Montana Field Guide

Cusick's Horse-mint is an herbaceous, long-lived perennial with numerous stems, woody at the base arising from a branched rootcrown surmounting a taproot. Plants are 1-2 dm high, but the stems often lie prostrate beneath the surface spreading from the rootcrown. Opposite leaves have blades that are 1-2 cm long and triangular with a rounded base and a petiole that is up to 1 cm long. Foliage is covered with fine, short hairs. White flowers are borne amongst purple-tipped bracts in a head-like inflorescence that is 1.5-4 cm long. The tubular corolla is 8-12 mm long, and the purple-tinged calyx is tubular with 5 pointed lobes. The 5 stamens are exserted from the mouth of the flower tube.

Flowering occurs in late June and July.

The related nettle-leaf horsemint (Agastache urticifolia) is taller (at least 4 dm) and has larger leaves.

In the Tendoy Mountains, Cusick's horesmint grows on open slopes with little vegetation cover. The populations are primarily on steep, loose talus below limestone outcrops, often in chutes, with a few plants usually occupying talus deposits at the base of the slope. On the Beaverhead National Forest, this species is confined to the south-facing slopes of narrow canyons, across a wide range of elevations (Vanderhorst 1995). One lower-elevation occurrence occupies the slope above a broad valley.

Woody dominants include limber pine, Douglas fir, mountain mahogany (Cercocarpus ledifolius), big sagebrush (Artemisia tridentata), and a gooseberry (Ribes setosum). Major grasses include bluebunch wheatgrass and spike-fescue (Leucopoa kingii). Frequently associated forbs include the talus-adapted perennials Arenaria nuttallii, Oenothera caespitosa, Phacelia hastata, and Penstemon montanus, and annuals such as Collinsia parviflora and Polygonum douglasii.

Agastache cusickii seems to be confined to areas of smaller-size talus. The substrate is constantly settling and shifting and is easily disturbed. Soil development is minimal on the limestone talus where the species grows in the Tendoys, however at one site where the banks have been cut by quarrying, some horizonation was noted (Vanderhorst 1995). The upper zone of the talus is extremely well-drained, however clays provide a higher water-holding capacity in the lower zone. This steep habitat is not generally susceptible to fire; the species may sprout from the caudex following disturbance (USDA, Fire Effects Information System, http://www.fs.fed.us/database/feis/).

POLLINATORS
The following animal species have been reported as pollinators of this plant species or its genus where their geographic ranges overlap: Bombus appositus, Bombus centralis, Bombus flavifrons, Bombus nevadensis, Bombus ternarius, Bombus griseocollis, Bombus impatiens, Bombus insularis, and Bombus suckleyi (Thorp et al. 1983, Johnson 1986, Mayer et al. 2000, Colla and Dumesh 2010, Williams et al. 2014).

This species' habitat is by nature easily disturbed, however all occurrences on the Beaverhead National Forest are in remote, relatively undisturbed sites (Vanderhorst 1995). The steepness of this habitat tends to make it inaccessible to cattle, and the timber on these slopes is not of commercial quality (Vanderhorst 1995).

One large population on a lower-elevation valley slope lies along a major gravel road, where past quarrying of limestone talus has reduced the habitat extent, and road maintenance could impact bank stability. Roads are also found in some canyon bottoms where the species occurs, but these are infrequently traveled. Future road improvement or maintenance and other canyon-bottom activities could be managed to avoid impacting the populations and habitat.

STATE THREAT SCORE REASON
Reported threats to Montana’s populations of Cusick's Horsemint are related to some roadside populations where gravel mining and herbicide application are persistent (MTNHP Threat Assessment 2021). The largest population in addition to others are found along a remote county road near a rock quarry where gravels are regularly collected for road maintenance. Herbicide application is also known to occasionally destroy plants along this road.