South Texas Ambrosia
Other Scientific Names: None
Other Common Names: South Texas ragweed, Rio Grande ragweed
Status: Federal and State Endangered
Global Rank: G2
State Rank: S2
Global Location: South Texas ambrosia is only found in South Texas (Jim Wells, Kleberg, and Nueces counties, and historically in Cameron County) and the adjacent state of Tamaulipas in Mexico.
Description: South Texas ambrosia is a non-woody perennial with erect stems to 40 cm, which are connected by a network of underground root-like stems. Leaves on the lower stem are arranged opposite to one another. Above these leaves, the arrangement changes to alternating leaves on either side of the stem. Mature plants have oval shaped leaves, which lack leaf stalks and are usually 2-4 cm long (rarely 7+ cm). The leaf edges are shallowly divided in the young leaves, becoming entire and unlobed with age. The leaves have a thick layer of microscopic hairs, which make them appear grayish-green. The inconspicuous, creamy-yellow male and female flower clusters occur separately along the floral stalk and are either hanging and cup-shaped (male) or globular and upright (female). Male, pollen-producing flower clusters occur along the upper 5-10 cm of the flower stalk. The fruit-producing female flowers are located below the male flowers in angles created by the leaves and main stem. Four to five blunt spines surround the small (2-3 mm), bur-like fruits.
South Texas ambrosia has inconspicuous, separate male and female flower clusters. The male flower clusters are cup-shaped and hang downward from the upper portion of the flower stalk. Below this, the globular female flower clusters stand upright.
The mature leaves of South Texas ambrosia are oval shaped, unlobed, and have a thick layer of microscopic hairs, which make them appear grayish-green.
Similar Species: There are no other Texas ambrosias with leaves that are hairy, grayish, oval shaped, and unlobed. However, Western ragweed (Ambrosia psilostachya) can be confused with South Texas ambrosia when plants are immature. Western ragweed can have leaf stalks (0-2.5 cm long) and generally longer leaves (2-6 cm, but rarely 5+ cm long). Also, leaf edges are either shallowly or deeply divided.
Western ragweed leaf edges are either shallowly or deeply divided.Image courtesy of Aaron Arthur.
Habitat: South Texas ambrosia occurs in grasslands and mesquite shrublands of the Texas Coastal Plain.
Image courtesy of Alice Hempel.
Life Cycle Events: Flowers appear July through November.
Survey Season: Because South Texas ambrosia dies back every year during the winter, this species should be looked for during the active growing season. Even without flowers and/or fruits, mature plants of South Texas ambrosia can be recognized by their distinctive leaves.
Comments: Due to the plant's colonial nature, estimating the actual number of individuals of South Texas ambrosia is difficult. Because the plant reproduces by sending out root-like stems that develop into new clones, hundreds of above-ground stems could represent one individual. This could greatly decrease the genetic diversity available in South Texas ambrosia populations and, therefore, could decrease the plant's ability to adapt to changing environments. Furthermore, South Texas ambrosia's small, scattered populations exacerbate the threats from habitat destruction and alteration this species faces.
For additional information see:
- U.S. Fish and Wildlife Service
- Flora of North America
- Poole, J.M. and G.K. Janssen. 1997. Managing and monitoring rare and endangered plants on highway right-of-ways in Texas. Section 6 final report. Austin: Texas Parks & Wildlife Department.