[ field key — habit · leaf · flower · pod · distinguishing from S. hebecarpa ]
Habit & Stem
A shrub-like herbaceous perennial, typically 3–5 ft
tall and shorter than Senna hebecarpa. Stems are erect to
slightly arching, branched in the upper portion, and finely
pubescent (hairy) on the upper stems and inflorescence branches
— a key difference from the glabrous stems of S. hebecarpa.
The plant has a dense, bushy form with multiple stems arising from the
woody crown, giving it a more substantial presence than its height would
suggest. Emerges slightly later than S. hebecarpa in spring
(early to mid-May in the Tulsa area) and grows steadily through early
summer.
Leaves
Even-pinnately compound, 4–8 in long, generally
smaller than those of S. hebecarpa, with 6–12 pairs
of oblong leaflets, each 3/4–1.5 in long. Leaflets are
medium green, smooth to slightly hairy on the underside, with
entire margins and a blunt or slightly notched tip with
a tiny mucro. The petiolar gland between the lowest
leaflet pair is nearly sessile, globular or dome-shaped
— broader and less stalked than the club-shaped gland of
S. hebecarpa — and is a reliable identification feature
under a hand lens. Foliage is somewhat coarser in appearance than
S. hebecarpa.
Inflorescence & Flowers
The flowers of S. marilandica differ in arrangement from its
cousin: rather than terminal racemes, the flowers are borne in
short axillary racemes or clusters in the upper leaf
axils, making the inflorescence appear tucked among the foliage rather
than displayed above it. Individual flowers are essentially identical to
S. hebecarpa: five yellow petals, 3/4–1 in
across, with prominent dark brown stamens forming a
contrasting center eye. The petals are somewhat irregularly arranged
and drop within a day or two. Bloom period is July through
August, often starting a week or so later than
S. hebecarpa when the two are grown side by side.
Fruit & Seeds
The fruit is a flat, papery-brown legume pod 3–4 in
long and about 3/8 in wide, slightly shorter and narrower than those of
S. hebecarpa. The pods are held at an angle from the stem and
contain 8–14 flattened, rectangular seeds. Pods
dry to a dark brown and often persist on the plant through winter, splitting
along both sutures to release seeds. The seeds, like those of
S. hebecarpa, have a hard seed coat and require scarification
for reliable germination. The rattle of dry pods in winter wind is a
diagnostic sound of a senna planting at rest.
Distinguishing from Senna hebecarpa
The two eastern US perennial sennas overlap in range and appearance but
segregate by habitat and morphology.
S. marilandica is generally shorter (3–5 ft
vs. 4–6+ ft), has pubescent (hairy) upper stems vs.
glabrous, bears flowers in axillary clusters rather than
prominent terminal racemes, and has a short, globular petiolar
gland vs. the stalked, club-shaped gland of S. hebecarpa.
Ecologically, S. marilandica prefers drier, upland
sites — open woods, prairie edges, and dry meadows of the
Cross Timbers and Tallgrass Prairie — while S. hebecarpa is
found in moister bottomlands, streambanks, and wet meadows. If you're
standing in dry post-oak savannah, you're probably looking at
S. marilandica; if your feet are wet, it's
S. hebecarpa.
Seasonal Cycle in NE Oklahoma
Breaks dormancy in early to mid-May, one of the last
prairie perennials to emerge. Grows rapidly through late May and June,
reaching full height by early July. Flowers open in July
and continue through August into early September in wet summers. Seed
pods ripen from September through October, turning from green to brown.
Foliage is killed by the first hard freeze (late October to
early November), and stems die back to the crown but often
remain standing as winter structure. The plant can take two to three
full growing seasons to reach mature flowering size from a young
transplant — patience is essential.
