Forward thinking

Greenhouse Management: What factors do growers need to consider when choosing a greenhouse covering?

David Morrison: I always encourage growers to seek the knowledge of greenhouse designers or manufacturers when choosing a greenhouse covering. However, it can help to know the predominant factors and how to evaluate them for yourself.

It starts with understanding the light needs of the prospective crops and the target year-round production schedule. This information will define the required Daily Light Integral (DLI) for each month of operation. The location of the greenhouse, its latitude, and its local climate determine how much DLI is available each month. Next are the energy costs to operate the greenhouse. These costs include the local electric rates for the lighting load and the cost to heat the greenhouse. With this information in hand, there are tools to help growers evaluate glazing options. For example, the USDA-funded LAMP project has lighting calculators to estimate supplemental lighting costs. This resource can show the impact of different glazing materials on production costs.

GM: How does greenhouse glazing affect production costs?

DM: I recommend growers consider light as a process input. As water, growing media, and nutrients have a cost, so does bringing light to the crop. There are capital costs to install glazing on the structure and to add supplemental lighting. Then there are ongoing operational costs. These ongoing costs include electricity to run supplemental lighting and fuel to overcome heat loss through the glazing. The choice of greenhouse glazing affects both costs. The more light transmitted through the glazing, the lower the supplemental lighting costs. The more thermally insulating the glazing, the lower the heating costs.

GM: Can you give an example of how glazing impacts costs over time?

DM: Recently, I looked at a medium-sized greenhouse growing tomatoes in Kalamazoo, Michigan. Tomatoes need a high DLI for maximum production—about 30 umol per square meter per day. I used the LAMP calculator to estimate the return on investment by covering the structure with Roehm’s ACRYLITE® Alltop 16 mm double-wall glazing. This glazing provides exceptional light transmission compared to many others on the market. The additional light transmitted was predicted to reduce electrical lighting costs by 20% during a single year of crop production. The simple payback on that investment was 16 months. Over ten years of operation, the investment in ACRYLITE® multi-skin glazing was equivalent to investing cash at an interest rate of 75%.