Giant Sunflower is a tall, erect, native perennial forb growing on unbranched hairy stems from 3 to 10 feet high. The stems are frequently reddish, the hairs spreading, but some plants may have few hair or none and may have only reddish tints on the stem as species of Helianthus are quite variable.
The leaves are all stem leaves, opposite on the lower stem and becoming alternate in the upper portion. The shape is lanceolate, 3x as long as wide, with 3 main nerves from the leaf base. Leaves taper to a short stalk which has prominent small growths at the base. Margins on larger leaves are generally flat with a few fine widely spaced teeth. The lower leaf surface will be rough from fine stiff hairs, some of which may be gland-dotted.
The floral array is a loose branched cluster of 1 to 12 flower heads at the top of the stem. Individual flower stalks are less than 4 inches long.
The flowers are composite, about 1-3/4 to 3-1/4 inches wide, consisting of an outer ring of 12 to 20 pale yellow ray florets which are not fertile and an inner disc of 60+ bisexual fertile disc florets whose corollas are tubular with five yellow lobes which spread when the floret opens. The back surface of the ray floret laminae are not gland-dotted. The five stamens of each disc floret have dark brown to black anther appendages which surround a branched yellow style. The flower head is hemispheric in shape with 20 to 25 green phyllaries, in several series, that are linear in shape, loosely arranged or spreading, with pointed to abruptly tapered tips. The margins of the phyllaries are usually finely hairy as is the outer surface, but not gland-dotted.
Seed: Mature flowers produce a dry deltate shaped brownish cypsela, 3 - 4 mm long, with a pair of awl-shaped awns on the top, which quickly fall away. Seeds require 30 days of cold stratification for germination. If planted in the Autumn, Winter will do the work.
Habitat: Giant Sunflower grows from a fleshy root system that develops small rhizomes which allow the plant to spread vegetatively. It prefers full sun, and moist conditions (wet mesic to mesic).
Names: The genus Helianthus is from two Greek words, helios for 'sun' and anthos for 'flower'. The species giganteus refers to the tall height of the plant. The author name for the plant classification, 'L.' is for Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy.
Comparisons: This species has similarities to H. grosseserratus (Sawtooth Sunflower) and H. nuttallii (Nuttall's Sunflower) with which it can inter-grade as both exist is Minnesota. However, H. grosseserratus has leaves that have coarse teeth on the margins whereas H. nuttallii has leaves more like H. giganteus, but in H. nuttallii the anthers have yellow appendages, not black or dark brown. Neither have the small growths on the leaf stalk base.
Above: The yellow ray florets are not fertile but the 60+ disc florets are. The anther appendages are dark brown and surround the style and are greatly exserted from the yellow corolla tube which has 5 pointed lobes the spread outward when the flower is open.
Below: The outside of the flower head has 20 to 25 phyllaries, in several series, that are linear in shape, loose arranged or spreading, with long pointed tips. These and the flower stalk are hairy.
Above: The plant can reach to 10 feet high. 2nd photo - Stems may or may not have fine hair as seen here or may have only reddish tints. 3rd photo - Leaf stalks are hairy and there is a pair of very prominent small leaf-like structures at the base of the stalk on the upper leaves.
Below: Stem leaves, opposite on the lower stem (3rd photo) and becoming alternate in the upper portion (1st photo). The shape is lanceolate, 3x as long as wide, with 3 main nerves. The underside (2nd photo) has many short rough hair. The leaf nodes (3rd photo) are slightly swollen.
Notes: Giant Sunflower is not indigenous to the Garden but was first planted by Eloise Butler in 1911 with plants sourced from Gillett's Nursery in Southwick MA. She then planted twice in 1918 with plants from Mahtomedi Mn and from Hazel Park in St. Paul. It was still in the Garden at the time of Martha Crone's 1951 census.
Giant Sunflower is native to counties in Minnesota in the northern 2/3rds of the state and is found in North America from the Mississippi River area of the U.S eastward to the coast except for Florida and New Hampshire. In Canada it is known in Alberta, Manitoba, Ontario, Quebec, New Brunswick, Newfoundland and Nova Scotia. There are a total of 12 species of Helianthus native to Minnesota.
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"