Description: During the 1st year, this
native biennial plant consists of a rosette of basal leaves spanning about 6"
across. These basal leaves are up to 3½" long and ¾" across; they
are obovate or oblanceolate, shallowly lobed or dentate along the margins, and
nearly hairless. During the 2nd year, a flowering stalk develops up to 3' tall
that is unbranched or little branched; it often leans over to one side. This
stalk is pale green, hairless, and glaucous. The alternate cauline leaves are
up to 8" long and 1" across, becoming smaller as they ascend the central stalk.
They are usually lanceolate, hairless, and glaucous; some of the upper leaves
are linear. The margins of these leaves are smooth or sparingly dentate. The
base of each cauline leaf clasps the central stalk with a pair of basal lobes.
Normally, the cauline leaves are some shade of green, but they can become
reddish or yellowish green in bright sunlight. The central stalk (and any
secondary stalks) terminates in an elongated raceme of flowers up to 1½'
long. The small flowers bloom near the apex of this raceme, while the siliques
(slender cylindrical seedpods) droop from their pedicels below. Each flower is
about 1/6" across, consisting of 4 white petals, 4 light green sepals, a pistil
with an undivided style, and several stamens. The petals are barely longer than
the sepals. The pedicel of each flower is about 1/3" long, hairless, and
ascending. The blooming period occurs from mid-spring to early summer and lasts
about 2 months. Each flower is replaced by a slender silique up to 3½"
long that is hairless; it contains a single row of small seeds with winged
margins. The siliques spread widely or droop downward from their pedicels. When
the siliques split open to release the seeds, they can be carried aloft to some
extent by the wind. The root system consists of a taproot. This plant spreads
by reseeding itself.
Cultivation:
The preference is medium shade to partial sunlight, moist to slightly dry
conditions, and a soil that is loamy or slightly rocky. The size of a plant and
the appearance of its foliage can be strongly influenced by its growing
conditions.
Range & Habitat:
Smooth Rock Cress occurs occasionally in most areas of Illinois (see
Distribution
Map); in some areas, it may be locally common. Habitats include mesic
upland woodlands, thinly wooded bluffs,
rocky slopes of bluffs, shaded
cliffs, and ravines near streams. This plant is generally found in hilly
woodlands where deciduous trees are dominant.
Faunal Associations: Small bees and flies
occasionally visit the flowers for nectar or pollen. Bee visitors include
Ceratina spp. (Little Carpenter bees), Nomada spp. (Cuckoo bees),
Osmia spp. (Mason bees), Halictid bees, and Andrenid bees, including
Andrena arabis (Rock Cress Andrenid Bee).
Photographic Location: The plant in the upper
photograph was growing in a wooded ravine near a stream in Vermillion County,
Illinois, while the plant in the lower photograph was growing on top of a
wooded bluff in the same county. The latter plant was exposed to some direct
sunlight because of a small opening in the canopy.
Comments: This is one of the native Arabis
spp. (Rock Cresses) that occurs in wooded areas. It is not particularly
showy and is often overlooked. Smooth Rock Cress can be distinguished from
other Rock Cresses as follows: 1) It is completely hairless during the blooming
period, 2) It has siliques that are widely spreading or drooping, and 3) Its
cauline leaves clasp their stems with a pair of basal lobes. These three
characteristics are usually sufficient to distinguish this species from other
Rock Cresses. Some other characteristics that are occasionally helpful: 1) Its
siliques are round in cross-section, and 2) Each of its siliques contains a
single row of seeds. For example, another woodland species, Arabis
canadensis (Sicklepod), has an appearance that is similar to Smooth Rock
Cress; they both have drooping siliques. However, the cauline leaves of
Sicklepod are sessile, its lower foliage is often hairy, and it has flattened
siliques.