The ecology of common heliotrope (Heliotropium europaeum L.) in a Mediterranean dry-land cropping system

© 2005 Dr. James Robert HuntCommon heliotrope (Heliotropium europaeum L.) is an herbaceous Mediterranean summer annual that grows on areas devoid of vegetation in the dry-land cropping region located in the north-west of the state of Victoria, Australia. This region is known as the Mallee, and common heliotrope is considered a weed here because it transpires soil water that could otherwise be used by ensuing crops, and is toxic to livestock. In this study, laboratory experiments have shown that germination of seeds of common heliotrope is not limited by light, cold treatment, or a leacheable inhibitor. Temperature and water potential (and perhaps depth inhibition) are the principal environmental factors that limit germination. Although 100 % of seeds will germinate under optimal conditions, these conditions are rarely met in the field, and the percentage of seeds that germinate at sub-optimal conditions changes seasonally and between Australian populations. This is the principal mechanism of dormancy in the species, ensuring that seeds do not germinate when conditions are unfavourable for continued growth and that in the field, germination of a seed population is fractional, spreading risk temporally. Germination of seeds of common heliotrope does not conform to the assumptions of the hydrothermal time model frequently used to predict field emergence of weed species. Field observations and simulated rainfall experiments on a consistent soil type (Calcarosol with a clay loam surface texture and clay subsoil) indicated that approximately 20 mm of rainfall is the minimum amount required for germination and emergence of common heliotrope. Field experiments using lysimeters indicated that this is also the amount required for minimal reproduction on the same soil type. Although common heliotrope can successfully reproduce upon the rainfall event which causes its germination, its growth is indeterminate, and further access to moisture will result in massively increased reproductive output. Plants will continue to grow and produce seed over summer until they are killed by drought or senesce in autumn. Laboratory and field studies showed that root growth in common heliotrope is relatively slow, and is opportunistic in areas where moisture becomes available. Simulation of soil water and temperature fluxes showed that soil type has a large impact on the amount and duration of water potential that seeds and plants are exposed to. In the case of common heliotrope, this is critical in determining regional prevalence and distribution in the north-west of Victoria. It is recommended that studies of arid species reliant on isolated rainfall events for emergence should consider absolute water availability and not in terms of rainfall amount alone. Analysis of long term summer rainfall data from the Victorian Mallee indicates that there is potential for reducing the cost of controlling common heliotrope by using residual pre-emergent sulfonylurea or triazine herbicides, instead of the traditional post-emergent herbicides or cultivation. More information is needed on the efficacy of such residual herbicides on common heliotrope and other summer weeds, and their impact on subsequent crops before the full potential of this management option is known. Reduced cost of control could also be achieved by economic analysis of the likely benefits of controlling each emerging cohort of common heliotrope, and a framework for decision making is suggested