Hieracium pilosella is a perennial, stoloniferous flatweed with extensive underground root mass. Patches of H. pilosella usually range from 0.5 to 1.2 m in diameter, with almost 100% cover within the patch. Patches are often surrounded by a 15 cm wide “halo” of bare ground which is drier, lower and nutrients and higher in acidity than soil outside the halo (Scott et al., 2001). H. pilosella is genetically and morphologically highly variable. Zhan (1923 in Trewick et al., 2004) reported 624 variants of the species.

The two direct children listed under Hieracium pilosella are Hieracium pilosella var. niveum Muell.-Arg. and Hieracium pilosella var. pilosella L. (ITIS, 2010).The systematics of the genus Hieracium is extremely complex and contentious, probably due to recent speciation, hybridization, polyploidy, and diverse reproductive strategies. In its native European range five cytopes of H. pilosella have been identified, ranging from diploid to heptaploid, but the range is dominated by sexual tetraploids and asexual pentaploids (Trewick et al., 2004).

In its invasive range in New Zealand initial studies indicated that most H. pilosella populations were pentaploid. However further studies revealed aneuploids and hybrids. Tetraploid sexual plants have been confirmed in some populations. New Zealand populations are morphologically and genetically diverse, apparently surprisingly so, given that the initial introduction is thought to have been recent, small and asexual. Hybrids of a related taxon (probably H. praealtum have also occurred at least three times (Trewick et al., 2004 and references therein).

Hieracium pilosella is a perennial plant.

Invasive populations of Hieracium pilosella in Australia, New Zealand and North America occupy different climatic niches to those realized in their native ranges. Particularly in eastern North America its invasive regions tend to be wetter than its native range. Furthermore, geographically separate invasive populations of these two species have distinct climatic niches (Beaumont et al. 2009a).

Hieracium pilosella spreads by seed and stolons; it also produced seeds asexually by apomixis. Hawkweed seeds are wind dispersed, with most seeds falling within 2 m of the parent plant, and long-distance events are rare. This limited dispersal may hinder the ability of hawkweeds to realise their full geographic potential under current and future climates (Williams & Holland, 2007 and references therein). Plants are highly fecund producing up to 40,000 seeds/square metre of cover/year (Burton & Dellow, 2005 in Beaumont et al., 2009b).

Hieracium pilosella plants can tolerate heat, frost and snow (Beaumont et al., 2009b).

Introduced by acclimatisation societies.

In New Zealand, Hieracium pilosella is considered a “sleeper weed”, as it was first introduced to the country in 1878 but remained localised for around 80 years. After this “lag” phase of at least 80 years the population suddenly increased its range dramatically. It has now spread significantly into tussock grasslands used for grazing and into conservation areas (Klöppel et al., 2003; Groves, 2006).

This weed forms dense patches of small, flat rosettes than can cover up to several hectares in area. H. pilosella excludes native species by outcompeting them. The ability of H. pilosella to outcompete native New Zealand plants has been confirmed in laboratory experiments (Moen & Meurk, 2001). Although H. pilosella is thought to be eradicated in Australia, Hieracium potentially threaten tussock grasslands and tablelands in alpine and temperate regions of the eastern states of Australia (Barker et al., 2006 in Beaumont et al., 2009b).

H. pilosella affects soil properties and nutrient cycling in areas it inhabits. Soils under H. pilosella are higher in P (Beaumont et al., 2009b), N and C (Saggar et al., 1998; McIntosh et al., 1995), higher in soil microbial biomass C, N and P and have greater microbial decomposition (Saggar et al., 1998). H. pilosella alters the nitrogen cycle under patches, converting more mineral N into organic microbial forms, and may be one of the reasons for its success in invading N-deficient environments in New Zealand. Indeed soil processes under this plant are profoundly different from those occurring in areas it invades, which may only be a metre away (Saggar et al., 1998). One reason for these differences may be due to the fact that H. pilosella is a perennial and thus returns more C and N to the soil from dying leaves and roots than do surrounding herbfield annuals (McIntosh et al., 1995)

The ability of H. pilosella to extract large proportions of moisture and nutrients from the surrounding soil results in a “halo” of soil around patches that is drier, more acidic and less favourable for growth of other species (McIntosh et al., 1995). McIntosh et al (1995) found that H. pilosella patches their diameter c. 13cm each year by expanding the halo of bare soil surrounding them. The halo appears to be a zone in which nutrients are depleted to the benefit of the plant.

Furthermore, increased acidification, may increase labile aluminium in the soil, which could further enhance the spread of H. pilosella and reduce establishment of other species (Boswell & Espie, 1988 in Scott et al., 2001).

Glasshouse experiments have recorded that increased carbon dioxide levels have a fertilisation effect on H. pilosella, suggesting increased competitive ability of this species in future climates (Leadley & Stöcklin, 1996 in Beaumont et al., 2009b).

The weed also has agricultural impacts, which may have serious financial consequences for farmers in New Zealand (Scott, 1993 in Moen & Meurk, 2001). It is a serious weed for high country farmers in New Zealand as it reduces species richness and of short tussock grasslands and total forage available to sheep. Although Hieracium is palatable, the low growth form makes it unavailable to sheep (Moen & Meurk, 2001)

Preventative measures: Predicting Distribution- Ecological niche modelling (ENM) is a way of assessing potential spread of non-native organisms ENM was carried out on Hieracium pilosella using just native range data versus entire range (invasive and native) showed that climatic niches of invasive populations differed substantially from their native ranges. A number of populations were shown to exist in areas where there is no current climate analogue in its native European range. In eastern North America H. pilosella invasive regions it inhabits tend to be wetter than in its native range. In western North America it occupies a subset of its native range, and in Australia and New Zealand populations occupy a climatic niche more similar to their native range than those of North American populations. Modelling using only native range data or only invasive range data is not sufficient to accurately predict distributions, and failed to capture known distributions in North America, New Zealand and Australia. Entire range data was more effective at predicting current distributions. These differences are likely to occur due to release from biotic constraints in invasive range, allowing species to fulfil their entire fundamental niche (Beaumont et al., 2009a).

For example, in Europe Hieracium has several insects which are specialist feeders and cause significant damage, whereas specialist herbivores are less common in its invasive range and do not cause significant damage (Sárospataki, 1999 in Beaumont et al. 2009a; Syrett & Smith, 1998 in Beaumont et al., 2009a). In its native range geographical barriers may also limit Hieracium spread; little is known about limiting range margins in its introduced range. Populations of H. pilosella in New Zealand have a high degree of genetic and genome size variation due to interspecific hybridisation, which may have contributed to its invasive success in New Zealand (Morgan-Richards et al., 2004 in Beaumont et al., 2009a).

Biological control In New Zealand a biological control project was initiated in the 1990s. Five insect species associated with H. pilosella in central Europe were selected for introduction into New Zealand that were chosen. The insects were chosen as they have narrow host ranges and are damaging to the plant (Syrett et al., 2001 in Klöppel et al., 2003). There were: Oxyptilus pilosellae, a plume moth feeding on the above-ground plant parts, Aulacidea subterminalis, a gall wasp causing galls at stolon tips, Macrolabis pilosellae, a gall midge attacking the stolon tips and rosette centres, and Cheilosia urbana and Cheilosia psilophthalma, hoverfly species which feed externally on the roots and on the above-ground plant parts, respectively. All five insect species were released in New Zealand, and the two gall-forming insects established in the field (CABI, 2010).\r\n

Laboratory trials of the gall wasp A. subterminalis were conducted on stressed (water, nutrient and plant competition) and non-stressed plants. The authors concluded that galling by the wasp is likely to reduce vegetative reproduction of H. pilosella whether or not the plants are stressed, indicating that the wasp may be a successful biocontrol agent (Klöppel et al., 2003).

Grazing: Grazing by sheep has been suggested as a low-cost method of removing H. pilosella inflorescences in order to reduce seedling establishment. Three studies in New Zealand have provided evidence that sheep grazing can reduce the percentage of plants with flowers (Norton & Reid, 2009 and references therein). However there is evidence that grazing of inflorescences can result in increased stolon production. Further research is necessary to confirm whether grazing is an effective management tool as suggested by Espie (2001 in Norton & Reid, 2009).