Yellow Giant Hyssop is an erect native perennial forb growing from 3 to 6 feet high on sturdy green 4-angled heavily ridged stems. Stems can be much branched near the top of the plant. The plant is aromatic with the odor of catnip.
Leaves are opposite, thin, coarsely toothed, somewhat egg shaped with a rounded base on long stalks. Leaf size is up to 6 inches long by 3 inches wide. The underside is pale in color due to fine whitish hair. Upper leaves will be smaller.
The inflorescence is a dense spike of whorl-like flowers, up to 8+ inches high, atop the stem with shorter spikes on side branches, usually not interrupted. In the mint family this whorl-like arrangement is called a 'verticillaster' where the flowers look like a whorl arrangement but are actually in cymes that rise from the axils of opposite bracts. Large plants can produce many spikes. Flowers open in various spots around the spike, not from the bottom to the top.
The small flowers are 5-parted, about 1/3 inch long with a calyx that forms a tube with 5 sharply pointed teeth. The tube is green in color and the flowers are yellow to greenish. Protruding from the corolla are 4 stamens with white filaments and yellowish anthers and a single white style with a bifurcated tip. Flower clusters in the spike are interceded with ovate green floral bracts. Many species of bees are attracted to the flowers. Ontario Wildflowers reports that research has shown this to be one of Ontario's top 20 plants for feeding pollinating insects.
Seed: Fertile flowers produce a dry nutlet containing one oblong brown seed about 1 mm x 2 mm. These disperse by wind shaking the stem. Seeds are small and need light for germination plus 60 days of cold stratification. Seeds that fall from the plant will readily self-germinate in the Spring and the seedlings transplant easily and larger seedlings will flower the same year as Eloise Butler reported in 1914.
Habitat: Yellow Giant Hyssop prefers partial sun to light shade in moist to mesic conditions. In full sun there must be adequate moisture. The root system is fibrous with rhizomes allowing the plant to populate by the root system in addition to seeding, but it is not an aggressive spreader via the root system - seeds spread the plant around.
Names: The genus Agastache is derived from two Greek words - agan, meaning 'very much' and stachys, meaning 'an ear of wheat' which together refer to the flower spikes of this genus having many flowers, like grains of wheat. The species nepetoides means like "Nepeta" referring to an aromatic plant, which in this case is the aroma of catnip, hence the alternate common name of Catnip Yellow Hyssop. The author names for the plant classification cover two people; first to classify was '(L.)' which refers to Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy. His work was updated by ‘Kuntze’ who is Otto Kuntze (1843-1907) German botanist, who edited the collections in Berlin and Kew Gardens and then published Revisio Generum Plantarum which laid out new rules for nomenclature which were rejected at the time, but some acceptance came long after his death.
Similar plant or comparisons: A similar looking plant but shorter, and whose flowers have a purplish calyx is Blue Giant Hyssop, A. foeniculum. It tolerates slightly more dry soils and spreads by reseeding. Another plant but as tall or taller, and whose flowers have a green calyx is Purple Giant Hyssop, A. scrophulariifolia. That plant's roots also produce stolons allowing it to reproduce that way.
Above: The inflorescence showing the method of flowering at various spaced around the spike. The drawing from Britton, N.L., and A. Brown. 1913. An illustrated flora of the northern United States, Canada and the British Possessions. 3 vols. Charles Scribner's Sons, New York.
Below: 1st photo - the flowers have an exserted style and 4 stamens. The calyx tube is green with 5 pointed lobes. 2nd photo - the stem is 4-angled and heavily ridged (or winged as it is sometimes called). 3rd photo - the underside of the leaf - pale due to fine whitish hairs.
Below: Two views of the egg shaped leaf.
Below: An example of a mature large plant with numerous side branches at the top each of which end in a flower spike.
Notes: Yellow Giant Hyssop is native to the eastern half of the United States except the Gulf Coast, New Hampshire and Maine. It is known in Ontario and Quebec. In Minnesota it is rare in the wild, only previous collected in Chippewa county some years ago, but it has been reported in various other places where it has either been planted or volunteered from a seed source. It is considered native even though the population area is quite restricted but is not on the endangered or threatened plant list. It is listed as threatened in Wisconsin, Vermont and New York.
It is not planted for beauty but for pollinators in native gardens. It was first planted in the Wildflower Garden in 1914 by Eloise Butler - on Sept. 19 from Minnetonka and on Oct. 15 from Fort Snelling - and again in 1917 and '18. Martha Crone listed it on her 1951 plant census. While missing for some years it has again made its appearance in the Garden and outside the Garden.
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"