Portulaca pilosa

Photo courtesy of Attila Kapitany.

Portulaca pilosa L. is one of the more commonly encountered Portulaca species in the northern and eastern parts of Australia, particularly during the summer months.

This naturalized species probably arrived with ship ballast at one or more coastal ports between 1870 and 1920. The Australian Virtual Herbarium website shows several sporadic records from the late 1800s. There may have been multiple introductions over many years in different places. In Queensland it was first recorded  in 1932 at Victoria Park in the inner suburb of Spring Hill, Brisbane.

P. pilosa is an annual or short-lived perennial of sprawling habit. It belongs to a group of Portulaca species found throughout the Americas that are commonly called "Moss Roses". The group is characterized by a low, sprawling growth habit. The plants often form solid mats of soft, green, tightly-packed foliage on the ground, giving the appearance of moss; hence the common name. The common garden annual, P. grandiflora Hook. from South America also belongs to this group.

P. pilosa forming a dense mat of soft, green foliage.
Photo courtesy of Attila Kapitany.

The leaves of P. pilosa are linear or narrowly ovate-elliptic and slightly flattened. They are 0.5-2.7 cm long and 0.1-0.4 cm wide. The axillary hairs are numerous, long and dense.

Branch of P. pilosa showing  crowded leaves and long, dense axillary hairs.
Photo courtesy of Attila Kapitany.

The flowers of P. pilosa are pink, purple, magenta, or crimson and borne in terminal clusters of 1-6. They are about 10-15 mm in diameter and may be numerous.

Photo courtesy of Attila Kapitany.

The pyxis of P. pilosa is about 5-7 mm long and the operculum dehisces at or below the middle.

Terminal cluster of pyxides showing one operculum dehisced
and spilling numerous, black seeds, and one empty lower capsule.
Photo courtesy of Attila Kapitany.

The seeds are kidney-shaped (i.e. reniform) shiny black in the fresh state, minutely stellate and colliculate, and may be slightly iridescent under magnification. The tubercles are in an ornate series of concentric rows and are minutely nippled.

P. pilosa seed under 20X and 40X magnification.
Photo by Ian Menkins.

P. pilosa differs from P. australis by the free stamens (cf. always connate or "fused at their base" for P. australis) and the greatly exserted stigma that exceeds the length of the stamens (cf. the stigma is lower than the anthers for P. australis). The stigmatic lobes are pink or purple (occasionally crimson) for P. pilosa, but yellow or orange for P. australis.

P. pilosa flowers. Photo courtesy of Attila Kapitany.

P. pilosa differs from P. filifolia by its more prostrate growth habit (cf. upright to diffuse for P. filifolia), very crowded linear or narrowly ovate-elliptic leaves which are often dorsoventrally flattened (cf. terete and sparse for P. filifolia), and its purple to pink flowers (cf. yellow for P. filifolia). The stigmatic lobes are pink or purple for P. pilosa, but generally yellow or greenish-yellow for P. filifolia. The seeds of P. pilosa are shiny black (cf. steely grey-iridescent for P. filifolia). The testa (i.e. seed surface) cells are fine and colliculate for P. pilosa and the tubercles are only minutely nippled (cf. testa coarsely tuberculate with most or at least some cells prominently nippled for P. filifolia). The seed shape is reniform (i.e. kidney-shaped) for P. pilosa (cf. comma-shaped for P. filifolia). The axillary hairs on P. pilosa are generally more dense and numerous than those found on P. australis and P. filifolia.

The roots of P. pilosa are fibrous and may be deep or spreading. The epidermis of the main roots is somewhat papery and flaking (like the bark on a paperbark tree). The roots may be slightly enlarged in places but rarely to the point of being tuberous. Mature plants generally do not have a distinct taproot. Instead, the root system is frequently made up of several main roots that branch and spread laterally.

P. pilosa  generally has spreading fibrous roots, however slightly
 tuberous roots may sometimes occur, such as in the example at right.
The above examples show a clustered network of fine feeder roots
close to the surface. Photo courtesy of Attila Kapitany.

A plant in full flower can be very showy. However due to its ability to produce abundant seed and spread rapidly in warm climates it is more often than not considered a weed. It has a preference for recently disturbed ground, particularly in sandy or gravelly areas. It is often found on coastal dunes and beaches above the high tide mark, as well as on road edges, footpaths and gravel driveways. In gardens it loves to invade the spaces between pavers or rocks. In Australia it is seldom found far from habitation or the activities of humans, although it has become naturalized in many coastal and desert situations in the tropics. It generally fades away in very cold climates in the absence of bare ground and disturbance.

Friend or foe? A weed can be defined as any plant that is out of place.

So is the plant in the above photo: 

1) an attractive ground cover that is heat and drought tolerant? or

2) a spreading, potentially invasive weed?

P. pilosa has the potential to be both.

Below: A particularly showy specimen of P. pilosa.

Photos courtesy of Attila Kapitany.