Solidago gigantea is usually the least hairy species of the S.canadensis complex. The stems may be somewhat glaucous and the array is usually somewhat more open than in S. canadensis and S. altissima, and less leafy than S. lepida. Its blooming season begins and ends earlier than S. altissima in eastern North America. The species is diploid, mostly east of the Appalachian Mountains, tetraploid throughout the eastern forest area, and hexa-ploid on the prairies. The broader-leaved hexaploids on the prairies have been treated as S. shinnersii; G. H. Morton (1984) indicated that the differences are not diagnostic. Reports of hexaploids in the mountains from Alberta, Idaho, Oregon, Washington, and northwestern Wyoming all have minute stipitate glands on the phyllaries, peduncle bracts, and sometimes the distalmost leaves; such plants belong in S. lepida, as do plants from British Columbia. Stems sometimes have 1–2 elongate insect galls near the base (S. Heard, pers. comm.). Although reported from much of Florida, specimens were seen only from Liberty County; all other collections are S. leavenworthii.
This native perennial wildflower is 3-7' tall and largely unbranched, except at the apex, where the flowering stalks occur. The central stem is light green or pale purple, terete, glabrous, and sometimes glaucous. The alternate leaves are 3-5" long and 1/32/3" across, becoming only slightly shorter while ascending the central stem. Sometimes there are leafy lateral stems that develop from the leaf axils of the central stem, but they are very short and insignificant. The alternate leaves are narrowly lanceolate to elliptic, slightly to sharply toothed along their margins, and sessile; each leaf tapers gradually towards its tip and base. The upper surfaces of the leaves are medium to dark green and glabrous, while their lower surfaces are a slightly lighter shade of green and glabrous or nearly so (sometimes there are fine hairs along the major veins below). Each leaf has 3 veins (a central vein and 2 lateral veins) that are nearly parallel to each other. The central stem terminates in a panicle of flowerheads up to 1' long and 1' across. There is some variability in the shape and size of the panicle across different populations of plants. Individual branches of the panicle are light green, slightly to moderately pubescent, and recurved. There are usually some leafy bracts along the branches of the panicle; these leafy bracts are similar to the leaves, except they are smaller in size. Individual flowerheads are a little less than ¼" across, consisting of about 7-15 yellow ray florets and 5-11 yellow disk florets. At the base of each flowerhead, there are appressed floral bracts in 2-5 overlapping series; individual floral bracts are linear-lanceolate in shape. The blooming period occurs from late summer to early fall for about a month. Both ray and disk florets produce small achenes with sessile tufts of hair. The achenes are bullet-shaped, flat-topped, and finely pubescent. The root system is fibrous and rhizomatous. Colonies of plants are often formed from the rhizomes.
It flowers from July-September. (Peattie, 1930) It blooms August through October.(Hultman, 1978) The active growth period is summer. Fruit/seed period begins in the summer and ends in the Fall. (USDA PLANTS, 2009)
Giant Goldenrod is occasional to locally common throughout Illinois. However, some decline in populations across the state may have occurred from habitat destruction. Habitats include openings in floodplain woodlands, borders of woodlands, low partially shaded areas along rivers and ponds, riverbottom prairies, wet prairies, prairie swales, soggy thickets, fens, and ditches. The preceding list includes both sandy and non-sandy habitats. In general, Giant Goldenrod is an adaptable plant with low fidelity to any particular habitat. Its main requirement is the availability of adequate moisture.