonly have sporangia also developing on many pinnae (supernumerary). Spores mature starting in late June.

Notes:

Swamp Moonwort was previously considered a var of (or lumped into) Botrychium simplex (Least Moonwort, var. tenebrosum). Its global range is not currently well defined, primarily due to taxonomic differences, but there are reports of it across the northern US and southern Canada from Minnesota to Nova Scotia and as far south as Ohio and Pennsylvania, possibly extending to Iceland and perhaps Europe. In Minnesota it is found in a variety of habitats, from hardwood forests to cedar swamps to prairies.

Its form can vary depending on habitat. In heavy shade it tends to be spindly, the tropophore typically short with few pinnae, the sporophore with few (or no) branches and few sporangia. In more open areas it can be quite robust, the sporophore more heavily armed with branches and sporangia. In either case, the pinnae are variable in shape but are often rectangular, wider than long, the tip edge smooth or scalloped. Note that, while many Botrychium species may develop sporangia on the basal pair of pinnae, B. tenebrosum is the only Minnesota species known to develop them on any or all pinnae.

Spindly, forest-grown plants can resemble Botrychium mormo (Little Goblin), most easily distinguished by its sporophore, which has sporangia partially sunken into the branch or stalk; it also has a stouter, fleshier appearance and matures in late summer, where B. tenebrosum is more slender and matures in early summer. B. simplex is also very similar and distinguishing the two is not always easy, but a reasonably consistent characteristic is the length of the stalk below the tropophore: in B. simplex, the tropophore tends to split off near the base of the stalk, where it splits off about half the length of the plant in B. tenebrosum, though this is not always the case for either species.

Keep in mind that Botrychium identification can be difficult even for the experts so don't be discouraged if you struggle to get it down to species.