Flower:

Branching clusters of numerous small, stalked, yellow flower heads at the top of the plant. Cluster shape is variable, from pyramidal with spreading branches and up to 1200 flower heads, to much smaller and more compact with upright branches, appearing club shaped with as few as a 100 heads on smaller plants. Flowers are less than ¼ inch across with usually 8 to 15 petals (ray flowers) surrounding a center disc with usually 3 to 6 disc flowers.

Surrounding the base of the flower are 3 or 4 layers of narrow, lance-linear, hairless greenish bracts (phyllaries), the entire set of bracts (involucre) 2 to 4.5+ mm (to 1/6+ inch) long. Flower stalks are hairy, usually shorter than the involucre, all arranged on one side of the branch (secund) and curving upward.

Leaves and stems:

Leaves are all alternate without a basal rosette, relatively thick and firm, strongly 3-nerved, 2 to 6 inches long, ¼ to ¾ inch wide, broadest near the middle, tapering to a sharp point at the tip and tapering to a stalkless base. Edges are variable, from toothless, to minutely toothed nearly to the leaf base, to toothed only on the tip half. Leaves are largest around mid-stem, becoming much smaller above, and lower stem leaves often withered away by flowering time.

The upper surface is rough textured, the lower short hairy, more densely hairy along major veins.

Stems are erect and rigid, generally short stiff-hairy throughout, though hairs may wear off with age especially on the lower stem. Upper stem hairs are persistent and may be glandular into the flower cluster. Insect galls are often present on the upper half of the stem. Stems are single or up to 40+ stems in a group from underground creeping rhizomes, though this may be hard to determine when entire fields can be filled with evenly spaced stems.

Fruit:

Fruit is a dry seed (achene) with a tuft of whitish hairs (pappus) 2.5 to 3.5 mm long attached at the tip to carry it off in the wind.

Achenes are brown, sparsely to moderately hairy, oblong to narrowly cone-shaped, .5 to 1.5 mm long.

Notes:

The “Canada Goldenrod complex” in Minnesota consists of Solidago canadensis, S. altissima, and S. gigantea (related S. missouriensis and S. juncea are not technically included but are also similar except have basal leaves). They can be a tough group to crack.

Tall Goldenrod and the closely related Canada Goldenrod (S. canadensis) are very similar and both have two recognized varieties or subspecies; the two of Canada Goldenrod are var. canadensis and var. hargeri, and S. altissima has subsp. altissima and subsp. gilvocanescens. Some authors treat them all as four varieties of the single species S. canadensis and they will always be a challenge to readily separate. While multiple other references all have their two cents worth, our descriptions are based closely on those found in Flora of North America and based on our own field observations over time.

Plants with involucres (the set of bracts surrounding the base of flowerhead) 2 to 3 mm long are either S. canadensis or S. altissima susp. gilvocanescens, where S. altissima subsp. altissima has involucres 3 to 4.5+ mm long. Numbers of involucres on mature plants should be measured to obtain a reasonable average. Plants with hairs on the lower leaf surface as well as the veins are either S. altissima or S. canadensis var. hargeri. Beyond that, S. altissima has longer pappus hairs and thicker, firmer leaves that are minutely toothed or mostly toothed in the tip half where S. canadensis leaves are thinner, more lax, mostly toothed nearly to the leaf base, and is generally less hairy throughout. S. altissima also frequently has stem galls where S. canadensis never does.

• S. altissima var. altissima stems are moderately hairy to the base, though hairs may wear off later in the season, leaf undersides are moderately hairy on the surface and more densely so on the veins. Involucre 3 to 4.5+ mm (1/8+ inch) long, pappus hairs more than 2.4 mm long.
• S. altissima var. gilvocanescens stems are moderately hairy to the base, though hairs may wear off later in the season, leaf undersides are moderately hairy on the surface and more densely so on the veins. Involucre 2 to 3 mm (max 1/8 inch) long, pappus hairs more than 2.4 mm long.
• S. canadensis subsp. canadensis stems are hairless to sparsely hairy below mid-stem, and leaf undersides are hairless or hairy only along major veins. Involucre 2 to 3 mm long, pappus hairs less than 2.3 mm long.
• S. canadensis subsp. hargeri stems are sparsely to moderately hairy below mid-stem and leaf undersides are hairy along veins as well as minutely hairy on the surface. It also has a distinctive graceful, cascading panicle that is wider than long. Involucre 2 to 3 mm long, pappus hairs less than 2.3 mm long.

The third species in this complex, Giant Goldenrod (S. gigantea), has stems hairless below the flower clusters that often have a waxy coating, its leaves are toothy and hairless except sometimes on the veins, and it also frequently has galls on the lower stem, but they'll be hairless on the surface where galls on Tall Goldenrod will be hairy. Contrary to popular belief, Tall goldenrod appears to be the predominant species throughout the greater Metro area, typically tolerating drier sites than Canada or Giant Goldenrod and it—not Canada Goldenrod—is usually the species you'll see completely filling up old fields or open disturbed areas.