Plugs are seedlings that retain their undisturbed root system within a core of media. They are planted as seeds into shallow, chambered flats known as plug sheets (Figure 14-3), which vary depending on the size of the plug being produced. Typical plug sizes range from Уш" X 3/4" X 3/4" deep to 13/іб" X 1V4" X 7/e" deep on 21" X 11" plug sheets that contain from 98 to 800 chambers per sheet. The plug chambers may be either square or round, resulting in a root plug of the same shape.

While plugs are used in many areas of horticulture, in the produc­tion of ornamentals they are most widely used by the growers of bed­ding plants. A majority of growers prefer to use plugs over other seed propagation techniques, for several reasons:

• transplant shock, common to bareroot seedlings, and transplanting time are reduced

• plugs do not overcrowd as quickly as seedlings in germination flats, so they can be held longer awaiting transplant

• sowing of the seed can be automated and there is no need to thin the seedlings after germination, resulting in the need for fewer seeds and in time saved

figure 14-3. Flat with 98 separate cells for seedlings (Delmar/ Cengage Learning)

• plugs can be transplanted automatically, further reducing time and labor costs

• shorter crop production time permits more crops to be produced in the same facility

Although some growers do start their own plugs, the trend is toward specialization in plug production by large wholesale growers who can provide the controlled environments necessary to produce high-quality plugs at reasonable costs. They then sell the plugs to other growers for transplanting and further growth.