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Traditionally, tobacco seedlings have been produced in plant beds covered with perforated plastic or woven materials in temperate regions or mulched with gravel, sand, rice hulls and other materials in tropical regions. In recent years, labor shortages and the probable ban on the use of methyl bromide for soil sterilization have led to the use of containerized (intact-root) seedlings in many areas of the world.
The most common method for the production of intact-root seedlings is the float system. The float system was introduced by Speedling, Inc, a large producer of vegetable and other transplants in the USA, in the mid 1980s. Since its introduction, the float system has become the most common method of seedling production in the USA. For example, approximately 73% of the tobacco seedlings in North Carolina were produced in greenhouse float systems during 1996. Various adaptations of the float system are also used in many Western European countries, Australia and Mexico as well as countries in Central and South America.
In the float system, tobacco seedlings are grown in polystyrene trays that contain a soilless growing medium. These trays are often called Todd cells because they were developed by Mr George Todd, founder of Speedling Inc. The trays are floated on a bed of water that has been fertilized with a soluble fertilizer. Each tray contains 200-392 cells that are shaped like an inverted pyramid. In the flue-cured production areas of the USA, the float plants are grown in heated greenhouses. In the burley production regions, where production units are often smaller than with flue-cured, small heated greenhouses or covered-outdoor water beds are used. In tropical regions, conventional greenhouses are not necessary for float systems.
Intact-root greenhouse seedling production has been examined for several years as an alternative to field or greenhouse plant beds. In Canada, seedlings produced in Todd cells were more uniform, under went less transplanting shock and survived better than bareroot seedlings from conventional plant beds in greenhouses. The proportion of root to shoot was much higher for seedlings from Todd cells than for bareroot seedlings (Walker, 1980). In the field, plants from Todd cells flowered earlier than bareroot seedlings.
Currently, cell numbers range from 200 to 392 cells per tray.
Economic studies have shown greenhouse float systems to be slightly more expensive than field plant beds. However, float systems offer significant labor advantages as compared to plant beds. In particular, the float system saves labor during the critical planting period, allowing tobacco to be transplanted in a timely manner.
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