Yaupon blacksenna (Seymeria cassinoides) is a sprawling annual herbaceous plant, native to the northern two-thirds of Florida and to most of the Southeast Coastal Plain from Texas to Virginia. Throughout its geographical range, it occurs in dry to moist open pinelands and savannas. These photos were taken in a xeric flatwoods dominated by longleaf pines (Pinus palustris) in Alachua County. There are two species of blacksenna in Florida. I have written previously about S. pectinata. They are similar in many ways, but yaupon blacksenna is distinguished by its narrow, needle-like leaves.
As an annual, yaupon blacksenna emerges in the spring and quickly produces a number of thin, wiry stems that may each 1.5 - 2.5 feet tall and extend for that distance in all directions away from the main stem. These stems are reddish purple in color and are covered with fine hairs. Flowering tends to occur in early fall. Large numbers of the small lemon yellow blossoms are produced in the leaf axils. The flowers are comprised of 5 petals and together they may be 1/2 inch wide. Tiny reddish dots and markings occur at the base of each petal. They are visited primarily by bees. Small rounded seed capsules follow in late fall to early winter.
Blacksennas are fairly showy wildflowers, but have not been propagated for several reasons. As an annual it does not lend itself well to typical landscape settings. Second, it and its close relative are root parasites on southern pines, like longleaf and slash (Pinus elliottii) pines. It is not clear whether these plants cause the pines any significant harm by their parasitism, but most such plants do poorly in a landscape without an opportunity to function as a parasite. These are interesting plants that should simply be admired when encountered in the field.