Plant Atlas 2020 is the most comprehensive survey of plants (flowering plants, ferns and charophytes) ever undertaken in Britain and Ireland. It is based on over 30 million records, collected mainly by volunteer recorders of the Botanical Society of Britain and Ireland (BSBI) between 2000 and 2019, as well as previous nationwide surveys undertaken in the 1950s and 1990s. This resource provides the data behind the phenological diagrams (flowering and leafing) presented on the Plant Atlas 2020 website (www.plantatlas2020.org) and in the Plant Atlas 2020 book. Note that for non-flowering plants included in the atlas (e.g. ferns, horsetails etc.), the “flowering” fields in the phenology file included here are equivalent to the months when spore-bearing structures are visible.