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.
There are annual and perennial species of Sunflower, but very few are grown extensively for market. The best perennial kind is H. rigidus (formerly known as Harpalium). This grows 3-5 ft. high, has roughish purple stems, and yellow flower heads about 3 in. across with a darker centre. Until its fine variety "Miss Mellish" with taller stems, larger and brighter yellow flowers, made its appearance, H. rigidus was often grown as a catch crop in any out-of-the-way part of the garden for cut flowers in late July and August. The variety is a much better plant, but to get the best results the snake's-head-like rhizomes should be planted at least 1 ft. apart. Once established, the plants look after themselves, and increase with astonishing rapidity - so much so that they are apt to choke themselves in the struggle for air and light. They should be thinned out to 1 ft. apart every spring. Bunches of twelve sprays realize from Is. to 3s. per dozen - sometimes much less. Other species of Perennial Sunflower worth growing as catch crops for cut flower are H. decapetalus, 4 ft.; H. giganteus, 10-12 ft.; and H. multiflorus, 3-5 ft. The last has fine double-flowered varieties like Bouquet d'Or, flore pleno, and Soleil d'Or or grandiplenus - all worth a place for cut bloom, as they are fine in colour and last well when cut. H. sparsifolius is a newer plant about 8 ft. high, with fine leaves, and very large bright yellow flowers with pointed petals.