Some greenhouse plants get too tall to make desirable potted plants. Poinsettias, Easter lilies, chrysanthemums, and some hydrangeas can all get leggy if started too early or subjected to overly warm tempera­tures during their periods of most rapid vegetative growth. The applica­tion of growth retardants to the plants will help to regulate their size. For a listing of the most commonly used florist crop growth retardants, their mode of action, and the crops with which they are effective, refer to Chapter 4. The use of growth retardants and the time of their applica­tion must be included in the production schedule for each crop.

PEST CONTROL_________________________________

Greenhouses provide an ideal environment for plants and pests alike. The conditions of high temperature and high humidity, coupled with close spacing of a monoculture, make pest control a necessary element in the greenhouse production schedule. Also necessary is employees’ cooperation and understanding of how they can personally assist in the control effort. Pest control should be included in the job training for all greenhouse workers.

Chapter 6 describes the pests of ornamental plants: insects, infec­tious parasitic pathogens, weeds, and other injurious agents. It also describes the profitable control level sought by growers and the four principles of control. Efforts to prevent pests from becoming estab­lished in the greenhouse apply the principle of exclusion. For example, sources for plant materials that guarantee disease-free and insect-free stock are essential. Eradication, another principle of control, seeks to remove or eliminate pests that are already present in the greenhouse. Destruction of infected or infested plants, elimination of alternate host weeds beneath benches, soil pasteurization, and use of chemical sprays all apply the principle of eradication. Protection is the third principle of control. It seeks to place a barrier between the crop and the pests. Screens on greenhouse vents and doors, and growth conditions that favor the hosts more than the pests, are methods of protection. Spraying plants with chemicals before insects or pathogens arrive also applies the principle of protection. The principle of resistance is applied each time a grower selects a resistant variety for production.

Pathogenic inoculum, insects, and weed seeds enter the greenhouse in numerous ways. Insect eggs, viruses, fungal spores, or bacteria can be on cuttings or stock plants purchased from another grower. Oxalis, other weed seeds, and nematodes can be in the soil of plants purchased for forcing. Pests may be carried in by the wind, on equipment, or on the shoes and clothing of greenhouse workers. In retail operations, every customer who comes through the door is a potential source of inoculum of many common greenhouse pests.

Once inside the greenhouse, pests are disseminated by irrigation water, air currents, workers, hoses, and other contaminated tools.

Pest control in greenhouses is complicated by two factors:

1. The complex life cycle of many insects and some fungi that allows them to be unaffected by many chemical pesticides throughout much of their lives.

2. The rapid development of resistance to particular pesticides that characterizes some pests, especially insects.

For example, many insecticides are only effective against the adult stages of an insect. The larval, nymph, and egg stages may not be affect­ed at all by a fumigant or spray. Hence, two or three days after spray­ing, the crop can again be overrun. With insects such as whiteflies, an insecticide may have to be applied every two or three days for a month to control a succession of adult populations before they are able to lay eggs.

The problem of natural resistance develops when the same pesti­cide is used repeatedly. No spray, dust, or fumigant kills 100 percent of a pest population. A few are always unaffected. As they survive and breed, a pest population builds within the greenhouse, immune to the pesticide that once controlled effectively. To avoid the buildup of resis­tant pest populations, a grower must rotate the choice of pesticides fre­quently. This should be part of the production schedule for each crop.