This common plant grows rapidly to maturity, and then continues to flower as long as conditions are favourable, but dies off as the summer becomes hotter and drier. It is an annual herb about 200-500 (sometimes as much as 700) mm high, sometimes becoming woody at the base. Plants can be single-stemmed or more branched, and the stems are held upright. The leaves and stems are bright green, quite fleshy, sticky, and aromatic. Leaves are largest at the base of the plant, where they taper as they reach the stem, and become smaller and narrower towards the tops of the stems. At the point where the leaf joins the stem, the leaf wraps slightly around the stem. Leaves are borne at a large angle to the stem. The leaves are quite large, about 20–90 (and rarely up to 130) mm long and 10–20 (rarely up to 30) mm broad, with wavy or toothed margins.

Fig. 2. Osteospermum monstrosum, close-up of flowering and fruiting plant (Photo Desertnaturalist).

The flowerheads are borne at the top of the plant on long, branching flowering stalks with few, small leaves (see Fig. 2). Each flowerhead is surrounded by a row of green, leaflike flowerhead bracts (involucral bracts; see Fig. 3), which have very broad dry translucent margins (most visible in the fruiting head).

Fig. 3. Side view of flowerhead of Osteospermum monstrosum, showing involucral bracts (Photo Sally Hey).

When looking at the flowerheads face-on (see Fig. 4), they are smallish (10–15 mm across) and form a circle, with few spaces between the ray flowers. The ray flowers (outer flowers of the flowerhead, with large petals) are yellow with reddish-maroon/brown centres, and are fairly short, from just a few mm longer than the flowerhead bracts to up to twice the length of the flowerhead bracts (see Fig. 3). The tiny disc flowers in the centre of the flowerhead are black when in bud, yellow when they open (see Fig. 4).

Fig. 4. Face-on view of a flowerhead showing the short yellow ray flowers with their dark maroon bases, and the black central disc flower buds (Photo Kevin Thiele).

Very noticeable for this species are the fruiting heads (see Figs. 1, 2 & 5), many of which are already developing while the plant is still in flower, and are the same size or larger than the flowering heads. The fruiting heads hang downwards from the tips of the flower stalks, and while still developing are silvery-green with distinctive darker stripes formed from the flowerhead bracts and the edges of the developing seeds (technically, fruits). As they mature the seeds develop a striking red-pink tinge on the wings, but are pale brown with transparent wings when ripe (see Figs. 5 & 6). At this point, the flowerhead bracts shrivel up and point back away from the fruits.

It’s worth taking the time for a closer look at the seeds, which are about 9 mm long and consist of a wrinkled, sculptured central section housing the embryo, and three thin, broad, translucent wings (see Fig. 6). At the top of the central section is a round, thin-walled “window” over an internal air cavity. Once mature, the fruits are pale brown and dry, and the wings serve to keep them tumbling in the wind for greater dispersal.

The one-eye monster can be confused with a few other closely-related species that also occur in the Namaqualand region, in particular the dassiepee boneseed, Osteospermum hyoseroides, which shares the very broad transparent margins on the flowerhead bracts, and has very similar fruits, around the same size and with a single “window”. However, O. monstrosum has ray flowers that are only a few mm longer than the flowerhead bracts, or up to twice as long, while O. hyoseroides has ray flowers that are two- to three times the length of the flowerhead bracts, and also usually lack the darker basal ring.

Very similar to O. hyoseroides is the dassie boneseed, O. amplectens, but it is also easily differentiated from O. monstrosum by ray flowers that are two- to three times the length of the flowerhead bracts. Osteospermum amplectens differs from both the previous species by having much smaller fruits, only 4 – 5 mm long, which also do not have any “windows”. In addition, both of these species, with their large attractive flowerheads, usually have ray flowers that are orange rather than yellow, and lack the dark basal ring.

There is one more similar species in this group, Osteospermum breviradiatum. It is, however, rare and not very well known; it has short ray flowers like O. monstrosum, but in other respects, its fruits and its flowerhead bracts are exactly like those of O. amplectens.