Short-Day Plants

Short-day plants need a specific number of solid hours of darkness to form flower buds. Under natural conditions they will do this as autumn approaches. But there are two ways to change their flowering period. You can hasten it by shading the plants to supply them with extra hours of darkness. You can also delay flowering by breaking the darkness-induced budding cycle with additional or intermittent light.
Covers of heavy black sateen,  shade cloth, or building paper can be used to shut out light. This covering is placed over the plants in late afternoon, perhaps 5 o’clock, and left on until 7 o’clock the following morning. Plants requiring such treatment should be grown in an area that will accommodate the erection of a wire frame around them to facilitate placement of the shading material. If you will be growing plants that need short days (mums) for multiple years, consider shading that could be hung like drapes and just unhooked and taken down or tied back when not needed.

The time to start shading depends on when you want to sell the plants. Chrysanthemums require 2 to 4 weeks of artificially induced darkness to start flower buds ahead of normal season schedule; poinsettias shaded about 2 weeks beginning the first part of October will flower for Christmas sales. Shade the Christmas begonia (Melior) from October 10 to October 20 to make it produce a heavy crop of flowers for Christmas sales. If you supply short-day plants with 2 to 6 hours of additional evening light from 25-watt bulbs spaced 2 feet above the plantsI you can break and delay their budding cycle. With proper manipulation of light and darkness you can have them in flower through most of the year.

Many commercial plant suppliers, especially chrysanthemum specialists, furnish exact data for each variety they sell.
You can change and control the flowering periods of such important profit-makers as kalanchoe, poinsettia, gardenia, China aster, and Christmas cactus, by shading or lighting them.