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.
This genus comprises the various kinds of Poppies - annual and perennial. The Iceland Poppy (P. nudicaule), a native of Siberia and North America, is the finest of all for market. Although really a perennial it is treated as an annual or biennial. The seeds are sown in April and May, and the young plants are pricked out and afterwards transplanted by September about 1 ft. apart. There are several varieties, but the tallest are the best for selling. The flowers have white, yellow, orange, etc, shades, and if cut before open they last well. Large quantities are grown for market, the flower stems being cut as long as possible and stood in water a few hours before they are packed for market.
The Oriental Poppy (P. orientate and its variety bracteatum) is a true perennial, is quite hardy, and produces large crimson-scarlet flowers in early summer. The plants are best raised from seeds, and will flourish in any garden soil.
P. pilosum from South-east Europe is a handsome perennial Poppy, 1-2 ft., with pale-scarlet to deep-orange flowers. There are several fine hybrids between this and other species, the flowers having many shades of colour.
The "Shirley Poppy" is said to be a form of the common Corn Poppy (P. Rhoeas). It has lovely blooms of all shades of delicate colour except blue. The trade is usually in the seeds in spring. These are best sown in March where the plants are to bloom, the seedlings to be thinned out to 9 in. apart.
The Opium Poppy (P. somniferum), a beautiful annual, 3-4 ft. high, with white, rose, or lilac flowers, is chiefly grown for the heads or capsules, that are sold to chemists and herbalists. The seeds are sown in spring in rows 2 ft. apart; the plants are afterwards thinned out about 1 ft. apart and kept free from weeds during the summer. The capsules are ripe in August.
In addition to the above there are numerous kinds of garden Poppies, the seeds of which are sold in spring. There are single and double varieties of all shades of colour, and special names like the "Danebrog", the "Tulip", "Japanese Pompon", etc, have been given to some of them.