Michael J. Buschermohle,

Samuel J. Ray

James B. Wills

Biosystems Engineering and Environmental Science

Hydroponic greenhouse tomato growers are experiencing reductions in yield due to detrimental environmental conditions. At maturity, the plants are five to seven feet tall and form a dense canopy that restricts air movement throughout the greenhouse. The stagnate air around the plants is often higher in relative humidity (RH), lower in CO₂ concentrations and colder that required for optimum production.

Ideally, the relative humidity of the air surrounding hydroponic tomato plants should be maintained around 70% at night and 85% during the day. Because the air around the plants is stagnated due to the dense canopy that forms as the plants grow, the plants are often exposed to relative humidity levels of 90% or higher. At these higher humidity levels, the tomato plants are more vulnerable to diseases such as grey mold, leaf mold and powdery mildew. Humidity levels above 90% can also affect yields. Above 90% RH transpiration is impeded. Without adequate transpiration, the nutrient enriched water at the roots will not reach the stems and fruit. Poor pollination prevails at these humidity levels because pollen grains tend to coagulate and are not well dispersed. Insufficient transpiration and pollination can result in smaller fruit, irregularly shaped fruit which ultimately reduce yields.

Optimum CO₂ concentrations are difficult to achieve with poor air distribution in the plant canopy. The lack of CO₂ in areas of poor ventilation reduces the amount of CO₂ that is available for plant uptake to achieve optimum production.

Growing temperature influences vegetative growth, cluster development, fruit setting and fruit yield. Optimum growing temperatures range from 80 to 90F during the day with a minimum of 64F at night. Poor air circulation in the plant canopy results in inadequate heat distribution throughout the greenhouse which inhibits the air around the plant to receive the necessary heat to maintain proper growing temperatures. Below optimum growing temperatures results in slower plant development, lower yields and can delay harvesting.

Project Description

The goal of this project was to design and test a low-cost system that would control relative humidity levels and reduce temperature stratification by circulating air within the plant canopy. The design concept was to move warmer, less humid air from the eave of the greenhouse through the plant canopy using blowers and perforated ducts that run along the length of the row. The blower is placed at one end of the double row of plants. Air is suppled to the blower through a vertical intake pipe that extends above the plant canopy. The blower pulls the warmer, less humid air from the eave of the greenhouse and forces it through the perforated duct running along the bottom of the canopy within the double row. The prototype air distribution system was tested in a 30' x 96' commercial hydroponic greenhouse during a spring growing season. Yield data, air temperature and air humidity levels were compared in the experimental greenhouse to an adjacent 30' x 96' greenhouse that was used as a control.

Preliminary testing of the prototype air distribution system during the spring growing season indicates the system has the potential to effectively control the environment in a hydroponic tomato production greenhouse. The amount of time the environment in the plant canopy remained in the danger zone where conditions were favorable for disease infestations and plant physiological damage (above 90% RH and below 60F) were calculated for each house. The growing tomato plants in the experimental house remained in the danger zone less than 10 hours as compared to over 245 hours in the control house over the test period. These danger zone times are an indication why disease problems were observed in the control house, but were not observed in the experimental house.

The tomato plants in the experimental house yielded approximately 2.5 pounds/plant more than plants in the control house. This represents a 14.5% yield increase. In addition, earlier ripening occurred in the experimental house. Based on the cost to install and operate the system, only a 5% increase in yield is required to make this air distribution system economically feasible for growers to adopt this new technology.

A research project is currently underway at the Knoxville Experiment Station to test, under more controlled conditions, the effectiveness and feasibility of using this prototype air distribution system to better control the growing environment in hydroponic tomato production greenhouses. Questions such as will this new technology consistently increase yields for both fall and spring crops, does this system promote earlier ripening, will the system reduce the incidence of diseases and thus reduce chemical application costs, and can this system be used to reduce heating costs will be addressed over the next two years.

Email all comments and suggestions to ghonea@utk.edu
Copyright © 1999 by The University of Tennessee. All rights reserved.

This research represents one season's data and does not constitute recommendations.  After sufficient data is collected over the appropriate number of seasons, final recommendations will be made through research and extension publications.