Gastrointestinal Nematode (GIN) parasitism is the main health challenge small ruminants face with detrimental consequences on their health, welfare, and economic returns. Gastrointestinal parasite control is usually achieved with the use of anthelmintic drugs. Use of anthelmintics is strictly regulated in the organic sector. However, due to the global spread of anthelmintic resistance and their impact on the environment, the organic sector is seeking for non-chemical complements or alternatives. Examples of alternatives used by farmers include grazing management strategies, selective breeding for resistance and protein supplementation for improved productivity and immunity. The primary focus of our group is to investigate the anthelmintic efficacy of bioactive plants as an alternative for the control of GIN. Bioactive plants contain plant secondary metabolites (PSM), which are compounds not essential for plant growth or reproduction, but are thought to play a role in plant defences. One group of PSM are the condensed tannins and they have been associated with significant anthelmintic activity. The perennial shrub heather is rich in condensed tannins and is abundant across northern Europe. The objective of our work was to quantify the anthelmintic efficacy of heather extracts. To do so, we have performed a comparative analysis of extracts obtained from two heather species – Calluna vulgaris and Erica cinerea – collected in five different European countries, across two seasons and tested against two GIN species. Calluna vulgaris samples were collected in the UK (Scotland), Germany, Norway, Switzerland, and Spain, where a second heather species (Erica cinerea) was also collected. Extracts were tested against the egg hatching and the larvae motility of the GIN species Teladorsagia circumcincta (abomasal) and Trichostrongylus colubriformis (intestinal). Our results show that heather extracts from different countries and seasons had a significant impact on egg hatching and larvae motility. In addition, T. circumcincta eggs were more susceptible compared to T. colubriformis. Our results indicate that heather could be a contributor to parasite control management in small ruminants. The variation in efficacy may be explained by differences in environmental factors impacting heather growth, the presence and levels of condensed tannins, and their interactions with the parasite species. Demonstrating anthelmintic activity in living animals and identifying the sources of variation in anthelmintic efficacy is fundamental in optimising heather as a viable alternative in the control of GIN.