Each year the Rare Plant Monitoring Program picks a plant deserving special attention. Monitors are not required to survey for the species of the year, but the idea is to gain a statewide status update for a particular plant while also building excitement and allowing participants to become more familiar with one plant species.
Missouri rock-cress goes by the Latin binomial Boechera missouriensis. The genus Boechera was once considered synonymous with Arabis until genetic studies showed that these groups were in fact distinct. In the broad sense, Boechera is a large genus, with over 100 species spread across North America, Greenland and eastern Russia. More recent studies suggest that most plants found in eastern North America, including Missouri rock-cress, should be placed in the genus Borodinia while the name Boechera would almost exclusively apply to western species. In Wisconsin, 50 populations of Missouri rock-cress have been reported. But of these, only 25 have been seen in the last 40 years. Seven populations have been searched for recently without success.
Missouri rockcress is found in rocky or sandy sites, usually with relatively little canopy cover. These sites can be oak-pine barrens, dry woodlands, bedrock glades or even sand prairies. Populations are scattered across the northern half of the state, though most occur in the northeast. Common associates of these habitats include blueberries, huckleberry, bracken fern, poverty oat grass, sweet fern, hazelnut, Hill’s oak and pines.
Though most species in the genus are perennials, Missouri rock-cress is a biennial. After germinating, it produces a basal rosette its first year. In June of the second year, plants flower, and fruit matures as a slender flat pod called a silique in July. After going to fruit, the plant dies. No studies have examined self-compatibility in Missouri rock-cress, but related plants are self-compatible and pollinated by small bees and flies.
Rock-cresses, like all mustards, have 4-merous flowers, meaning they have four petals and four sepals, and these are radially symmetric. Distinguishing the rock-cresses from each other, though, can be difficult. Among the 10 or so species in Wisconsin, Missouri rock-cress can be identified by its basal rosette leaves and flower size. The basal leaves are pinnatifid, or lobed more than half way to the leaf midrib. Lyre-leaved rock-cress (Arabidopsis lyrata) has similar leaves but is much smaller than Missouri rock-cress. Next, look to the flowers. Smooth rock-cress (Boechera laevigata) may look similar to Missouri rock-cress, but it does not have lobed basal leaves and its flowers are smaller (3-5 mm vs. 5-10 mm). Also, the petals of Missouri rock-cress are twice as long as the sepals while the petals of smooth rock-cress are only slightly longer than the sepals.
Missouri rock-cress is considered imperiled or critically imperiled throughout its entire range except for a stronghold in the Interior Highlands of Oklahoma, Arkansas and Missouri. Because it requires an at least somewhat open habitat, encroachment is always a threat, and fire suppression or tree planting for timber production will result in habitat loss. When open canopy is maintained, though, Missouri rockcress seems to be able to handle other disturbances and can persist in otherwise low-quality sites like quarry edges or recovering sandy fields. Take this into consideration if you find a population of Missouri rock-cress this summer. Canopy closure may be more of a threat than the presence of weedy species like Kentucky bluegrass, sheep sorrel or even spotted knapweed. As a biennial seed, production is critical to maintaining population viability, but seeds may not remain viable in the seed bank for very long, reinforcing the importance of maintaining suitable habitat. That said, there is a lot we don't know about Missouri rock-cress and why it may be declining.