There’s something refreshing about seeing all these green, grassy-looking plants growing right in the Potomac River every summer. They aren’t grasses, though; they’re water willow (Justicia americana; Acanthaceae), an emergent aquatic whose rhizomes form vast stands of plants in the shallow waters of ponds and streams.
The plants will grow to as much as three feet tall, sending up long stems with tight clusters of flowers on the ends. Only a few stems at a time will bear flowers, but the blooming period of a colony can last two months or more.
Water willow is native to eastern North America, ranging from Texas and the eastern Great Plains northeastward to New York, Ontario, and Quebec. It’s endangered in Iowa and threatened in Michigan.
Of the roughly dozen and a half species of Justicia found in North America, this one is by far the most northern species. One other occurs in the southern part of the Mid-Atlantic, and one in the Mid-West; the rest seem to be found only in Florida, Texas, or the Southwest.
Last spring I found the closely related Justicia californica in Anza-Borrego Desert State Park, which grows as a shrub or tree in the Sonoran and Mojave Deserts.
Hard to believe they’re in the same genus.
aka Carolina wild petunia
Despite the common name, and despite appearances, this plant is more closely related to water willow (FotD June 20) than to the common garden petunia. The latter plant is in the genus Petunia, family Solanaceae, and is therefore closely related to tobacco and tomatoes.
Actually Ruellia and Petunia are quite far apart taxonomically; not only are they in different families, they’re in different orders (Scrophulariales and Solanales, respectively).
About 15 species of Ruellia can be found in the continental US; of these, four are found in Maryland, and of those four, this is the only one you’re likely to see. The other three are critically imperiled here.
R. caroliniensis ranges from New Jersey (where it’s endangered) west to Illinois, and then south to the Gulf coast. It grows one to three feet tall. I’ve seen references to it liking both dry, sandy soils and moist soils; the three places in the Gorge where I know to find it have sandy soils that are prone to minor flooding, so I’m not sure what to make of that.
See the partly submerged rock on the lower right? Yeah, that’s one of the places where this plant grows.
aka American water-willow
As spring turns into summer and the water levels in the Potomac drop, the stems and foliage of this aquatic plant appear along the river’s edge, growing out of the water.
Water willow spreads by rhizomes, forming large colonies that help stabilize shorelines and provide habitat for small invertebrates. Many types of bees, flies, and butterflies feed on the nectar or pollen. I don’t know if water-willow could properly be considered a keystone species, but it certainly is ecologically important.
Hundreds of species of Justicia grow in tropical and temperate zones of the Americas and parts of Asia and Africa, but only about two dozen are native to the US. American water-willow is by far the northernmost growing of these species, and can be found as far north as Ontario and Quebec, though no further west than Texas. It’s threatened in Michigan and endangered in Iowa.
Although each flower is relatively short-lived, and only a few are produced at a time, the overall blooming period of the plant can be several months long.
evening clouds reflected in the Potomac