according to floristic surveys ~10: in a township in Massachusets ~3: in the British Isles ~25 %
||Yakimowski SB, Rieseberg LH. The role of homoploid hybridization in evolution: a century of studies synthesizing genetics and ecology. Am J Bot. 2014 Aug101(8):1247-58. doi: 10.3732/ajb.1400201 abstract & p.1248 left column 2nd paragraphPubMed ID25156978
||Abstract: "[Investigators] revisit Stebbins' views on the frequency of hybridization, the evolution of hybrid sterility, and the evolutionary importance of transgressive segregation, introgression, and homoploid hybrid speciation in the context of contemporary evidence. Floristic surveys indicate that ∼10% of plant species hybridize, suggesting that natural hybridization is not as ubiquitous as Stebbins argued."
||P.1248 left column 2nd paragraph: "Although the ubiquity of hybridization across plants was accepted into the 1990s (Raven, 1976, Whitham et al., 1991), surveys of floras suggest that this is not the case, with estimates of the proportion of plant species that hybridize ranging from ∼3% in the Concord Township flora in Massachusetts (primary sources Mayr, 1992, Mallet, 2005) to ∼25% for the flora of the British Isles (primary sources Stace, 1975, 1997 Mallet, 2005), with an overall frequency of 0.09 hybrid combinations per nonhybrid species (BNID 113466)."