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)