Evaluating field-bred lettuce varieties for hydroponic greenhouse production
University of Arkansas researchers trialed 65 lettuce varieties to determine their potential for production in greenhouse hydroponic systems.
By David Kuack
An increasing number of greenhouse ornamental plant growers are looking to expand into edible crops. There are also field vegetable growers who would like to expand their production to include greenhouse crops.
Some of the easier and faster crops for growers to try to produce in a greenhouse are lettuce and other fresh greens.
One of the issues these growers are facing is what varieties of lettuce can be grown in a greenhouse environment. Much of the commercial lettuce breeding is focused on outdoor field production. Growers looking to expand their lettuce offerings beyond commonly produced greenhouse varieties usually have to do their own trials looking for field varieties that can be adapted to a greenhouse environment.
Need to expand greenhouse varieties
University of Arkansas horticulture professor Mike Evans said he is constantly receiving inquiries from growers about what lettuce varieties can be grown in greenhouses.
“At Cultivate’14 we surveyed growers who participated in one of the greenhouse vegetable seminars about their educational and research needs,” Evans said. “One of the growers’ responses was the need for variety information.
“If you look at seed catalogs, most of the information describing lettuce varieties is based on field production, not greenhouse. So if a grower wanted to grow lettuce hydroponically in a greenhouse during the winter there is little information available. If a grower wanted to use nutrient film technique or deep flow floating systems in a greenhouse, there’s basically very little information on how lettuce varieties would do in these production systems. Most of the production information is field-based.”
Evans said there is also a need for evaluating lettuce varieties for fall, winter and spring greenhouse production. He said these variety evaluations need to be done in different regions of the country to see how they perform under different climates.
Lettuce variety evaluations
University of Arkansas researchers selected 65 lettuce varieties for evaluation in greenhouse production systems. A nutrient film technique and deep flow floating system were used for the trials.
“Our goal with the variety trials was to generate better and more variety information and to determine which varieties would work best in climates similar to ours,” Evans said. “We especially wanted to be able to make variety recommendations across a production year. That is, varieties which work well in the fall, winter and spring.
“There are certain varieties that do well during winter. But as soon as the days start getting longer, the variety begins to bolt. Or a variety may do well in the fall and spring, but during the lowest light levels of winter, it has some type of production issue.”
University of Arkansas researchers selected 65
lettuce varieties for evaluation in greenhouse
Photos courtesy of Mike Evans, Univ. of Ark.
Evans said the information that has been collected is for lettuce varieties that perform well in a glass greenhouse in Arkansas.
“These varieties may not respond the same way in Michigan, Arizona, Florida and Texas,” he said. “They also won’t respond the same way in locations where the light and humidity levels are different. These trials are probably good recommendations for growers in climates similar to ours.”
Lettuce varieties were planted from September through May. No crops were grown in June, July and August. Four crops were produced during the fall to spring cycle.
“Some growers try to grow during the summer months by chilling the nutrient solution,” Evans said. “We weren’t set up for summer production. Having trialed 65 varieties we will probably select 15 of the best performing varieties to evaluate for summer performance. For the summer evaluations we will have to use a different greenhouse set up in order to chill the nutrient solution.”
Measuring growth rate
Evans said one of major growth parameters measured was biomass production or growth rate.
“The quicker the plants grow, the shorter the production cycle,” Evans said. “Every day on the bench is cost to the grower. We looked at fresh weight and dry weight, two measures of growth.
“Some growers let lettuce grow for a specific amount of time. Other growers try to achieve a specific weight.”
Evans said the lettuce crops were grown on a 42-day production cycle in both the NFT and deep flow systems. At the end of the 42-day cycle the lettuce was harvested and measurements were taken.
“Sometimes if a variety is a fast grower, the lettuce might exceed the weight that a grower would want,” Evans said. “That tells us this variety could have been grown in a much shorter period of time. Or a variety that didn’t reach a minimum weight at the end of the 42-day cycle was considered a slow grower. Fresh and dry weights were used as a measure of how fast a variety can grow. How fast can a variety put on biomass? That is what growers are selling—biomass.”
Lettuce varieties that did well in a nutrient film
technique system tended to do well in a deep
flow float system.
Evans said there were similarities in how varieties performed in the two production systems.
“If the varieties did poorly in NFT, they tended to perform similarly in deep flow too,” he said. “If a variety did well in NFT, odds were high that it did really well in deep flow.”
Evans said the two most common problems he hears about lettuce from growers are powdery mildew and tipburn.
“Ninety percent of the calls I receive are about these two problems,” he said. “We rated the lettuce varieties we trialed for tipburn and powdery mildew. Powdery mildew, in our region of the country, is the disease that can often give growers fits. It can really wallop a lettuce crop. We also measured the incidence of tipburn, which can be a problem on a number of greens.”
Evans said semi-heading and heading (butterhead) types seem to be more prone to tipburn.
“What happens is that as these varieties start to form heads there is an area of high humidity,” he said. “There is this little microclimate of high humidity. If a grower is growing under real high humidity, has structures with poor air circulation or the nutrition levels aren’t right, a calcium deficiency can occur. These can create a tipburn problem. We saw much less tipburn on varieties that tend to be loose leaf types.