Description: This perennial herbaceous plant is 2½-6' tall with an erect central stem that branches occasionally. The stems are light green to dark purple and sharply 4-angled. The stems are sparsely to moderately pubescent with more or less spreading hairs, especially along the upper half of a plant. The leaves are opposite, about 1-6" long and ½-2½" across; they are medium to dark green above and hairless to slightly short-pubescent. The lower surface of the leaves is light whitish green and moderate to densely short-pubescent. The leaves are ovate-lanceolate, ovate, or ovate-cordate in shape; they have a wrinkled appearance from the veins that can be seen on their undersides. Leaf margins are crenate-serrate. The petioles of the leaves are ½-3" long and channeled above; they are light green to dark purple and slightly to moderately short-pubescent.
 
The flowers are produced in cylindrical spikes from the upper stems. Individual spikes of flowers are 2-6" long and about ¾-1" across. Each spike is packed with a multitude of small flowers facing in all directions. An individual flower consists of tubular corolla with a short upper lip and a slightly longer lower lip that is recurved; the corolla is about ¼" long. Across different ecotypes, the corollas vary in color from white to pale lavendar or pale purple. At the base of each flower, there is an open cylindrical calyx with 4-5 teeth; the latter are triangular-ovate in shape. The calyces are about the same length as the corollas; they are typically light green, sometimes with a slight pink or purplish tint along their upper margins. Individual flowers have 4 strongly exserted stamens with light purple anthers and 1 strongly exserted white style that is divided into 2 parts at its tip. The flowering period occurs from mid-summer to autumn, lasting about 1 month. Individual flowers open up seemingly at random along the floral spike during this time. Each flower has a fruit (a dry schizocarp) that is divided into 4 parts, each part containing a nutlet. The small chunky nutlets are up to 1.5 mm. long and yellowish brown. The root system is fibrous and short-rhizomatous. Clumps of clonal plants often develop from the rhizomes.

Cultivation: The preference is partial sunlight to light shade, moist to mesic conditions, and a loamy to silty soil with decayed organic matter, or sandy soil with decayed organic matter. This plant will wilt if it is exposed to sunny dry conditions, and it doesn't tolerate much competition from other plants. This plant can be propagated from seed or by division of clonal plants.
 
Range & Habitat: The native Giant Purple Hyssop is uncommon to occasional in central and northern Illinois, while in the southern part of the state it is rare or absent (see Distribution Map). Habitats consist of edges of bottomland woods, openings in wooded areas, small meadows in wooded areas, edges of sandy woodlands, openings in sandy woodlands, banks of streams, swales in upland prairies, moist typical savannas and sandy savannas, thickets, and woodland borders along roadsides. Areas with a history of disturbance are preferred if it reduces excessive competition from woody vegetation and tall herbaceous plants.

Faunal Associations: The nectar and pollen of the flowers attract honeybees, bumblebees, other long-tongued bees, Halictid bees (Lasioglossum spp. and green metallic bees), masked bees (Hylaeus spp.), flower flies (Syrphidae), and bee flies (Bombyliidae); see Robertson (1929) and Wilhelm & Rericha (2017). Other information about floral-faunal relationships for Giant Purple Hyssop are scant. A polyphagous aphid, Macrosiphum euphorbiae (Potato Aphid) has been observed to suck plant juices from this plant (Pepper, 1965). As for mammalian herbivores, little is known, but White-tailed Deer probably browse on the foliage sparingly, or not at all, because aromatic members of the mint family are generally not preferred browse for these animals.

Photographic Location: A woodland border in east-central Illinois.

 
Comments: Wild plants of Giant Purple Hyssop have only slightly purple flowers (or even white), although their stems are often purple. Some nurseries sell cultivars of this plant of unknown parentage that have more showy purple flowers. Giant Purple Hyssop (Agastache scrophulariifolia) is similar to the more common Yellow Giant Hyssop (Agastache nepetoides) in its overall habit of growth, but the latter plant is glabrous rather than hairy, and its flowers are pale yellow. Another species, Anise Hyssop (Agastache foeniculum), barely occurs in the wild in Illinois (where it may be adventive), but it is more common in cultivation. This latter plant is shorter than either Giant Purple Hyssop or Yellow Giant Hyssop, and its leaf undersides are distinctly whitened by canescence (dense, very fine, very short white hairs). In addition, the calyces of Anise Hyssop are tinted blue-violet throughout, whereas the calyces of the other two hyssop species are light green (although the upper margins of the calyces for Giant Purple Hyssop are often tinted light pink or light purple). The scientific name of Giant Purple Hyssop refers to the similarity of its leaves to a figwort, such as Late Figwort (Scrophularia marilandica), but their flowers are quite different. Sometimes the scientific name for the plant species being discussed here is spelled Agastache scrophulariaefolia, but it seems this may be a mispelling.