Description: This adventive annual plant is about
½–2' tall and more or less erect, although individual plants may
sprawl. The stems are usually light green, round, and glabrous or slightly
pubescent. The
alternate
leaves are up to 6" long and 1" across, although usually smaller. They are
lanceolate or linear-lanceolate, hairless, smooth along the margins, and
sometimes slightly ciliate. Each leaf has a short petiole or is nearly sessile;
there is a membranous sheath (ochrea) that wraps around the stem at its base.
This sheath may have a few longitudinal ridges and there are bristles along its
upper rim that exceed 1 mm. in length. The upper surface of a leaf often has a
black smudge that is more or less triangular-shaped; this smudge may be dark
and conspicuous, or it may be faint and barely perceptible. The upper stems
terminate in 1 or 2 spike-like racemes of flowers; there are often shorter
racemes from the axils of the upper leaves. Each raceme is about
½–1½" long, more or less erect, and oblongoid in shape from
the crowded whorls of small flowers. The flowers may be pink, greenish white,
or purple, even on the same raceme; usually pink flowers are the most common.
Each flower is about 1/8" long and shy to open; it consists of 5 sepals, 6
stamens (usually), a style that is divided into 2 or 3 parts toward the middle,
and no petals. The sepals are not glandular-punctate, and the stamens are not
exerted beyond the sepals. The blooming period occurs from late spring to early
fall; a colony of plants will typically bloom for 1-2 months during the summer.
There is no noticeable floral scent. Each flower is replaced by a seed that
ovoid, flattened or slightly 3-angled, black, and shiny. The shallow root
system doesn't produce rhizomes. This plant often forms colonies, particularly
in disturbed wetland areas, or it may occur in drier areas as scattered plants.
Reproduction is by seed only.
Cultivation: The preference is full sun or partial
sun, wet to mesic conditions, and fertile soil with organic matter. However,
this adaptable plant will also grow in light shade and poor soil containing
clay, gravel, or sand. After the seeds germinate, it develops quickly and may
bloom while only a few inches tall.
Range &
Habitat: Lady's Thumb is a common plant that has been observed in
most counties of Illinois (see Distribution Map); it probably occurs in every
county of the state. Habitats include marshy areas, edges of streams and
drainage canals, mudflats, roadside ditches, moist weedy meadows, vacant lots,
fallow fields and edges of cultivated fields, edges of yards and gardens, moist
areas along railroads, and waste areas. This species prefers disturbed areas,
but can invade higher quality wetlands to a limited extent. Lady' Thumb is
native to Europe.
Faunal
Associations: The nectar of the flowers attracts Halictid bees,
wasps, and Syrphid flies primarily. Less comon visitors include small
butterflies and bumblebees. Halictid bees also collect pollen occasionally. The
foliage is eaten by the caterpillars of some Copper butterflies and several
species of Moths, while the flowers and fruit are eaten by the caterpillars of
the Gray Hairstreak butterfly (see
Butterfly & Moth Table).
Japanese beetles are also quite fond of the foliage of this and other
smartweeds. Mammalian herbivores rarely feed on the foliage of Lady's Thumb
because the foliage is pungent, peppery, and slightly bitter. However,
White-Tailed Deer may chomp off the tops of young plants upon occasion. The
seeds of smartweeds are very popular with waterfowl and granivorous songbirds.
The seeds of Lady's Thumb, in particular, are eaten by birds in both upland and
wetland habitats (see Bird
Table). The ecological value of this little plant is rather high,
notwithstanding its weedy nature.
Photographic
Location: A vacant lot near Urbana, Illinois. The black smudge on
the leaf of the photographed plant is darker and more conspicuous than usual.
The flowers of the raceme are variously colored, including pink, white, and
purple.
Comments: Lady's Thumb is
probably the most common and adaptable Persicaria sp. (Smartweed) in
Illinois. It occurs in both wetland habitats and surprisingly dry habitats. I
have even found small plants of this species flowering underneath the shade of
lawn trees. It is fairly easy to distinguish Lady's Thumb from other
Persicaria spp. (Smartweeds) because its leaves often have black smudges
on their upper surface. Sometimes these black smudges aren't produced, however.
The spike-like racemes of Lady's Thumb are somewhat distinctive: they are
rather stout, erect, and densely crowded with small flowers; these flowers
often have conspicuously different colors on same raceme. Other Smartweeds
often have racemes that are more slender, nodding, and less densely crowded
with flowers. For these species, the flowers of individual plants are more
likely to be homogenously colored. Another scientific name for Lady's Thumb is
Polygonum persicaria.