Greenhouse management wasn’t my career forte. My expertise is in ornamental plants and landscape design. Being involved with the nonprofit The Greenhouse Project since its inception has given me a great opportunity to improve my understanding of how greenhouses work. You would think growing plants in a greenhouse would be a breeze, since the environment is completely controlled. However, greenhouse growing has as many challenges as growing outdoors, although the challenges can be somewhat different. For example, there are often lots of insect pests in a greenhouse, especially one managed organically. Unlike outdoors, where nature might help manage pests with beneficial insects and predators, beneficial insects must be purposefully released into a greenhouse and then nurtured to keep them alive. Maintaining optimal day and nighttime temperatures can be extremely difficult. Greenhouses can get quite hot during the day and it always seems the heater breaks in the middle of a freezing cold night risking the demise of the entire crop.
Water management is imperative in a greenhouse, particularly when plants are grown in containers rather than in the ground, since containers have a reduced water-holding capacity. Not only is the amount, duration and timing of irrigation critical to plant health, atmospheric evaporative demand is too. Moisture in the air is necessary to keep plants from drying out, especially during the heat and intense sun of summer. However, air circulation is also essential to prevent water collecting on leaves, which encourages disease organisms. There’s a fine line between not enough moisture in the air and too much.
If an environment is too dry, plants don’t thrive. Greenhouse growers measure vapor pressure deficits (VPD) to tell how much moisture is in the air. Cory King, The Greenhouse Project manager, has found tomatoes really suffer if the VPD is not optimum. Measuring the VPD tells him when to mist to raise moisture content in the air to prevent the tomatoes drying out. With a slightly elevated VPD, research has shown an increase in photosynthesis and in yield in tomatoes. Yet, too high a VPD can cause tomatoes to crack (Iraqi, Gagnon, Dubé and Gosselin, http://hortsci.ashspublications.org/content/30/4/846.5.abstract). It’s all about balance.
There’s so much more to greenhouse production than sowing seeds, watering and waiting for perfect tomatoes. Using scientific methods allows plants’ needs to be met most efficiently, which increases productivity.
JoAnne Skelly is Associate Professor & Extension Educator, Emerita at University of Nevada Cooperative Extension. She can be reached at firstname.lastname@example.org.