Many natural control agents of olive pests need pollen and nectar from non-crop plants in order to complete their life cycles. However, a deep knowledge about the occurring plant species in the agroecosystem is necessary to select the plant species to be maintained or enhanced from a conservation biological control approach. Thus, in this study the goal was to increase the understanding about the plants biodiversity in Mirandela, an important olive producing region in the northeast of Portugal. For that, in a weekly basis during spring and every other week in summer and autumn blooming plant inventories were accomplished in three olive orchards with spontaneous vegetation and its surroundings (woody and herbaceous vegetation areas) from April to December of 2012 and 2013. The percentage ground cover for each flowering plant species was recorded following the Daubenmire cover scale modified by Bailey. From total of 258 plant species belonging to 47 families, 172, 169 and 180 species were identified in the herbaceous, olive and woody plots respectively. The most abundant family was Asteraceae followed by Poaceae, Fabaceae and Brassicaceae and the most abundant species was Coleostephus myconis (L.) Rchb.f. followed by Echium plantagineum L. (Boraginaceae), Chamaemelum mixtum (L.) All. (Asteraceae) and Crepis capillaris (L.) Wallr. (Asteraceae). Several species were specific of each land use and presented different flowering periods, representing a potential variety of food sources across the seasons. Additionally, some of the identified species, such as Foeniculum vulgare L. (Apiaceae), are known for their implications as providers of key requisites for natural control agents. These result provided with valuable information for the implementation of conservation biological control measures.