In The Vascular Plants of Iowa Eilers and Roosa report that fourteen species of Solidago have been found in Iowa. Three of these species S. ptarmicoides, S. riddellii, and S. rigida are sometimes placed in the genus Oligoneuron —as Oligoneuron album, Oligoneuron riddellii, and Oligoneuron rigidum, respectively (See USDA website). One species, S. altissima, was previously considered a separate species, then it was considered to be a variety of S. canadensis (S. canadensis var. scabra). Now, according to the Flora of North America (FNA) it is considered a single species again, but with two subspecies (subsp. altissima and subsp. gilvocanescens. The grass-leaved goldenrod (S. graminifolia) has moved to the genus Euthamia and is now known as Euthamia graminifolia (common goldentop).
Distribution information for the U.S. is available at BONAP and for Iowa at An Illustrated Guide to Iowa Prairie Plants. Seven species—indicated below in bold font—have been reported in Johnson county. Finally, at the top of this page is a navigation bar with a link to "Plant inventories". If your unknown plant was found in one of the inventoried areas you can reduce the probable matches to your plant even further.
Solidago, like Aster, is one of our most difficult genera. Natural hybridization frequently occurs and the species are also highly plastic. For proper study full specimens, showing subterranean parts and basal leaves as well as the whole flowering stem, are essential. Identification of fragmentary specimens is safe only after long familiarity with the group.
Gray's Manual of Botany Eighth Edition (footnnote Page 1381).
Solidago altissima - Tall goldenrod, Late goldenrod
1) Three species are short plants usually less than 3 feet tall—S. flexicaulis (prefers mesic woodlands), S. missouriensis (prefers open plains), and S. nemoralis (prefers sterile soils and old fields). S. flexicaulis may have a zig-zag stem and its leaves are more than 3 cm wide (ovate, petioled, and dentate). S. missouriensis leaves are less than 2.5 cm wide and linear-lanceolate.(cauline leaves oblanceolate decreasing in size from base to inflorescence).
2) The four taller species are generally taller than 3 feet—usually around 5 feet tall. Three of these are often confused: S. gigantea, S. altissima, and S. canadensis—sometimes collectively referred to as "the tall goldenrods" or as the "canadensis complex". Some herbariums (e.g. The Freckmann) prefer to treat these as two species by making S. altissima a variety of S. canadensis. Since the FNA regards them as three separate species, we will take their approach here. They are usually distinguished by the hairiness of their stems and the size of their floral involucres. S. gigantea has smooth (hairless) stems below the inflorescence. S. canadensis has hairy stems below the inflorescence but the hairs diminish and disappear further down the stem. S. altissima has hairy stems throughout the plant.
The floral heads are smaller on S. canadensis. the length of its involucres range from 1.7 to 2.5 mm. Those of the other two species are 2.5-4 mm long. The number of disc flowers per head provide another distinction. For S. gigantea the number is 7-12 compared to 3-6 in the two other species. Finally, the laminae (fused petals) of the ray flowers tend to be longer than 1.5 mm for S. gigantea and 1.5 mm or less in the other two species.
The fourth member of the tall species is S. speciosa usually less than 4 feet tall with thick narrow leaves and reddish stems.
3) Two species seem different enough from the rest to be considered for inclusion in different genera.
These are first, S. rigida (Oligoneuron rigidum) which has two kinds of leaves, relatively large and petioled basal leaves and smaller cauline leaves which are often directed upward along the stem; and second, S. graminifolia (Euthamia graminifolia) which has three veins running the length of its grass like leaves.
4) S. speciosa and S. nemaralis have leaves that are largest at the bottom and are progressively smaller moving up the stem. Beneath the inflorescence, S. speciosa is smooth stemmed while S. nemoralis has pubescent stems.
5) Only two species of goldenrod are known to routinely play host to the gall forming fly (Eurosta solidaginis). These are S. altissima (with hairy stems) and S. gigantea (with smooth stems). Apparently, the fly finds S. canadensis rarely suitable for its purposes. There is information about this relationship on the 37 minute video - The Goldenrod and the Gallfly.