Summary
Cirsium arvense (L.) Scop. is a clonal plant originally native to Europe and Asia. Today, it has spread throughout the temperate zones of the world and is one of the most important perennial weeds. Creeping thistles grow in patches. They can spread vegetatively via their extensive root system and generatively via wind-dispersed seeds. Surveying the growth strategy of C. arvense makes it possible to optimize indirect and direct control measures.

This thesis analyzes the growth strategy of C. arvense throughout its life cycle using results from field experiments and model experiments.

The strategy of C. arvense proved to be typical for a perennial species. Under favourable growth conditions the thistle patches spread extensively. But even in years with unfavourable conditions C. arvense survived with at least a few shoots. Investment in generative spread was comparatively low. The production of flowers depended on the success of vegetative growth and the thistle population did not develop a large persistent seed bank. C. arvense proved to be highly susceptible to competition for light in the vegetative cycle as well as in the generative cycle. Seedlings in particular turned out to be vulnerable. In model experiments, they displayed lower rates of germination, higher mortality, and reduced growth rates when growing in the shade. But C. arvense responded fast to changes in competition: When the shading was removed in late summer, a second wave of seedlings appeared. Vegetatively, the C. arvense patches were smaller and less dense in highly competitive crops. After harvest, the population regenerated by producing new shoots. A first phase of vegetative growth in established patches appeared in spring in young crop stands with low competitive ability.

The effect of cultural measures on the growth of C. arvense was studied in a long-term field experiment. In the first phase of this experiment, from 1988 to 1999, shoot densities of C. arvense were compared between fertilized plots and unfertilized plots in the rotation beets – winter wheat – winter barley. In spite of mechanical control in the beet crops in all plots, shoot densities and patch size increased in the unfertilized plots. In the fertilized plots, shoot densities remained constant. In the second phase from 1999 to 2003 the rotation was changed to grass/clover (annual) – winter wheat – winter wheat. Previously unfertilized plots were transformed into fertilized plots and plots fertilized in the previous phase were turned into unfertilized plots. As in the first experimental phase, C. arvense responded with rapid vegetative spread to the absence of fertilizer. But the effect turned out to be reversible as well: The patches established during the previous years disappeared almost completely within a few years of applying fertilizer. In addition to the impact of fertilization, the grass/clover mix had a pronounced regulative effect and differences in weed suppression between wheat cultivars became apparent.

Observations in an oilseed rape – oats – winter wheat – winter barley rotation supported these results. The development of thistle patches and individual shoots resembled that of the long-term experiment. In the highly competitive rape crops only few shoots developed, which usually did not reach the flowering stage before harvest. In the subsequent oats crops extensive thistle patches developed in spring. A period of fallow after harvesting the oilseed rape reinforced the effect of the less competitive oats crops. Seedlings were very rare in all field experiments. Only in 2003, it was possible to observe seedling development under field conditions in an oats crop. As seen in the model experiments, competition for light in areas with high crop density led to higher mortality and growth retardation in the seedlings. All seedlings failed to establish beyond the end of the growing season.

The results show that it is possible to control C. arvense by non-chemical cultural measures. Especially successful are measures aiming at enhancing the competitiveness of crops. Dense crop stands that exert competitive pressure early in the growing season limit vegetative spread of C. arvense and prevent seedling establishment. Isolated measures, including one-time herbicide applications, are not sufficient for sustainable thistle control. As a perennial, C. arvense is well adapted to outlast unfavourable periods. An effective control strategy should comprise fertilisation, crop rotation, cultivar selection, and soil cultivation as well as direct control methods.