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Summary Mouse‐ear ( Pilosella officinarum ) and king devil ( P. praealta ), introduced to New Zealand in the 1920's, have rapidly increased in abundance in the modified short‐tussock grasslands of the dry montane regions of South Island. Allelopathr and competition effects were investigated to explatn the success of these hawkweed species in ousting other species and to find an agronomically useful species to out‐compete them. Evidence of allelopathic effects was found in both laboratory and field experiments. The phytotoxic agents in mouse‐ear and king devil are phenolics of which umbelliferone is the most active. Umbelliferone, present in mouse‐ear leaves only, is washed into the soil from recently dead leaves but not from living leaves, and is deleterious to root development. Mouse‐ear and king devil compete for nitrogen, phosphorus, and water. Pot competition trials with several grass and clover species showed alsike clover to be the best competitor against the two hawkweeds. The nutrient and moisture status of the soil modified the allelopathic effect. In field conditions, allelopathy is effective as an interference phenomenon only under certain circumstances.
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Makepeace et al. (1985) studied this question.
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