There are around 170 (Mabberley) species of Day flowers.
They are annual or perennial herbs whose roots often have tubers.
The mainly prostrate stems are up to around 2 m long and erect stems 30 to 40 cm high.
The stems are often thin and roots grow from the nodes.
Stems are usually smooth but in some species they have hairs.

The alternate leaves are linear, lanceolate, ovate to narrowly ovate with no petiole or one a few mms long.
Blades are up to 15 cm long and 4 cm wide.
The tip is pointed and the leaf base forms a closed sheath down the stem.
There are usually hairs on the upper edge of the sheath.

Axillary inflorescences typically have 1 (3 to 4) spathe enclosing 2 flower clusters (cymes).
A cyme is a branched inflorescence with the terminal flower on each branch opening first.
The spathe is a bract folded lengthwise and its base may be open or closed.
The spathe can be triangular, sickle or heart-shaped and from 0.5 cm up to 4 cm long.
They are on a peduncle 1 to 2 cm long.
There are sometimes hairs on the outer surface.

In each spathe one cyme is on a longer pedicel and the usually male flower(s) are outside the spathe.
The second shorter cyme, often enclosed in the spathe typically has male flower(s) at the top and bisexual at the bottom.
Cymes can have 1 or 2 (4 – 6) flowers.

There are 3 thin sepals (outer tepals) that are free or the lower 2 may be fused.
They are only a few mms long.
The 3 free petals or inner tepals can be blue, white or yellow.
They can all be about the same size or the lower or anterior petal may be a lot smaller and colourless.

There are usually 3 fertile stamens and 3 (2) staminodes all on thin, hairless filaments.
The central fertile stamen is larger than the other two – especially noticeable in the anther.
The basifixed anthers open via longitudinal slits.

The staminodes are variously shaped but often resemble a cross.
They have nectar in the anthers bees can obtain by piercing the anther.

The superior ovary, of 3 carpels has 3 (2) locules with 1 or 2 ovules in each.
The fruit are capsules with smooth grey to black seeds.
There are 2 dehiscent chambers each with 2 small seeds and a larger indehiscent posterior chamber with a larger seed.
The surface of the seeds may be wrinkled or pitted.

Three species are found in S. E. Queensland.
Two are native – Commelina diffusa (C. cyanea) the Blue Commelina and C. lanceolata the Queensland Wandering sailor.
The introduced Commelina benghalensis or Hairy commelina is a weed.

There are some cultivars.
The flowers of some species are the source of a blue dye.