At the Mitchell Mill State Natural Area, heavy summer thunderstorms in early July have caused the Little River to overrun its banks, creating rivulets which coalesce to completely cover much of the surface of the granite flat rock with shallow, flowing water.
Outside the main channel, the water runs gently and quietly across the surface. The frequent rains have created an optimal environment for rapidly growing vegetation. Several invasive species are thriving under these conditions. Two are pictured below, in large masses. In the foreground is Common Water-primrose (Ludwigia grandiflora ssp. hexapetala), with the yellow flowers, and behind it is a huge mass of Marsh Dayflower (Murdannia keisak), which is not yet in bloom. They completely fill huge areas of the flat rock pools, crowding out the native species that normally live there.
Common Water-primrose, in spite of its invasive predilections, can be a beautiful plant, with dark green, symmetrical foliage and showy yellow flowers.
The flowers have 5 petals, 5 sepals, 10 stamens, and a single, central style and stigma. They bloom on upright stems, unlike some close relatives that bloom directly from creeping or decumbent stems.
Most references treat Common Water-primrose as an introduced species from South America. However, there is evidence that populations were present in the Southeast prior to foreign introductions. Weakley’s Flora (May, 2015) classifies it as a native.
There are a number of closely related species of Ludwigia that look very much alike, requiring a combination of field characteristics to separate them. One simple characteristic is the relative length of the floral tube to the stem of the flower (Petiole). In Ludwigia grandiflora ssp. hexapetala, the floral tube (FT) is much shorter than the Petiole (P). See below.
The Marsh Dayflower (Murdannia keisak) is a rapacious invasive plant, one of the worst wetland invasives we face today. Ironically , the area that it now occupies was the site of a large colony of Parrot Feather (Myriophyllum aquaticum) just three years ago. Parrot Feather is itself an invasive plant, introduced from South America. The Marsh Dayflower, introduced from Asia and pictured below, appears to have won the battle of the alien invasives. Whether it will begin to displace the Water-primrose remains to be seen. It looks like a stand-off for now.
On a happier note, a huge shrub of the rare but native Swamp Titi (Cyrilla racemiflora) is now in full bloom. Although it is common in the Coastal Plain, Mitchell Mill is one of the few places it can be seen in the Piedmont. It has large, glossy leaves and flowers in long racemes.
A raceme is a cluster of flowers arrayed along the length of a common stem. On the flowering raceme seen below, the oldest flowers, now turning to fruit, are found near the base, the open flowers in the middle, and the unopened buds at the end.
On the day these photographs were taken, the flowers were attracting large numbers of small to mid-size bees, bumblebees, and the Ailanthus Webworm Moth below.
For those interested, a new version (May 2015) of Flora of the Southern and Mid-Atlantic States by Alan S. Weakley is now available for download from the UNC Herbarium website. Here is the link: http://www.herbarium.unc.edu/flora.htm