While mowing a path through the meadow this week I noticed a bright pinkish-purple flower in the grasses. I remember seeing the unusual flowers of Eastern Sensitive Briar a few years ago in the Sandhills habitat at the North Carolina Botanical Garden in Chapel Hill. And now I am thrilled to discover it here, growing in several patches under the power line. This native species, Mimosa microphylla, grows throughout the southeast in sandy or gravelly soil in open woods, fields, and disturbed habitats like roadsides and power lines. And yes, this is related to the non-native invasive tree we call Mimosa, and both are members of the pea family.
Unlike its invasive tree cousin, Sensitive Briar is a semi-woody plant that spreads in a vine-like manner among other vegetation. Stems are 3 to 4 feet long. The delicate-looking leaves are bipinnately compound, meaning that each leaf is divided into leaflets, which further are divided into leaflets, giving the plant a somewhat airy or lacy appearance.
Initially, the flower is the most noticeable part of the plant. The round flower head is pinkish to purplish and about the diameter of a nickel. The flowers consist of clusters of individual tube-shaped flowers. Long, brightly-colored stamens tipped with yellow pollen protrude from these flowers, giving them a puff-ball appearance. Unopened flowers look like an unripe blackberry.
Another common name for the plant is “Cat’s Claw”. A close look at the re-curved spines that cover the stems, leaf petioles, and even the seed pods, and you can see how it got that name. These tiny thorns may also play a role in how this plant got its other common name, Sensitive Briar.
The reason for this plants’ name is because the leaves curl closed when touched. The left photo above shows the typical leaf position – the right shows that same leaf after it has been lightly touched. This is a prime example of what is called rapid plant movement. This movement of the leaf is caused by the rapid movement of water in the cells of the leaf. When touched, the affected leaflets quickly lose water pressure (osmotic cell pressure or turgor) and the leaflets collapse or seemingly wilt. So why would the plant bother to do this? One theory relates to the re-curved spines. As soon as a plant-eater trying to make a meal of the Sensitive Briar touches it, the leaflet collapses out of the way, exposing the sharp prickles to the would-be diner, making the plant much less appealing. The closure takes only a second or so. Collapsed leaflets will regain their position within a few minutes if left alone.
While I was photographing the plant, I noticed several insects on the flowers – a hover fly, a tiny grasshopper, and a brightly colored nymph with black and white bands on its antennae. A quick search on Bug Guide ( a great web site for insect identification) leads me to believe this is the nymph of a Bush Katydid. The unusual flowers of this plant made a perfect back drop for this tiny bug – both look like they are dressed for a festive occasion.
Spending some time with this unusual plant and the associated fauna made for a pleasant hour on an otherwise hot and humid morning. I’ll be checking the plant in a few weeks for the seed pods in the hopes of getting some Eastern Sensitive Briar started near my garden.