The headword pignu/h is otherwise unattested (and not in LSJ). The entry as a whole, however, occurs in other lexica, with the headword differently spelled: e.g. pigu/nh in Photius (pi869, but obelized there by Theodoridis), puti/nh or pitu/nh in Hesychius, phgnu/nh in Lexica Segueriana 342.27. Theodoridis quotes Dindorf's view that Toup was probably right to regard the authentic version as pitu/h.
On the botanical issue see LSJ s.v. knh=kos.
cf. generally pi 1540.