Ecological Factors

Mouse-Ear Hawkweed

Ecology:

Hieracium pilosella, mouse-ear hawkweed, is a stoloniferous rosette plant which forms dense patches to the exclusion of other vegetation. It occurs in grasslands in England, Canada, the United States, and New Zealand. Native to Europe, it was first reported as a weed in North America in 1902 and in New Zealand in the1870's. It grows in a range of grasslands in Europe, particularly favouring low fertility, nitrogen deficient soils (Bishop & Davy 1994). In England it occurs in areas of low rainfall with well-drained, sandy soils of low soil water potential in summer which are also low in major organic nutrients (Bishop and Davy 1984). In New Zealand it has appeared in a wide range of grasslands, with particularly rapid expansion in short tussock grasslands previously dominated by fescue tussock (Makepeace 1985a, Hunter et al. 1992).

Its major mode of reproduction is asexual but is tightly linked to ploidy level and flowering, as only plants which initiate inflorescences produce stolons and daughter plants (Bishop et al. 1978). After daughter plants have been produced, the mother plant dies. Average plant age varied from study to study and site to site. In some populations individuals were replaced every one to two years, while others contained individuals up to 16 years old. If the soil in which mouse-ear hawkweed is growing is dry, or is infertile, then a dense advancing front of plants will radiate from a bare center (Boswell & Espie 1998). On richer soils, inner gaps are filled when they occur. Grazing (by rabbits) was found to increase the length and double the number of stolons formed (Makepeace 1985a). Similarly grazing by sheep increased, or had little effect on the cover of mouse-ear, but did significantly reduce flowering (Espie 1994).

Makepeace (1985b) found that mouse-ear hawkweed tends to establish on open sparsely-vegetated sites, and that seedlings sprouted only in years of higher-than-average spring rainfall. Seeds germinated most rapidly at 22 C. In the field, seeds germinated after moist intervals in spring and autumn, and establishment during a wet summer. In instances where seedlings survived the initial few weeks, they reached adult size in 8 to 10 weeks. Later research showed preferential establishment in grassland microsites compared with bare ground (Espie 1992, Rose & Frampton 1999) and confirmed the importance of seasons with high spring-summer rainfall for seedling establishment (Espie 1994). In East Anglian English grasslands Watt (1962) found that a series of dry years led to the establishment of mouse-ear hawkweed, through reduction of sward cover. Mouse-ear hawkweed's dominance in an area once established is due in large part to its rosette growth pattern and stoloniferous reproduction, but it may also be allelopathic. Makepeace et al. (1985) found that umbelliferone, a known inhibitor of root growth, is present in mouse-ear hawkweed leaves and extracts inhibited the growth of several species of clover.

Watt (1981a, 1981b) has made long-term observations of English mouse-ear hawkweed populations in East Anglia and observed the establishment and rapid spread of an even-aged population of plants, followed by debris accumulation, surface levelling and stabilisation coupled with plant senescence and subsequent species diversification. In his study area, a Festuca ovina dominated microplot was subsequently taken over by Hieracium pilosella, which in turn was being replaced by Thymus, although it was not clear whether Thymus was actively displacing the hawkweed or was filling in gaps left by the hawkweed's senescence. In New Zealand, it has shown its greatest increase in dry intermontane basins with a semi- continental climate. In unimproved rangelands pasture and conservation, it displaces both native species and agronomically important forage plants. It can spread rapidly and widely, increasing to over 80% of ground cover (Makepeace 1985a, Hunter et al. 1992). Mouse-ear first appeared in New Zealand's South Island montane grasslands around the 1950's, and thought the two study areas are not identical, Table 1 gives a broad indication of how rapidly it has increased. There is some evidence (Watt 1981b) that over time a population of mouse-ear hawkweed may peak, then senesce and is replaced by other species.

Table 1 Hawkweed % frequency of occurrence in Canterbury & SI High country sites 1960's and 1990's (n = 401 and 133 sites).

If grasses are dominant in a selected area, fertilization can tip the balance in favor of grasses . Davy and Bishop (1984) studied the effects of the addition of inorganic nutrients on Hieracium in Breckland grass-heaths. Quantities and compositions of salt mixtures used were the same as those used by Willis (1963), i.e., (weight in grams for treatment of one square yard of vegetation) 21.2 g (NH4)2HPO4; 10.6 g (NH4)2SO4; 21.1 g K2SO4; and 42.4 g MgSO4.7H2O. Nitrogen alone was added as NaNO3 (Davy and Bishop 1984). Applications were made by hand sprinkling dry mixtures on 11 occasions at about four-month intervals between October 1975 and March 1979. Fertilization on plots which were 50% or greater grasses was found to prevent the spread of mouse-ear hawkweed, with the effectiveness of NPK > NP > NK > N > no treatment (Davy and Bishop 1984). The decline of hawkweed populations in nutrient treated quadrats was rapid and striking due to two different mechanisms: (1) direct physiological and demographic responses to the increased nutrient supply; and (2) indirect effects arising from responses of competitors to increased nutrients, as it is not shade tolerant (Davy and Bishop 1984). Reader and Watt (1981) found a similar response to 336 kg/ha/yr NPK fertilizer for Hieracium floribundum. One application increased growth of Poa compressa and temporarily halted hawkweed patch formation.

Fertilisation should only be used to control hawkweed in areas where it can be expected to give preferred and established grasses a competitive edge. In this case, hawkweed can be shaded out. In New Zealand Scott et al. (1990), Scott (1993) and Cossens & Boswell (1993) have shown that oversowing and fertilising Hieracium piloselladominated grassland successfully controls the hawkweed. Alsike clover (Trifolium hybridum) was the most effective competitor with mouse-ear hawkweed.

Control by chemicals is effective but expensive. Makepeace (1985b) found that 2,4-D ester + "versatil" (active ingredient chloropyralid) at 1000 + 400 g/ha or 750 + 300 g/ha gave good control of hawkweed but also reduced the vigor of alsike clover. The clover recovered over time, the hawkweed did not. Three years of herbicide treatment with oversowing of clover were necessary to essentially eliminate the hawkweed. Cossens & Boswell (1993) applied 2,4 D and mecoprop-MCPA-dicamba herbicides, resulting in a 40% reduction in hawkweed ground cover in after 3 years.

The main outcomes of the Hieracium Management Review workshops were a shared understanding of the main ecological factors and the management implications that arise from these. The focus related largely to the undeveloped tussock grasslands, and to H. pilosella in particular.

Two important interpretative factors were:

1. the stage of Hieracium invasion;
2. the dynamics of tussock grassland vegetation.

Diagrams were used to provide a common concept (or language), and more easily enable farmers and scientists to share and build on their collective experiences.