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Sunday, January 13, 2013

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.













6 comments:

wendikaa said...

I found these growing here in Malaysia. So pretty by the road :)

wendikaa said...

WE have lots of variety here in Malaysia - some I've posted here.
https://www.facebook.com/media/set/?set=a.254137558087042.1073741837.188142281353237&type=1

Grey_Gum said...

Thank you for sharing the photos! Yes lots of interesting variation among these different species and cultivars.

wendikaa said...

Would be great if we could trade but I think we cannot send plants to Australia ya :(.

Daisy Hedge said...

Thank you for sharing this information. I found this little wonder this morning while walking my children to school. I have a side yard at home that grass has trouble growing in and I have started planting ground covers that I find growing wild in les than hospital places. This one looks ideal for my needs here in Brisbane.

Grey_Gum said...

A DISCLAIMER and a kindly word to those intending to leave comments.

The comments section is moderated which means I get to pick and choose which posts get published and which do not.

I sometimes receive comments related to summer-active weeds. Although most of these comments have been genuine inquiries, a few have unfortunately been snide in their intent. I generally just delete any ill-informed remarks like the following example: "So you grow weeds in your garden?".

I also will not reply to requests like "How can I get rid of this plant out of my lawn?". I am sorry but I do not offer advice on the control of weeds.

The definition of a weed is often a very subjective thing. In my experience, the weedy Portulaca species almost always prefer bare ground. Some of the species are summer "pioneer" species that take rapid advantage of a lack of competition brought about by bare earth. So if your lawn is infested with one of these species it more than likely means you have bigger problems than just pig weed. Portulaca seldom do very well if there is any kind of competition from lawn grasses or other plants that shade them out. So your problem, if it could really be called that, is most likely related to drought or grubs/fungus killing patches out of your lawn. So rather than blame the weed that you think is taking you over, you may need to take a closer look at your horticultural practices. If you use a whipper snipper or mow without a catcher for instance well you are just going to throw the seeds about and scatter them far and wide.

You can spray the weeds, but that will only defeat the purpose by opening up the ground even more. It will let in more light and reduce competition, so by spraying all you do is put yourself back to square one. Also most chemical sprays result in negative effects like drift and are not very successful on succulent weeds like Portulaca in any case. The plants die slowly and can still set seed and occasionally the wilted branches will recover, take root and re-grow. Nevertheless there are chemicals which offer good control and I can only advise that people see their local chemical supplier, hardware store, or Department of Agriculture for advice on the most appropriate chemical to use in your situation.

Most Australian farmers know the scourge of Portulaca oleracea only too well. I know from experience that it can be a nuisance plant that covers entire ploughed paddocks in summer, particularly those left fallow due to an absence of rainfall. And if you do get rainfall it reproduces and grows extremely quickly. By the time the paddock has dried out enough for you to get a tractor in there, the pigweed has well and truly taken over. It can form such massive beds that I have seen it clog farm implements repeatedly, meaning a lot of down time for farmers who have to stop and clear the great mass of tangled strands off their ploughs.

But if people read these pages carefully, they should see that it mainly taxonomic and horticultural in its approach. So providing advice on weed control is outside the scope of this blog and therefore not really relevant. If people wish to examine this subject in more depth I am sure there are plenty of websites on the internet that deal exclusively with weeds and can provide the answers that you seek.