• Herbicides and mechanical control of C. macrocephalum is likely to have neighbourhood effects on Thesium species. • The biocontrol rust on C. macrocephalum had no impact on T. utile survival. • T. utile is not at risk as it has many hosts and C. macrocephalum is a new association. • Hoeing as a control method is detrimental to T. utile survival. A herbicide trial in a Highveld grassland revealed, serendipitously, a positive association between the root hemiparasite Thesium utile and the noxious weed Campuloclinium macrocephalum (pompom weed). The parasite was unable to survive in plots where the weed was selectively removed by hoeing, but persisted and increased in the untreated control. Herbicides were more effective at killing Thesium than suppressing C. macrocephalum. A host-specific rust attacking the foliage of C. macrocephalum had no impact on T. utile survival. Weed control measures against C. macrocephalum are not likely to threaten native Thesium species as they have a large range of host species. These types of interactions should be investigated further in other species to improve our understanding of the ecology of root-parasites.