Mouse-ear hawkweed (Hieracium pilosella) is distinguished by production of stolons, a prostrate mat growth habit, a single lemon yellow flower, and in most situations, by the dense white under-surface of its smooth-edged leaves. Leaf size depends on habitat, varying from small (2 x 0.5 cm), in low fertility exposed situations, to large (10 x 2 cm) in high fertility or shaded sites.Regarded as the most serious problem hawkweed (Hunter et al. 1992), it occurs in both the North and South Island and is found from sea level to 1500 metres. It is abundant in the South Island montane zone from Marlborough to Southland (Webb et al 1988). In the high country it is predominant in the 500-1000 mm rainfall zone, though it can occur in both dryer and wetter regions (ca. 350 - 3000+ mm). Its optimum environment is probably in the 600-800 mm rainfall zone where it has sufficient soil moisture and adequate soil fertility. Mouse-ear appears to favour soils with moderate soil fertility, particularly as related to phosphate levels. It establishes best in sites with within low statured vegetation (Rose & Frampton 1999), from where, in the presence of grazing it spreads to dominate the surrounding plants and bare ground. Vegetative spread is not restricted by grazing (Espie 1994). Initially mouse-ear invades and displaces the inter-tussock vegetation, and then on some sites subsequently displaces previous dominants such as fescue tussock. Dense swards appear to depress the growth of sweet brier and anecdotally are considered to displace even established tall snow tussocks. In developed pastures, competition from forage species can suppress H. pilosella (Scott et al. 1990, Scott 1993).
H. pilosella ecology in New Zealand has been reviewed by Scott (1984) and in Hunter et al. (1992). European ecology is reviewed by (Bishop & Davy 1994).