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.
Few hardy border perennials have enjoyed a greater run of popularity than the double- and single-flowered varieties of Pyrethrum roseum. Botanically, however, Pyrethrum is now included under Chrysanthemum, where our old-time friend figures as G. coccineum. The wild species - the progenitor of the present lovely race - belongs to the Caucasus, where it is virtually and practically an Alpine of a few inches in height. The numerous varieties which now beautify our gardens have been evolved during the past fifty years or so (fig. 239).
The plants are valuable in the border by reason of their early summer gaiety and profuse flowering, and alike valuable - the single-flowered sorts more particularly - in the cut state by reason of brilliant or delicate colouring and general comeliness. Because of their great length of stem and graceful heads of flowers the florist decorator regards them as ideal; and the flower that is in demand by these latter is the flower for the commercial gardener to produce, and that in quantity. The great vases of the flowers seen in the windows of leading florists are as a free advertisement, and, while well merited, tell a tale that the producer of flowers cannot afford to ignore.
The Pyrethrum, more so than the majority of hardy perennial border flowers, requires to be dealt with from the cultural standpoint in a systematic manner, albeit, generally speaking, it is not fastidious. The plant itself is of tufted growth, its rootstock being made up of numerous crowns, often a hundred or more to a single clump. It happens, too, that the plant readily submits to division, the spade, the crudest of all implements, being often requisitioned for the purpose. No success worthy of the name has, however, ever followed this practice, and it is cited here as an instance of what not to do.
There are two seasons in the year when the plants may be lifted and divided with advantage - the early spring, when the new leaf growth is a few inches high, and again, in July or early August, when the plants, having got over their flowering, are seen to be pushing a new leaf growth. At either season the plants may be lifted, washed free of all soil in water, and divided up freely. Old flowered plants are best if placed on their sides on the potting bench, the clumps being wrenched asunder by hand assisted by the point of a knife, or, better still, by placing two small hand forks back to back, and pressing each in an outward direction. By repeating the process a clump is readily reduced to small units - single crowns if you will - and so long as each portion is possessed of root fibres it is capable of making a good plant. Indeed these single-crown portions make the best plants and yield the finest flowers in their season. When divided in this way the plants require potting singly, and the protection of a cold frame for a few weeks before being again planted out.
The Pyrethrum being naturally of a voracious appetite the soil cannot be too well enriched or too deeply cultivated. In growth the plants delight in abundant supplies of moisture. Where comparatively light soils obtain, the surface of the beds should be kept at ground level, or rather below, and not raised. Where space permits, those permanently planted should be set out 2 ft. asunder. In respect of soils, avoid the more sandy and those of a tenacious water-holding character. In planting, keep the crowns level with the soil, to bury them would be wellnigh fatal to the plants. On heavy and retentive soils a small black slug is most destructive to the crown growth, to which a sprinkling of soot now and again or fine coal ash is a useful deterrent. The raising of these plants from seeds is a matter deserving attention.
Fig. 239. - Pyrethrums, single and double.
Doubles: Ne Plus Ultra, blush; Aphrodite, Carl Vogt, Princess de Metternich, and Mont Blanc, white; Alfred Kelway, crimson; J. N. Twerdy, reddish amaranth; La Vestate, blush white; Solfaterre, Vance, and Pericles, yellow shades; Captain Nares and Mons. Barral, crimson shades. [e. h. j].