Greenhouse crops are watered either by hand with a hose or with semi­automatic watering systems. In large greenhouse ranges, hand-watering is too time-consuming and automated systems are necessary.

Watering done correctly becomes irrigation, the maintenance of a proper balance of both moisture and air in the soil of the crops. Too much water leaves too little air space, and the root system may rot or at least be dwarfed. Too little water does not sufficiently drive old air out through the bottom and draw fresh air in from the top. Too little water can also cause shallow root development, soluble salt buildup, and reduced plant and blossom size.

The amount of water applied to greenhouse plants is totally under the control of the grower. Nature can be blamed for a lot that goes wrong in a greenhouse, but not a water excess or deficiency. Correct irrigation techniques are not the same for all crops, nor are they uniform for a sin­gle crop throughout the year. The first and perhaps most difficult thing that a new greenhouse worker must be taught is how to water plants correctly. Proper irrigation can be encapsulated as follows:

• Sufficient water should be applied each time to drain from the bench or pot slightly but not excessively.

• Potted plants must have room at the top of the container for water. The soil level in each pot must be the same.

• Surface drying of the soil between irrigations is desirable. It permits new air to enter the soil.

• Clay containers require more frequent watering than plastic containers.

• Larger plants require more water than smaller plants.

• Artificial media require more frequent watering than all-peat or field soil media.

• Less irrigation is needed during the winter and on cloudy days than in the summer and on sunny days.

• The need for water should not be determined by sticking a finger into the soil if a systemic pesticide has been used on the crop. The pesticide may be toxic and absorbed through the skin.

• Potted plants and benches do not dry out uniformly. Usually, the soil at the edge of a bench or the pots along the edges dry sooner than those at the center of the bench. Also, plants nearest the heating system or ventilators can be expected to need water sooner than other plants. The determination to water must be based on the needs of plants away from these early drying locations. Even semiautomated irrigation systems may require hand-watering of crops in the early drying locations.

• Several kinds of moisture meters are available to help determine when to water and how much to apply. They are not foolproof and should be regarded as guides only.

• Fertilizers and pesticides can be applied as liquids through the irrigation system. To do so saves time.

• Water should be applied at the base of the plants. Wetting foliage should be avoided.

• Watering should be completed early enough so that surface water can dry before nightfall.