The performance of Achillea millefolium L., from early summer until early spring, in the presence and absence of Hordeum vulgare L. was assessed by growth analysis. Growth analytical quantities were derived for a 307 d experimental period from splined cubic polynomial exponential functions fitted to dry matter and leaf area data. Ontogenetic drifts in the net assimilation rate and leaf area ratio of A. millefolium were highly modified by interference from H. vulgare resulting in a diminished total plant relative growth rate and maintenance of the vegetative state. The relative growth rate of A. millefolium increased during the autumn and early winter in response to rises in net assimilation rate and leaf area ratio induced by the harvesting of the H. vulgare. During the post-harvest period, the relative growth rate of the rhizome increased and exceeded those of leaf and total weight resulting in a substantial increase in rhizome dry matter during autumn and winter and a pronounced rise in the rhizome weight ratio.
Keywords: Achillea millefolium L. (yarrow); Hordeum vulgare L. (barley); competition; growth analysis; splined growth curves.