Polycarpon tetraphyllum
(fourleaf allseed)

P. tetraphyllum is a small and relatively insignificant annual plant. It is a widespread weed of cultivated and waste areas. Thought to be native originally to the Mediterranean region, P. tetratphyllum is now present on every continent except Antarctica. It is regarded as an invasive species in Australia, New Zealand, Japan, parts of South America and the USA, including Hawaii.

The exact native range of P. tetraphyllum is obscure, but most authorities suggest it is native to southern Europe and the Mediterranean region (Weeds of Australia, 2013). Using DNA sequence data, Kool et al. (2007) found that the P. tetraphyllum clade is most diverse in the Mediterranean region, where they deduced that it probably originated. Weedy forms of the species have been spread to every continent except Antarctica.

P. tetraphyllum has been introduced to India, Japan, the Americas, Australia and New Zealand, parts of Europe and a few countries in sub-Saharan Africa.

P. tetraphyllum has very small, dust-like seeds produced in large numbers. The transport of seeds to other countries or other parts of countries where P. tetraphyllum does not yet occur is therefore likely. Seeds can be transported as a contaminant in agricultural or horticultural seeds, in other agricultural produce or among the dust that tends to travel with passengers, their goods and luggage.

P. tetraphyllum is generally a small weed of waste places, cultivated land and open pastures. In its native Mediterranean, P. tetraphyllum is found in woodlands and shrublands, deserts and montane vegetation (Danin, 2013).

In California, Hartman and Rabeler (2012) described its habitat as ‘disturbed shaded areas [and] roadsides’ below 450 m.

Biological Records Centre (2013) described its habitat in the UK as ‘open, sunny sites that are droughted in summer and relatively frost-free in winter. It grows with other therophytes on steep south-facing banks, on compacted shingle or sand, in bulb-fields and gardens and at the base of roadside walls.’ Clapham et al. (1962) described its habitat in the UK as sandy and waste places in the southeast (Cornwall, south Devon and Dorset), but added that suitable habitats for P. tetraphyllum in the UK were rare.

In New Zealand it is found growing in gravelly or sandy sites, coastal and inland areas, riverbeds, roadsides, railway ballast, seashore, waste land, cultivated land and grassland (Webb et al., 1988).

The ability of annual plants, including P. tetraphyllum, to colonise open habitats (because of their annual nature and wind-dispersed seed) and to capture resources effectively (because of the flat architecture of their rosettes) means they are more likely to occur in disturbed sites in temperate grasslands of the New England Tablelands of Australia (McIntyre et al., 1995). Disturbance by heavy grazing can increase the proportion of such annuals in these grasslands.

Genetics

2n=32, 48, or 64 (Hartman and Rabeler, 2012).

Reproductive biology

The flowers of P. tetraphyllum are homogynous and automatically self-pollinated, and often cleistogamous (Clapham et al., 1962). In the Mediterranean region, Danin (2013) reported it flowering in almost every month of the year. In Britain it flowers in June and July (Clapham et al., 1962). In Australia P. tetraphyllum flowers in spring and continues into summer (Herbiguide, 2013).

In Australia, seed germinates in autumn to winter and plants then grow over winter through to summer (Herbiguide, 2013).

Kew Royal Botanic Gardens (2013) quoted an average 1000 seed weight of P. tetraphyllum as 0.05g. The seeds showed 100% germination at 10^o or 11^oC in 8 or 12 hours day length, although their age when tested was not mentioned.

A seed bank in Spain dominated by tiny seeds of P. tetraphyllum and Vulpia spp. contained up to 126,600 seeds m^-2 (Marañón, 1998; Olea and San Miguel-Ayanz, 2006). Lunt (1997) found seeds of P. tetraphyllum in the soil under intact remnants of Eucalyptus tereticornis woodland in temperate south east Australia, both in grassy forest or small grassland patches in the forest. P. tetraphyllum was not represented in the vegetation. More seeds were found in soil from the forest remnants than in soil from the grassland patches. Buisson et al (2006), working on abandoned fields in southern France, found seeds of P. tetraphyllum in few of the surface soil samples in winter (persistent seedbank), and even fewer in summer surface soil samples (transient seedbank).

Physiology and phenology

In Australia, P. tetraphyllum plants can be annual or biennial, with seed germinating in autumn to winter and the plants then growing over winter through to summer. They start flowering in spring and this continues into summer (Herbiguide, 2013).

Using old herbarium records, Buswell et al. (2011) investigated whether short-lived plants (including P. tetraphyllum) introduced to Australia before 1920 were evolving rapidly to the different conditions there. The only significant change in P. tetraphyllum was a reduction in plant height in specimens from the semi-arid region of New South Wales, but plant height in higher rainfall coastal New South Wales either did not change or increased over time. The authors suggested that this pattern ‘is consistent with the idea that the more arid environmental conditions in western New South Wales have selected for decreased height’ in this and a few other species.

Longevity

P. tetraphyllum is usually an annual, although there is a report of it as a short-lived perennial (Herbiguide, 2013). No information seems to be available on the longevity of seed in the soil.

Population size and structure

In Australia, P. tetraphyllum can produce a dense cover in moist, fertile areas (Herbiguide, 2013). Since individual plants tend to be quite small, this implies that plant densities can be very high in appropriate conditions. Gremmen and Halbertsma (2009) also found the species had high cover value at times in Tristan da Cunha, an island in the South Atlantic.

In Australia, P. tetraphyllum resembles Crassula spp., Stellaria media, Cerastium spp., Portulaca spp., Anagallis arvenis, Montia spp., Sagina apetala and Euphorbia peplus (Herbiguide, 2013). All are low-growing, small weedy herbs, but most become taller or have much longer sprawling stems compared to P. tetraphyllum. Popay et al. (2010) identified Sagina apetala as the species most likely to be confused with P. tetraphyllum, but S. apetala has very narrow leaves in pairs and the flowers are less densely clustered than P. tetraphyllum.  

P. tetraphyllum var. diphyllum can be distinguished by leaves only paired (as opposed to being in fours), contracted inflorescences and 1-3 stamens, instead of 3-5 (Stace, 2010). 

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Physical/mechanical control

Where P. tetraphyllum is established, Herbiguide (2013) suggested preventing seed set to control the plant.

Biological control

P. tetraphyllum is generally considered too small and insignificant to merit any major biological control effort.

Chemical control

The Novachem Manual (2012) lists 2,4-D ester for use in pastures, acetochlor in maize crops or terbacil in some fruit crops. There seems to be little information elsewhere on chemical control, other than the ubiquitous use of glyphosate in total weed control.

Control by utilization

Herbiguide (2013) suggests grazing the area where P. tetraphyllum is established. However, heavy grazing can increase the proportion of P. tetraphyllum and other small annual species in grassland (McIntyre et al., 1995). 

Little information is available on the germination or longevity of seeds of P. tetraphyllum, nor on the slightly puzzling references to the plants being sometimes ‘perennial’ (Herbiguide, 2013). Nor is there information on how soon after germination the plants can flower and produce viable seed.

15/11/13: Original text by:

Ian Popay, consultant, New Zealand, with the support of Landcare Research.