Diario de jharkness

Existing literature on the identification of Solidago canadensis and S. altissima, two common North American goldenrods, is inconsistent and at times unclear. Photographic evidence of these species is seemingly even less reliable for accurate identification, considering the frequency of misidentifications; it appears that many iNaturalist observations of these species (particularly S. canadensis) are misidentified.

I have found some literature to be plainly inaccurate for the identification of these species in my area (western New England and eastern New York). All sources I have been able to find describe S. canadensis as being a shorter-growing species with glabrous (or nearly glabrous) abaxial leaf surfaces; these are described as solid traits by some authors. My personal observations indicate that S. canadensis is (at least regionally) a generally somewhat taller-growing species than S. altissima.

The feature most often used to distinguish the two species is the presence of hairs on the abaxial (lower) leaf surface, which in S. altissima are said to be uniformly dense on and between the veins, while in S. canadensis the hairs are said to be restricted entirely to the veins, or even that the leaves are glabrous. S. canadensis var. hargeri is reported to have hairs between the veins, but with prominent teeth, denser capitula, dense pubescence on the lower stem, etc., both varieties may be present in my area, though S. canadensis var. canadensis clearly dominates and there is little observable difference of the hairs between these individuals and those suspected to be S. canadensis var. hargeri. Therefore, the presence of hairs between the leaf veins cannot be used alone to differentiate these species, but detailed observance of a comprehensive number of characteristics is necessary.

Haines (2011) describes S. altissima as having broad leaves, with subentire margins and S. canadensis as having narrow leaves with more prominently toothed margins (associated pictures on GoBotany appear to show both species), while Abrahamson (2020) describes S. altissima as having narrow leaves with subentire margins, and S. canadensis as having broader leaves with more prominent teeth.

It is apparent that numerous errors and inconsistencies occur in the scientific literature regarding the identification of these species.

I have found these two species most easily separable by their clonal habits, inflorescence size and structure, and in the size of their involucres and ray flowers. S. altissima commonly forms large, unbroken colonies, has relatively small, compact inflorescences, and long involucres and ray flowers, while S. canadensis appears to only form small clumps, has much larger, open inflorescences, and has smaller involucres and ray flowers. These two species also appear to "prefer" different habitats, with S. altissima being an extremely common species of mesic to xeric old fields, and S. canadensis being most common in slightly wetter sites, often occurring on the edges of shrub thickets, and wetland margins; it is also apparently more shade tolerant than S. altissima. The presence and abundance of galls of Eurosta solidaginis is also a reliable trait for the identification of S. altissima, which generally supports numerous galls even in a small colony; I have yet to observe one of these galls on S. canadensis.

Below is a comparison of the two species based on the traits I commonly observe in my region.

Solidago canadensis
170-240cm tall when in flower (may be shorter in stressful environments)
Involucres 2-3mm tall
Rays 0.5-1.2mm long
Inflorescences 35-50cm tall, 40-75cm wide
Stem nearly glabrous at base, hairs relatively sparse and long throughout
Leaves around 16cm long (2cm wide), with a wrinkled appearance
Leaves are oblanceolate, nearly as wide in proximal half, with short teeth, uncommonly with prominent teeth
Abaxial leaf surface thinly pubescent throughout, though hairs on veins denser and longer (hairs between veins are not visible at all angles and thus may be mistaken for being glabrous)
Leaf texture is generally somewhat rough, but sometimes smooth
Leaves and stems uniformly dark green
Does not form large colonies, rather small clumps
Flowers open in mid August
Tends to tip over without support

Solidago altissima
110-210cm (more typically 130-170cm - have only ever found one stem over 200cm) tall when in flower
Involucres 3-4mm tall
Rays 1.8-2.2mm long
Inflorescences 15-35cm tall, 20-40cm wide
Also nearly glabrous at base of stem, hairs short and quite dense
Leaves around 18cm long (1.5-2cm wide), without wrinkled appearance
Leaves are oblanceolate, narrow in proximal half, usually with prominent teeth, though some may be subentire
Abaxial leaf surface densely and uniformly pubescent, no difference in hairs on and between veins
Leaf texture is rough
Leaves light green (occasionally yellow-green), stems yellow, occasionally red
Forms large colonies often to the exclusion of other species, though new colonies are common and can appear to be individuals or small clumps
Flowers open in late August and early September
Stems on edge of colonies tend to tip over, but not those in middle