In mud alongside our irrigation canals right now a knee-high, somewhat weedy herb is issuing two-inch wide, four-petaled, yellow blossoms such as those seen below.
Wildflower fanciers up North should recognize this as a species of Water-primrose, or Seedbox, genus LUDWIGIA, of the Evening Primrose Family. At the right in the image you can see the plant's long, slender, immature fruit topped with four persistent sepals. When the fruit splits, large numbers of tiny seeds will pour out.
The name Water-primrose is appropriate because this species' flowers and fruits are very similar to those produced by evening primroses of the genus Oenothera, of the same family. The most obvious differences between the genus Ludwigia and Oenothera are that in Ludwigia the sepals arise from directly atop the fruit, nicely shown in the photo, while among evening primroses, genus Oenothera, sepals arise from a kind of collar, or calyx tube, crowning the ovary. Also, Ludwigia sepals usually are persistent, while those of Oenothera are deciduous.
Sometimes Ludwigia fruits are very short and squarish, at least squared immediately beneath the four sepals. In late fall if your leg brushes against a mature Ludwigia of this type hundreds of sandgrain-like seeds will spill from dozens of small, boxlike, capsular fruits, and the plant almost names itself, "seedbox." The neat word for the shape a fruit that's roundish on one end but squarrish on the other is "obpyramidal."
I associate Ludwigia's bright yellow flowers with hot, humid summers and, since they're usually in swampy places, mosquitoes. When I saw our Ludwigias flowering here I knew I needed to mention them, just to evoke that hot, sweaty, mosquitoey association for Northern, winter-bound readers