Use 2011 to cite phangorn in a publication; 2017 for plotting phylogenetic networks. As phangorn is evolving quickly, you may want to cite also its version number (phangorn 2.5.5).

Schliep K.P. 2011. phangorn: phylogenetic analysis in R. Bioinformatics, 27(4) 592-593

Schliep, K., Potts, A. J., Morrison, D. A., Grimm, G. W. (2017), Intertwining phylogenetic trees and networks. Methods in Ecology and Evolution, 8: 1212--1220.<doi:10.1111/2041-210X.12760>

Corresponding BibTeX entries:

  @Article{,
    title = {phangorn: phylogenetic analysis in R},
    author = {K.P. Schliep},
    journal = {Bioinformatics},
    year = {2011},
    volume = {27},
    number = {4},
    pages = {592--593},
    url = {https://doi.org/10.1093/bioinformatics/btq706},
  }

  @Article{,
    author = {{Schliep} and {Klaus} and {Potts} and Alastair J. and
      {Morrison} and David A. and {Grimm} and Guido W.},
    title = {Intertwining phylogenetic trees and networks},
    journal = {Methods in Ecology and Evolution},
    volume = {8},
    number = {10},
    issn = {2041-210X},
    url = {http://dx.doi.org/10.1111/2041-210X.12760},
    doi = {10.1111/2041-210X.12760},
    pages = {1212--1220},
    keywords = {exploratory data analysis, networks, phangorn, R,
      trees},
    year = {2017},
  }