High relative humidity is one of the major contributing factors to Botrytis blight, a common fungal disease of bedding plants. Warm air holds more moisture than cool air. During warm days, the greenhouse air is more humid. As the air cools in the evening, the moisture-holding capacity drops until the dew point is reached. Water then begins to condense on surfaces including leaves. Humidity can be reduced by exhausting the moist air and replacing it with cooler outside air that is drier. Activate the exhaust fans for a few minutes and then reheat the greenhouse to raise the air temperature to the desired night time level. Repeat this process if needed. The cooler, outside air will lower humidity levels as it is warmed in the greenhouse. A relay should be installed to lock out the furnace or boiler while the fans are running so that flue gases are not drawn back into the greenhouse. This will help to prevent air pollution damage to sensitive seedlings. Vent (for 20 to 30 seconds) and heat two or three times per hour in the evening after the sun goes down and early in the morning at sunrise.

Using horizontal airflow (HAF) can also reduce condensation. HAF fans keep the air moving in the greenhouse, helping to minimize temperature differentials and cold spots where condensation occurs. Air that is moving is continually mixed. The mixed air along the surface does not cool below the dewpoint so does not condense on plant surfaces.

In addition, cultural practices can be used to reduce humidity within the plant canopy. These include proper watering practices and spacing of plants. Since most vegetable bedding plants are grown in flats that are spaced flat to flat, reducing humidity within the canopy is difficult. Proper planting dates, plant nutrition, watering practices and height management techniques help to prevent lush, overgrown plants thereby reducing humidity within the canopy.

Handwatering and overhead irrigation systems are the primary methods of watering vegetable bedding plants. Always water in the morning to reduce the length of time the leaves stay wet after irrigating to prevent foliar diseases. Rising temperatures during the day will evaporate water from the foliage, so the leaves stay dry. Avoid watering late in the day or when water will sit on leaf surfaces for long periods of time.