Description: This native perennial plant is largely
unbranched and up to 6' tall. The stout central stem is light green, and
slightly hairy in the upper stem and at the base of the upper compound leaves.
The compound leaves are even-pinnate with about 10-20 leaflets. The medium to
dark green leaflets are individually up to 2½" long and ¾"
across. Each leaflet is oblong, with smooth margins, and a pointed tip. Near
the upper base of a compound leaf is a small club-shaped gland; it is ovoid or
dome-shaped above a short stalk. This gland secretes nectar to attract certain
kinds of insects (see below for more information). From the axils of the upper
compound leaves develop upright racemes of yellow flowers. Each flower is about
¾" across; it has 5 pale yellow sepals, 5 yellow petals, 10 stamens with
black anthers, and a conspicuous pistil with long white hairs. The petals have
a tendency to turn white as they age, while the hairy pistil eventually
develops into a seedpod. There are 3 upper petals and 2 upper petals in a
flower; they have a tendency to become curved and have a claw-like appearance.
The blooming period is mid- to late summer, which lasts about a month. There is
no noticeable floral scent. The seedpods are about 4" long when fully mature;
each seedpod has 10-18 segments, and each segment is about as long as it is
across. Eventually, these seedpods become dark brown in appearance. The root
system consists of a central taproot and rhizomes. This plant often forms
vegetative colonies.
Cultivation:
The preference is partial to full sun, and moist to mesic conditions. A rich
loamy soil is preferred. This plant can become quite tall when the soil is
fertile and moist; it may flop over while the flowers and seedpods are
developing. Foliar disease is not a significant problem.
Range
& Habitat: Wild Senna occurs in scattered counties throughout
Illinois; it is perhaps a little more common in the east than the west (see
Distribution
Map). This species is occasional in some areas, and uncommon or absent in
others. Populations in the wild are probably declining as a result of modern
development. Habitats include moist meadows near rivers, savannas, fens,
pastures, and roadsides. It is often flourishes within the floodplain of
rivers. Occasionally, Wild Senna is found in flower gardens because of the
showy flowers.
Faunal
Associations: The flowers atttract bumblebees primarily, which seek
pollen from the anthers and possibly nectar. Halictid bees also visit the
flowers for pollen, but are less likely to achieve cross-pollination. The
extra-floral nectaries, on the other hand, attract primarily ants and a few
other insects, including ladybird beetles. It is possible that these insects
protect the plant from other insects that would attack the foliage; it has also
been suggested that the extra-floral nectaries discourage ants from robbing
nectar from the flowers. The caterpillars of some Sulfur butterflies rely on
Senna spp. as a source of food. In Illinois, this includes Eurema
nicippe (Sleepy Orange), Eurema lisa (Little Sulfur), and Phoebis
sennae eubule (Cloudless Sulfur). Mammalian herbivores usually avoid
consumption of the foliage, which has purgative properties. The seeds may be
eaten by some upland gamebirds.
Photographic
Location: The photographs were taken of plants growing in an herbal
garden at Meadowbrook Park, Urbana, Illinois. The lower photograph reveals one
of the extra-floral glands.
Comments: This is a striking plant while in bloom,
and it has attractive foliage. It is difficult to distinguish Wild Senna from
Senna marilandica (Maryland Senna), which has a very similar appearance.
Generally, Wild Senna has a more northern distribution than Maryland Senna, but
in Illinois their ranges overlap. On Wild Senna, the pistils of the flowers
have long white hairs, while in Maryland Senna these hairs are shorter and more
appressed. The nectary glands at the base of the compound leaves are also
supposed to shaped differently. In Wild Senna, these glands have a short stalk
and are club-shaped (although the shape of this 'club' can be variable), while
the glands of Maryland Senna can be short-cylindric, rounded, or dome-shaped,
but they are without short narrow stalks. The seedpods of these two species are
also supposed to be somewhat different in appearance in relation to the number
and shape of the seedpod segments. Another scientific name for Wild Senna is
Cassia hebecarpa.