Agricultural intensification is known to be one of the main drivers of pollinator decline, in particular because flower resources are often scarce in intensively cultivated landscapes. Agroecological practices such as wildflower strips implementation are seen as effective to mitigate this food scarcity by increasing the abundance, diversity and temporal continuity of the flower resources. However, in intensive cropping systems, these practices are often poorly adopted by farmers because of technical and economic barriers. We designed a practice based on the conservation of winter cover crop strips in the middle of spring crops and assessed the value of such undestroyed cover crop strips for the enhancement of pollinator populations, and the conservation of wild bee species. Cover crop strips on farmers’ fields were surveyed and compared to existing herbaceous field margins. Our results showed that within-field cover crop strips are effective in increasing the availability and continuity of flower resources for pollinators. We recorded a higher abundance, richness, taxonomic and functional diversity of bees in the covered crop strips than in the field margins, but no significant effect on hoverflies. Wild bees were supported by the sown flowers of the strips during spring, before the main blooming period of the spontaneous flowers of the field margins, and the combination of sown species of three different botanical families in the strips supported wild bees with different ecological traits, including oligolectic bees. Our findings indicate that this practice can support complementary flower resources, but also highlight the importance of protecting and extending spontaneous plant patches such as field margins for the conservation of rare and specialist bees.