The greenhouse gardener must attend to three equally important matters if he is to have healthy, pest-free, disease-free plants, and he must be alert to all three matters at all times and continuously: environment, good housekeeping, and preventive control of pests and diseases.
The first important step is to maintain a healthy physical environment for plants. Pay particular attention to watering, control of temperature, ventilation, sanitary working conditions, and introduction of new plants to the greenhouse, and safe storage of chemicals.
It is impossible to specify hard-and-fast rules about watering greenhouse plants because the plants vary in their water requirement. The general recommendations are: water when temperature is rising (usually before noon); try to avoid watering on dull, cloudy days; try not to let moisture remain on foliage overnight when greenhouse temperature is lower; and do not over water. Although you cannot always follow the recommendations and rules, and sometimes it is not essential for you to do so, it is beneficial to keep them in mind.
Fluctuations of temperature should be avoided. If at all possible, control the heat and ventilation in your greenhouse automatically. Set the heat thermostat for the night temperature you wish to maintain, set the ventilation thermostat 10 degrees higher than the heat thermostat and then let them alone. When I started greenhouse gardening, I was told that the daytime temperature should be 10 to 20 degrees higher than the minimum maintained at night, so every morning I raised the setting on the heat thermostat and every evening I returned it to its nighttime setting. Obviously, this was unnecessary the sun automatically takes care of the extra heat during the day; and anyway, if the sun does not shine, plant growth is slowed and plants do better without the extra heat.
Fresh air is extremely important in winter as well as in summer. Take care to avoid unnecessary drafts, but admit some fresh air to the greenhouse daily, though not so frequently and for shorter periods of time during winter. The limit control on most ventilation units can be set to permit partial opening of vents during winter weather. Plant diseases and pests thrive in a warm, closed, humid atmosphere. If the greenhouse is well ventilated and air circulation around plants is maintained, few problems of this nature should be encountered. Avoid crowding your plants space them sufficiently far apart in flats or benches to permit air to circulate around them, and leave space around each potted plant for the same reason.
Use sterilized planting mediums, keep work areas spotless and hands and equipment clean. Remove fading or deteriorating flowers and leaves from plants. Do not accumulate discarded leaves and blossoms destroy them immediately, preferably by burning, if there has been any evidence of disease. Sterilize cutting tools knife blades or scissors can be passed through a match or gas flame easily to provide a quick means of sterilization.
A clean-looking "gift plant" often harbors unhatched pests. As a precautionary measure, isolate or "put on probation" for at least three weeks all gift or newly purchased plants and cuttings before placing them near or among your other plants. Even if, at the end of three to four weeks, you see no evidence of pests or disease on the new acquisitions, dust or spray with pesticide before allowing them to join the plant community.
Chemicals used in pesticides, fungicides, and fertilizers may be poisonous to humans and animals, and these materials should be stored in a convenient but safe area, preferably under lock and key, so they are accessible only to the knowledgeable gardener for his own use.