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.
Although there are many distinct species of Paeony known in botanical collections, only one or two are grown extensively for market. Indeed the species themselves are not grown at all, but the magnificent double-flowered forms that have been raised from them. These have emanated chiefly from P. albiflora (or P. edulis), a Siberian plant, 2-3 ft. high with beautiful white or pale-pink flowers; and from P. officinalis, a red or crimson species from South Europe. P. corallina, a crimson or rose-red form, naturalized in parts of Britain, and P. pere-grina, bright crimson, both have had something to do with the production of the modern garden forms. What are known as Chinese Paeonies are derived from P. Pottsi and P. Reevesi, and the Anemone-flowered Paeonies are said to have come from P. officinalis and P. paradoxa.
These are all hardy herbaceous deciduous perennials, and are thus easily distinguished from the Tree or Moutan Paeonies (P. Moutan), a Chinese and Japanese species which has woody stems and large gorgeously coloured flowers having a wide range of colour from white to rose, salmon, lilac, scarlet, magenta, violet, etc. A newer kind, P. lutea, also from China, has yellow flower, and in the course of time we may expect the hybridist to utilize it for the creation of new varieties.
As market plants the herbaceous Paeonies are now grown extensively in some Middlesex market gardens, chiefly for cut flowers. They are planted beneath standard and half-standard Apple, Pear, and Plum trees about 2 ft. apart every way, thus giving about 8000 plants to the acre, after allowing for the trees and pathways. The plants like a good, rich and well-manured soil, and, once established, flower profusely year after year. A good plant will average a dozen fine flowers per annum, so that 80,000 to 100,000 blooms per acre may be taken as a fair crop. At 3d. per bunch of twelve flowers this means a gross yield of £100 to £125 per acre. Sometimes, however, there are enormous quantities sent to market, the bunches being simply taken up loose in van loads, and the prices may then average only 1d. per bunch. Only a few kinds - the deep-crimson, the white, and a deep-rose variety - are grown in market gardens, but there are now so many lovely shades of rose and pink that the market grower might well consider them. The cultivation is simple. So long as the soil is good, a little manure added in autumn or winter will give sufficient nourishment to the roots; and as the leaves and stems - weighing from 6 to 8 tons per acre - are allowed to die down and rot, they may be regarded as a natural annual dressing of manure. It would therefore be hard to find a crop more economic to cultivate than the herbaceous Paeony. The best time for planting Paeonies is about August or September, taking care not to injure the tuberous roots and fleshy shoots more than can be helped.
1, Summer form of fruit (magnified 400). 2, Minute black sclerotia formed in dying leaf-stalks close to the ground. When the leaf-stalk decays these sclerotia rest in the soil until the following spring, when they produce spores as shown in fig. 3, which infect the young leaf-stalks as they push through the soil (magnified 100). 4, Spores produced by sclerotium germinating in the soil (magnified 100).
Paeonies in market gardens are singularly free from disease. Nevertheless they are sometimes subject to attack from a fungus called Sclerotinia poeonioe, which produces the drooping disease. The fungus attacks the young crimson, bronzy, or purple growths in spring and destroys them very soon. In large areas spraying is almost out of the question, and the best plan would be to take up the affected plants and burn them, afterwards dressing the soil with flowers of sulphur. Fig. 232 shows what the Paeony disease is like.