Habitat
Ludwigia microcarpa is largely found in very wet, often calcareous settings such as stream banks, pond margins, and open wetlands. Rarely, L. microcarpa can be found in brackish marshes or tidal flats (Peng 1989).
Habitat comments by state follow:
In Alabama, L. microcarpa is found in hammocks, limesinks, and marshes (Hilton 1994).
In Arkansas, the sole element occurrence is in a "calcareous seep fen and along a spring branch, forming hummocks on sandy, gravely, marly ooze saturated by minerotrophic seepage" with Scleria verticillata, Rhynchospora capillacea, Parnassia grandifolia, Lysimachia quadriflora, Selaginella apoda, Solidago riddellii, Cynoctonum mitreola, and Fuirena simplex (AR NHC 1994).
In Florida, L. microcarpa occurs in the edges of mesic flatwoods, dome swamps, depression marshes, in the transition zones between wetland and woods, in ditches, along canals, and along the edges of fluctuating water levels (Hilsenbeck 1994). In addition, historic collections were made on muddy shores and ditches, around flatwood ponds, and in a roadside swamp (University of Minnesota Herbarium). L. microcarpa also has occurred in spartina marshes and grassland and savannas on Sanibel Island (Cooley 1955).
In Georgia, L. microcarpa is often found in the bottoms or margins of seasonal ponds as well as on mud banks and within roadside ditches (Patrick 1994).
In Jamaica, L. microcarpa occurs in swamps and ditches (Adams 1972).
In Louisiana and Texas, collections of Ludwigia microcarpa are from roadsides within formerly extensive wetland pine savannahs on the Montgomery and Beaumont Formations. Associated species include Rhynchospora colorata, R. divergens, R. perplexa, Scleria verticillata, S. georgiana, Fuirena breviseta, Centella asiatica, Pluchea rosea, Polypremum procumbens, Mecardonia acuminata, Mitreola peteolata, M. sessilifolia, and Helianthus angustifolius (Bridges and Orzell 1989).
Ludwigia microcarpa seems to be an obligate fen plant in Missouri where it is found in deep muck fens, prairie fens, and calcareous seeps. It is sometimes locally abundant around the springs and rivulets that run through fens in the southeastern Ozarks (Yatskievych 1994). Associates include Panicum agrostoides, Eleocharis calva, Fuirena simplex, Parnassia grandifolia, Galium tinctorium, Eupatorium perfoliatum, Lysimachia quadrifolia, and Calopogon pulchellus (MO NHD 1994, Steyermark 1963).
In North Carolina, L. microcarpa is mostly found along the outer Coastal Plain in wet calcareous settings, such as ditches and is fairly common (Weakley 1994). L. microcarpa has also been found in the Outer Banks in a slightly saline wetland (Gaddy 1994).
In South Carolina, L. microcarpa is found in ditches and marshes, chiefly along the Coastal Plain (Radford et al. 1968).
In Tennessee, the plant is mostly seen along streams, especially in disturbed edges. It is often found in areas with a limestone substrate, however it can also be found in ditches without this characteristic (Pyne 1994).
Ecology
In Missouri, flowering occurs in late July and fruiting occurs in mid-August. Since the petals are so minute, it is sometimes difficult to tell if the plant is in flower or in fruit. The fruit is capsular, containing minute seeds that are most likely water or wind dispersed when the capsule breaks open (Yatskievych 1994).
All species in Ludwigia sect. Microcarpium produce stolons from the base of erect stems, those of L. microcarpa are short and slender. These stolons creep along the ground or float along the surface of water late in the flowering/fruiting season. When the growing season resumes the stolons give rise to erect shoots from their tips (Peng 1989).
Ludwigia pilosa, a different species in the section Microcarpium with showy sepals and nectar discs and abundant nectar, has been observed being visited by honeybees, ants, bumblebees, wasps, and moths (Peng 1984). In L. microcarpa, the flowers are small enough to raise doubt that they would be bee-pollinated (Yatskievych 1994).
L. microcarpa is known to hybridize in nature with other Ludwigia spp. (Weakley 1994). This phenomena is documented for crosses between L. microcarpa, L. curtissii, and L. simpsonii, and the resulting hybrids were essentially sterile (Peng 1988).