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 a wedge-shaped leaf base which tapers to a very short stalk that has prominent small growths at the base. Small upper leaves may be almost stalkless. Margins on larger leaves are generally interrupted by only a few fine widely spaced shallow teeth. The lower leaf surface will be rough from fine stiff hairs; some leaf undersides may be gland-dotted.
The floral array is a loose branched cluster of 1 to 12 stalked hemispherical 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 when open, consisting of an outer ring of 12 to 20 ray florets with pale yellow rays, 15 - 25 mm long, which are not fertile. The back surface of the ray floret laminae are not gland-dotted. These surround an inner disc of 60+ bisexual fertile tubular disc florets with yellow corollas that have five triangular lobes which spread when the floret opens. The five stamens of each disc floret have dark brown to black anther appendages which tightly surround a branched yellow style. Stamens and style are exserted from the tube when the floret opens. The flower head is hemispheric in shape surrounded 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 disc florets 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.
Below: The deltate shaped brownish cypsela, 3 - 4 mm long, with awns fallen away. Photo courtesy Tracey Slotta, ARS Systematic Botantic and Mycology Lab.
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, and her last planting was in 1928. It was still in the Garden at the time of Martha Crone's 1951 census, but she made no notes of ever planting it.
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.
The other species of Helianthus currently in the Garden are: H. hirsutus, Stiff-haired Sunflower; H. pauciflorus ssp. pauciflorus, Stiff Sunflower; H. strumous, Paleleaf Woodland Sunflower; and H. tuberosus, Jerusalem Artichoke.
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.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"