Some flowers are used in the manufacture of perfumes, although they have been replaced largely by chemically manufactured products. The oils of lavender, thyme, and rosemary are three perfume oils taken partly from flowers. Flowers are also used to a certain extent as drugs. The drugs arnica and cannabis both come from the flowering tops of plants. The nectar of flowers is used by the bees in the manufacture of honey. But the principal value of flowers to man is for beauty and the fruits which follow them. Great business enterprises deal with seeds, the breeding of young plants in nurseries, and the sale of trees, shrubs, and flowering plants as well as cut flowers.
Florists are able to produce blooming plants all year round by planting in greenhouses and through artificial forcing. In forcing tulips as potted plants for an early Easter, the florist keeps them at a low temperature for a considerable period and then at a higher temperature to promote growth and flowering. In certain other plants, length of exposure to light each day is most important. For example, chrysanthemums can be forced into earlier bloom by covering them with a black cloth morning and night so that they receive light for only 10 to 12 hours a day; on the other hand, poppies bloom only when exposure to light periods are long. For greatest amount of bloom, plants should not be given an excess of nitrate fertilizer.
Flower production by potted plants can sometimes be increased by growing the plants in rather small pots so that root growth is limited. Flower production by shrubs can sometimes be increased by pruning, so that the remaining branches get lighter; but care must be taken not to remove so many leaves that food manufacture for the entire plant is decreased.