This section is from the book "Commercial Gardening Vol2", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
The Polyanthus as we know it to-day - in primrose and cream and white, sulphur and gold and orange, crimson and red and allied shades - has been gradually evolved from the above-named hybrid, itself the result of crossing Primula acaulis and P. officinalis. It is one of the most popular of spring-flowering plants, an old-time favourite, whose popularity is not a little due to perfect hardiness, great freedom of flowering, and simplicity of culture. The plants delight in rich soils and cool and shady spots; hence it is a success in orchard and woodland, or in soils where much moisture is found.
Though by nature, in common with its original parents, of perennial duration, it is for effective garden decoration far better if regarded as a biennial only, to be raised periodically from seeds. Seedlings, indeed, possess the greater vigour and provide that freedom of flowering everywhere admired.
The seeds should be sown in March or April in shady spots in the open, or in boxes or pans in frames, and be transplanted when three or four leaves have been formed. Not a few amateur and private gardeners postpone the sowing of the seeds to August, or even later, evidently working on the new seed-crop theory. Seeds sown at this latter date, while vegetating early and well, cannot overtake those sown a few months' earlier, and since size of plant in this instance means also a great flowering, the period named should be followed. Seeds of Polyanthus are most impatient of a deep soil covering. Far better that the seeds be thinly broadcasted on a well-prepared bed of soil, well watered in, and left alone to nature. In this way they are washed into the interstices of the soil and require nothing more. Rich food or abundant moisture, or their partial equivalent, shade, are essential to success, their opposites but court failure by starvation.
Seeds of many high-class strains are available at the present time, and among these the "Munstead" strain calls for special mention because of great size of blossom and a wide range of colours. A point of importance to the grower of this crop is the ever-present need for further improvement, best accomplished by cross-breeding, selection, and re-selection, which naturally includes the elimination of the degenerates. A profitable crop, of which the seeds and the plants are alike saleable. The Polyanthus is a great favourite with some market growers, who plant it between the rows of standard Apples, Pears, or Plums, about 1 ft. apart, or less, and sell the "roots" in boxes in the spring. Forty thousand plants to the acre may be taken as a fair crop. [e. h. j].