Purple Giant Hyssop is a native erect perennial plant growing to 5 feet in height, with a 4-angled stem like most members of the mint family. Stems usually have whitish hairs. Branching occurs near the top of the stem resulting is several flower spikes.
Leaves are opposite, stalked, with a rounded base, pointed tip, and coarse teeth. Leaf stalks and the lower surface of the mid-vein of the leaves have sparse long hair while the underside of the leaf has fine dense hair giving a whitish appearance.
The inflorescence is a 3 to 18 inch cylindrical spike containing whorl-like clusters of tubular flowers. In the mint family this arrangement is called a 'verticillaster' where the flowers look like a whorl arrangement but are actually in cymes that rise from the axils of a pair of opposite bracts. A series of these cymes and subtending bracts form close together on the spike and sometimes the series is interrupted leaving gaps. A pair of small ovate bracts will also be found at the base of the inflorescence.
The flowers are 5-parted, the corolla is whitish to purplish and exceeds the calyx in length. Flowers are not hairy. The corolla has an upper lip, formed by two fused petals and divided into 2 lobes which turn upward and a lower lip formed from 3 fused petals divided into 3 lobes that turn downward. The calyx, with 5 long pointed lobes, is green to whitish-purple compared to Agastache foeniculum where it is purple. There are 4 long stamens in pairs with white filaments and a style, all protruding from the corolla. A pair of stamens curve upward and a pair curve downward.
Seed: The mature fruit is a dry nutlet containing one oblong seed. These disperse by wind shaking the stem. Seeds are small and need light and 60 days of cold stratification to germinate. Put them on the ground or let them fall from the plant in the Autumn and let Winter do the work.
Habitat: Purple Giant Hyssop grows in open woods and thickets in soils that are sandy to loamy, with wet-mesic to dry-mesic moisture conditions and full sun. Partial shade is tolerated. The root systems is rhizomatous and the plant spreads by the underground stolons. It is very strong-scented.
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 name scrophulariifolia, is a reference to the leaves of the Scrophularias, (figworts) whose leaves resemble the leaves of this plant, that is, somewhat deltate shape with coarse teeth. That led to the alternate common name of 'Figwort Giant Hyssop.'
The author names for the plant classification cover two people: First to classify was ‘(Willd.)’ which refers to Carl Ludwig Willdenow (1765-1812), German botanist, a founder of the study of the geographic distribution of plants. He was director and curator of the Botanic Garden of Berlin. His work was updated by 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.
Comparisons: A similar looking plant but shorter, and whose flowers have a purplish calyx is Blue Giant Hyssop, A. foeniculum. It also tolerates slightly more dry soils and spreads by reseeding. Another is Yellow Giant Hyssop, Agastache nepetoides, which is also tall, branched, but with yellow flowers and a catnip odor.
Above: The flower spike may have interruptions as shown here in the 2nd photo which also shows that the clusters are separate cymes, not whorls. 3rd photo - The white to purplish corolla is longer than the calyx, which on this species is green.
Below: 1st photo - Stems are 4-angled with whitish hair. 2nd photo - The inflorescence. 3rd photo - Fall seed head.
Below: 1st photo - Leaves are opposite, stalked with regular coarse teeth, stalks and the underside (2nd & 3rd photos) midvein have hair while the undersurface had dense fine hair making a whitish coloration.
Notes: Purple Giant Hyssop is not indigenous to the Garden. Eloise Butler introduced the species in August 1917 when she collected some plants from a meadow near the parkway in Glenwood Park. Later on Oct. 2, 1917 she planted another from Minnehaha Park. It was in the Garden at the time of Martha Crone's 1951 census where she listed it as Figwort Giant Hyssop. It was absent on the 1986 census but is now present again. The plant has a limited range in North America, being found only in the eastern half of the United States, excluding the southern gulf coast states, and in Canada it is found only in Ontario. It is listed as threatened or endangered in six of the eastern states. In Minnesota it is restricted to the SE quadrant of the state including a few of the metro counties and a few scattered counties in the SW Quadrant.
Agastache scrophulariifolia is one of three members of the Agastache genus known in Minnesota, The other two are A. nepetoides, Yellow Giant Hyssop, which is rare and not collected since 1938, and A. foeniculum, Blue Giant Hyssop.
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"