Poor fens are distinguished by their weakly minerotrophic peatland soils influenced by surface and/or groundwater and relatively high species diversity. They are similar to open bogs, but have a higher pH, lack of pronounced leatherleaf and Sphagnum hummocks (often as high as two feet or more with intervening hollows in an open bog), and higher species diversity. In Wisconsin, these differences are all due to the fact that the vegetation in open bogs is slightly elevated above the influence of mineral-rich groundwater, usually by the growth and influence of Sphagnum hummocks, which despite wicking water upwards, actively acidify the rooting zone and cause nutrient availability to be extremely low. Poor fens may lie adjacent to muskegs, also known as treed bogs, but can be differentiated by typically having less than 10% cover of tree species.
Poor fens may be similar to northern sedge meadows in that both may be dominated by sedges, but sedge meadows usually lack calciphiles and have relatively few carnivorous plants and pink-flowered orchids. Poor fens may be distinguished from boreal rich fens by their lower pH, low abundance of strong calciphiles such as common bog-arrow grass (Triglochin maritima) and twigrush (Cladium mariscoides) and greater proportion of Sphagnum mosses. Poor fens often occur adjacent to Great Lakes shore fens, especially along the Lake Superior shoreline, and can be differentiated by their soils having slightly lower pH, higher coverage of Sphagnum moss (usually sparse in shore fens), and a grounded rather than floating mat not influenced by Great Lakes water levels. They also tend to have higher species diversity per unit area, perhaps due to the nutrient and elevation gradients provided by low Sphagnum hummocks. Poor fens may also resemble patterned peatlands, especially the minerotrophic hollows (flarks), but lack the patterning of alternating low ridges (strings) usually evident on aerial photos. Poor fens are also exceedingly more common than patterned peatlands.
