This perennial wildflower is usually an emergent aquatic that is
exerted 1-3' above the water line, otherwise it is a terrestrial plant
of similar height. The stems are usually unbranched, although larger
plants are sometimes branched below. The stems are light green,
angular-terete, and glabrous. At intervals along each stem, there are
pairs of opposite leaves about 2-6" long and ¼-1" across that are
either sessile or short-petioled. The deciduous leaves are narrowly
linear-lanceolate, elliptic, or narrowly elliptic in shape and usually
smooth along their
margins. Less often, the outer margins may be slightly undulate or
shallowly crenate. Both the upper and lower surfaces of the leaves are
medium green and glabrous.
From the axils of the middle to upper
leaves, there develops individual spikes of flowers on long peduncles.
Each floral spike is about 1" long and capitate (head-like) in
appearance; there are several overlapping flowers and buds per spike.
Each flower is about ¾" across, consisting of a short-tubular corolla
with 4 lobes, a short-tubular calyx with 5 teeth, 2 stamens with dark
purple or dark brown anthers, and an ovary with a slender white style.
The corolla has a shallowly notched upper lobe that curves backward, 2
lateral lobes that are widely spreading, and a lower lobe that
curves slightly downward. Except for the dark purple mottling at the
base of the lower lobe, the lobes are mostly white, otherwise they are
tinted pale purple or they are lightly speckled with fine purple dots.
The lobes of the corolla are longer than the corolla tube, and they are
oblong to oblong-oblanceolate in shape. The green calyx is about ¼"
long and glabrous; its teeth are narrowly lanceolate. The ascending
straight peduncles are
a little shorter to about as long as the leaves (up to 6" in length);
they are medium green,
angular, and glabrous. The blooming period occurs from early summer
into the fall, lasting about 2-4 months. Usually, only a few flowers
are in bloom at the same time. Afterwards, the flowers are replaced by
seed capsules up to ½" long that taper to stipe-like bases.
capsule has 2 cells, and each cell contains 2 seeds. The seeds are
about 1/8" (3 mm.) in length and warty. The root system is highly
rhizomatous, forming colonies of plants.
preference is full or partial sun, shallow standing water (up to 3'
deep) or wet conditions, and soil that is muddy, sandy, or gravelly.
The native Water Willow is occasional
in most areas
Illinois, except the NW section of the state, where it is uncommon or
absent (see Distribution
). Habitats include sandbars, gravelbars,
or mudbars of rivers, low islands in rivers or ponds, shallow water or
muddy banks of ponds and rivers, shallow water of rocky upland
streams, shallow water or wet areas of swamps, and sandy
Water Willow occurs in wetlands with either stagnant water or slow to
moderate currents of water.
The flowers are
cross-pollinated primarily by bees, including honeybees, bumblebees,
Anthophorine bees (Anthophora
), Little Carpenter bees (Ceratina
), Cuckoo bees (Epeolus
), Digger bees
Leaf-Cutter bees (Megachile
Green metallic bees, and other Halictid bees. Other floral visitors
include various wasps, bee flies (Bombyliidae), Thick-headed flies
(Conopidae), Syrphid flies (non-pollinating), small butterflies, and
skippers. These insects obtain primarily nectar from the flowers,
although some bees collect pollen and some flies feed
Water Willow is one of the host plants for the caterpillars of Darapsa
(Hydrangea Sphinx); they feed on the foliage.
This plant is
also a minor source of food for Muskrats.
A sandy marsh at the Heron
Vermilion County, Illinois.
The flowers of Water Willow are fairly showy and bloom
intermittently for a long period of time. In spite of its common name,
this is a non-woody herbaceous plant that is not closely related to
willows (Salix). Water Willow is the most northern member of its genus.
Another species that can be found in southern Illinois, Justicia ovata
(Southern Water Willow), is a slightly smaller plant with wider leaves
and more loosely flowered spikes. Otherwise, these two species have a
similar appearance and prefer similar habitats. A scientific synonym of Water Willow is Dianthera americana