What is 'Farina'?
Many species of Primula exhibit a white or yellowish powder on their leaves and even flowers. This is not a kind of fungus such as powdery mildew, but is a substance produced by healthy plants. In fact, to most Primula enthusiasts it enhances the appearance of the plant. For example, it is a delightful (and required) embellishment of the Show Self and Edged Auriculas. It even gives one of the species its name: Primula pulverulenta means 'powdery primrose'. In some species this powder is even fragrant!
This powder has more than one name. The term 'farina' means flour, which is what the powdery coating looks like. Another word is 'meal', which is just another word for flour! In some cases 'paste' is used, usually denoting a much thicker kind of white coating in the center of some Auricula flowers.
Excerpt from the Primroses Quarterly, Volume 2, Issue 2 (October 1944) p.23:
The Nature of Farina by Donald O' Connell
The word farina is taken from the Latin, and means flour, or meal, an apt description of its physical appearance. It is a phenomenon rarely found in Flora=s kingdom, being almost peculiar to the genus Primula.
This meal is formed by a multiplicity of microscopic hairs, shaped like slender, blunt-nosed rods. They are glandular in character and secrete an opaque wax, which in chemical structure in remarkably similar to the more common floral coloring pigments.
In color, farina is either white or very pale yellow, as in the leaf edges of P. marginata and the underside of the leaves of P. longiflora. Yellow farina is less commonly found, however.
The appearance of gray farina on the rare gray-edged Show Auriculas is caused by a thin overlay of white farina upon a green ground. The wax-secreting glands on these and other edged forms of Show Auriculas are identical with those on the foliage of the plants, whereas that found on the flowers of Border Auriculas and Selfs are of a slightly different shape.
Farina may occur on any part of the plant, or on several or all parts. It occurs quite consistently in the flowers and flower-heads of the Muscariods, Sphaerocephalas, and Denticulatas, and is further found on the leaves of nearly all Nivalids. It may appear so slightly as to seem a thin white dust, or so thickly as to flake.
Of the thirty-three sections into which the genus Primula is divided, twenty-one contain species that are to some degree farninose; while only twelve sections are efarinose, i.e. without farina. Among these twelve efarinose sections occur practically all the woodland Primulas, why we do not know. Witness the lack of meal on the Vernales section, to which belong P. Juliae and the Polyanthus, and the Cortusoids, the two sections containing the majority of commonly cultivated woodland species.
The purpose which is served by the farina on the various parts of the plants is an enigma as yet unsolved. It is generally accepted that meal is formed to prevent the plants from becoming burnt by the sun. The frequent presence of farina on the new growthBmore easily burntBof species in which the mature leaves are not farinose, or very slightly so, bears out this theory. Yet the presence of meal on such shade lovers as P. Winteri and many others raises a question which is not satisfactorily answered by this belief. As Farrer would say, it is hoped this short piece will amuse the uninitiated Aand not enrage the learned@, for the whole subject is more less one of contention and would undoubtedly reward more thorough research, which will be forthcoming as interest in this lovely genus grows.
Excerpt from the Primroses Quarterly, Volume 2, Issue 4 (follow-up article)