Ecological studies on reproduction and establishment of the woody weed, groundsel bush (Baccharis halimifolia L.: Asteraceae)

Abstract

Groundsel bush (Baccharis halimifolia) is spreading southward along the coastal range of eastern Australia where the annual rainfall exceeds 90 cm. This advance conforms with its cool temperature optimum for germination (15-20¦C) and its superior germination when given a cold pretreatment (5¦C for 1 week). The species has a facultative light requirement for germination, which suggests that the maintenance of a very dense vegetative cover in the herb layer could discourage establishment of the weed. Once established, however, B. halimifolia can produce viable seed in very dense shade (3 % of open light conditions), even though total seed production per plant is reduced. Prolific seeding occurs with high light intensity. Thus a tall plant (2 m) may release c. 1.5 million seeds. Seedlings possess an unusual ability to maintain growth under low nitrogen conditions during their first 13 weeks, and to survive even when the supply of all nutrients is poor. Seedlings showed no growth response when the strength of a standard nutrient solution was doubled. The fact that growth is not inhibited under enriched nutrient conditions, however, may account for the superior competitive performance of B, halimifolia on fertilized sites containing native coastal species adapted to low soil fertility. Implications with respect to integrated control of the weed are briefly discussed.