Annual herb, 60–520 mm tall. Leaves alternate, decreasing in length towards the upper part of the plant, 2- or 3-pinnatisect. Flowerhead solitary, 5–20 mm in diameter, peduncles becoming prominently inflated when in fruit; 2 or 3 series of bracts; 3- to 5-nerved. There are three types of florets in the flowerhead: disciform florets, in the outer most series; ray-like florets called pseudo-rays (which are modified disc florets where one of the lobes is elongated forming the pseudo-ray), follow the series of disciform florets and are yellow; and the disc florets, in the inner series and are also yellow. The disc floret fruits have a band of myxogenic cells, on the inner face of the fruit, these are specialized cells that become mucilaginous when wet. Flowering time is usually from late winter to mid spring but can occur in summer during favourable conditions.

Fig. 2. The pseudo-ray floret of Cotula pruinosa, note the elongated lobe that is roughly twice the length of the other three lobes. Photo Anthony Magee

Cotula pruinosa can be confused with C. turbinata and C. duckittiae. In the past, C. pruinosa was considered to be the yellow flowered form within C. turbinata, however, C. pruinosa differs from C. turbinata not only in the flower colour (C. turbinata has white pseudo-rays whereas C. pruinosa has yellow), but also the disc floret cypselae. The disc floret cypselae of C. pruinosa have myxogenic cells present on the inner side surrounding the fertile portion and they are absent in C. turbinata. The myxogenic cells are also present in C. duckittiae. Cotula pruinosa differs from C. duckittiae in having yellow disc florets and pseudo-rays whereas in C. duckittiae they are usually orange (rarely golden yellow). There are other characteristics that can separate these two species from each other, such as the flattened receptacle in C. duckittiae (slightly convex in C. pruinosa) and the elongated lobe of the pseudo-rays is four times the length of the other lobes in C. duckittiae (two times the length in C. pruinosa).

Fig. 3. The disc floret fruit of Cotula pruinosa, note the band of myxogenic cells. Photo Anthony Magee