Plant microfossils from the Montesano Formation have been studied qualitatively and quantitatively in the type section along the Middle Fork of the Wishkah River. The age of the Montesano Formation has been established independently by Foraminifera as Late Miocene (Mohnian-Delmontian).

Over 100 species of palynomorphs have been recognized from the Montesano Formation with the following taxa most common: Pinus, Picea, Pseudotsuga, Tsuga, Taxodiaceae, Cupressaceae, Taxaceae, Alnus, Betula, Carya, Castanea, Pterocarya, Quercus, Salix, Compositae, and Ulmus-Zelkova, Fagus, Juglans. This assemblage indicates that in late Miocene time elements of the Eastern deciduous forest and coastal plain provinces co-existed with elements of the Cordilleran forest province.

The major floristic difference between the Montesano Formation and the overlying Pliocene-Pleistocene sequence is the disappearance of elements of the Eastern deciduous forest and coastal plain provinces in the younger rocks. Also Artemisia, which is absent in the Montesano Formation, becomes an important element in the younger flora. The underlying Astoria Formation, of early to middle Miocene age, is similar floristically to the Montesano Formation with the major differences being an almost complete lack of Compositae as well as the absence of Polygonum californicum in the Astoria Formation.

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