Introduction The steep population decline of the emblematic monarch butterfly is caused mainly by the reduction of food sources for caterpillars and adults, as well as disturbance in its overwintering forests. Although feeding at the overwintering sites in Mexico has long been considered unimportant, observations show that butterflies feed inside and outside of their forests on sunny days. Nectaring close to the hibernation colonies may be relevant for their conservation, as their reserves decline faster in disturbed forests. However, there are no systematic studies on nectar sources in the Monarch Butterfly Biosphere Reserve, Michoacán, Mexico. Aims In this study, we identified and quantified the main plant species visited by butterflies for nectaring in the Reserve. Methods We collected systematic observational data on the flora and butterfly visits in three sanctuaries, around colonies formed between February and March of the 2019–2020 season. Results Butterflies fed on 29 plant species from 10 families. Most had white, yellow or blue flowers, were somewhat synanthropic and had their main flowering season in winter. The most visited species were Salvia mexicana and S. plurispicata, which were also the most abundant. By individual plant, Prunus serotina, Crataegus mexicana, Buddleja sessiliflora, Verbesina oncophora and Roldana albonervia were the most visited. Discussion/Implications for insect conservation The results point to possible interventions in support of the butterflies. The visited species are generally easy to encourage or cultivate and could be promoted in the surrounding agricultural areas.