Family Onagraceae has two Subfamilies – Ludwigioideae and Onagroideae.
The Evening Primrose Family has 17 to 22 genera and around 650 species.
Ludwigia is the only genus in the subfamily Ludwigioideae.
Subfamily Onagroideae is divided into 6 tribes with most genera in the tribe Onagreae.
Well known genera include the garden plants Clarkia, Fuchsia and Oenothera.

Onagraceae are mostly herbs and shrubs with a few trees.
Leaves are commonly opposite but sometimes in whorls or spirally arranged.
Biennial plants often retain just the basal leaves at the end of the first year.
The lower leaves may be opposite with the upper ones alternately arranged.
The simple lanceolate leaves may have a smooth edge or be toothed or lobed.
Stipules are sometimes present.

Inflorescences can be a single flower, a spike or a panicle (branched).
The bases of the sepals, petals and stamens are fused to form a hypanthium.
This is often long and it has nectar producing tissue inside.

There are 4 (2, 6) sepals inserted on the rim of the hypanthium.
In the bud the edges of adjacent sepals touch along their whole length (valvate).

There are 4 (0, 2 – 6) petals, sometimes with a narrow claw base on the rim of the hypanthium.
They can be valvate, imbricate (one edge is under the adjacent sepal and the other side is
    over the next sepal) or convolute (overlapping as in imbricate but coiled or twisted).

There are usually 4 (2 to 6) stamens or 8 in 2 whorls on the hypanthium rim.
The anthers open through longitudinal slits.
The inferior ovary typically has 4 locules.
Each locule has one to a few ovules with axile or parietal placentation.
The style usually has 4 stigma lobes.

The fruit are usually loculicidal capsules but sometimes a berry, drupe or nut.
The tiny seeds may have a tuft of hairs at one end.