Establishment of Woody Weeds in Western New South Wales. 1. Seedling Emergence and Phenology.

Abstract

Germination and survival of seedlings of four woody weed species (narrow-leaved hopbush Dodonaea attenuata, turpentine Eremophila sturtii, punty bush Cassia Eremophila var. Eremophila and silver cassia C. artemisioides) were examined, together with their phenology, at four sites which differed in shrub density, grazing pressure and topography in the Bourke - Wanaaring region of western New South Wales over the period 1979 to 1982. Although emergence varied between species (D. attenuata: 5,000-30,000 seedlings/ha; E. sturtii: 3,000-10,000 seedlings/ha; Cassia spp. : 2,000-4,000 seedlings/ha), the conditions favourable to large scale establishment depended mainly on rainfall during late autumn and winter for germination and that of the summer following germination for survival of all species. Turpentine required larger rainfall events or prior rainfall events for seedling germination. Soil disturbance enhanced seedling emergence, growth and survival of D. attenuata., by increasing soil moisture content deep in the profile. Sandhill areas showed significantly higher emergence levels for D. attenuata. The higher emergence of seedlings on densely shrubbed areas was most probably due to higher soil seed loads. Micro-depressions and sandplains favoured survival of all species. Grazing, mainly by rabbits, had no specific impact on emergence of any of the species, but reduced the survivorship of D. attenuata. Predation by insects had a negligible effect. Hopbush flowered in all years between July and October and turpentine flowered from May to September. Those that flowered were prolific. Seed loads had been dropped by late November. Generally, hopbush shrubs had attained 2 m before flowering but in one year 86% of those above 1 m high flowered. Turpentine generally flowered after it had attained 50 cm, although in one year one plant flowered when less than 25 cm. Encroaching populations of shrubs should be controlled before they mature and form dense stands. Hopbush and turpentine control programs should be completed before the end of winter, just before annual seed set. Control of young establishing stands of hopbush should be undertaken before they reach a height of one metre and 50 cm for turpentine. Rapid expansion outside treated areas is unlikely, however, occasional 'outlier' shrubs should be controlled before they mature and produce seed.